UgridDataset

Top-level container for UGRID NetCDF mesh data. Provides the user-facing API for reading, writing, inspecting, and operating on unstructured mesh data.

UgridDataset is a thin, GIS-aware facade: its public methods delegate the heavy lifting to the sibling modules of the subpackage (spatial ops, interpolation, I/O, plotting):

Hold "Ctrl" to enable pan & zoom

flowchart LR
    U["UgridDataset"]
    U -->|"clip · subset_by_bounds"| SP["spatial.clip_mesh<br/>spatial.subset_by_bounds"]
    U -->|"to_dataset"| IN["interpolation.mesh_to_grid"]
    U -->|"read_file · to_file"| IO["io.parse_ugrid_topology<br/>io.write_ugrid_topology"]
    U -->|"plot · plot_outline"| PL["plot.plot_mesh_data<br/>plot.plot_mesh_outline"]
    U -->|"metadata"| MD["UgridMetadata"]

See the subpackage overview for how UgridDataset composes Mesh2d, Connectivity, and MeshVariable.

pyramids.netcdf.ugrid.UgridDataset

Bases: _UgridFacades

Container for UGRID NetCDF mesh data.

Combines mesh topology, data variables, and global attributes into a single object with GIS-aware operations. Does NOT inherit from Dataset or RasterBase — the raster paradigm does not apply to unstructured meshes.

Attributes:

Name	Type	Description
_mesh		Mesh2d topology instance.
_data_variables		Mapping of variable name to MeshVariable.
_global_attributes		File-level NetCDF attributes.
_topology_info		Parsed UGRID topology metadata.
_crs_wkt		CRS in WKT format.
_file_name		Source file path, if read from disk.

Source code in src/pyramids/netcdf/ugrid/dataset.py

class UgridDataset(_UgridFacades):
    """Container for UGRID NetCDF mesh data.

    Combines mesh topology, data variables, and global attributes
    into a single object with GIS-aware operations. Does NOT inherit
    from Dataset or RasterBase — the raster paradigm does not
    apply to unstructured meshes.

    Attributes:
        _mesh: Mesh2d topology instance.
        _data_variables: Mapping of variable name to MeshVariable.
        _global_attributes: File-level NetCDF attributes.
        _topology_info: Parsed UGRID topology metadata.
        _crs_wkt: CRS in WKT format.
        _file_name: Source file path, if read from disk.
    """

    def __init__(
        self,
        mesh: Mesh2d,
        data_variables: dict[str, MeshVariable],
        global_attributes: dict[str, Any],
        topology_info: MeshTopologyInfo | None = None,
        crs_wkt: str | None = None,
        file_name: str | None = None,
        volume_topology: VolumeTopology | None = None,
    ):
        """Assemble a dataset from an already-built mesh and its variables.

        The low-level constructor. Prefer :meth:`read_file` for a file, :meth:`from_arrays`
        for raw arrays, or one of the ``from_*`` bridges -- each of those builds the mesh and
        the metadata record and then calls this. It is also what every derivation
        (:meth:`isel`, :meth:`reduce`, :meth:`copy`, ...) calls through ``type(self)(...)``,
        which is how a subclass survives them.

        Args:
            mesh: The topology the variables sit on.
            data_variables: Mapping of variable name to :class:`MeshVariable`. Held by
                reference, not copied, so a caller must not mutate it afterwards.
            global_attributes: The file-level attributes, e.g. ``Conventions``.
            topology_info: The record of the on-disk variable names and the declared
                topology dimension. ``None`` for a mesh built from arrays, where there are
                no file variables to name. Defaults to ``None``.
            crs_wkt: WKT of the mesh CRS. ``None`` leaves the mesh unreferenced, and
                :attr:`epsg` then answers ``None``. Defaults to ``None``.
            file_name: The path this dataset was read from, carried through derivations so a
                :meth:`to_file` round trip and ``repr`` can name it. ``None`` for an
                in-memory mesh. Defaults to ``None``.
            volume_topology: The ragged (per-column) vertical layering for a genuinely 3-D
                mesh — the map from faces to the flat ``volume`` axis that a
                ``location="volume"`` variable is indexed through (issue #1330). ``None`` (the
                default) for a 2-D mesh or a uniform-layer 3-D one, where the dense
                ``(layer, element)`` path applies and :attr:`n_volume` answers ``None``.
        """
        self._mesh = mesh
        self._data_variables = data_variables
        self._global_attributes = global_attributes
        self._topology_info = topology_info
        self._crs_wkt = crs_wkt
        self._file_name = file_name
        self._volume_topology = volume_topology
        self._cached_crs: Any = None

    @classmethod
    def read_file(
        cls,
        path: str | Path,
        mesh_name: str | None = None,
        driver: str | None = None,
        lazy: bool = True,
    ) -> UgridDataset:
        """Open a mesh file.

        UGRID NetCDF is read here. Any other format — 2DM, Selafin, DFSU, XMDF, … — is read by a
        third-party driver registered into the ``pyramids.mesh_drivers`` entry-point group. A
        non-NetCDF suffix is dispatched by detection; a NetCDF suffix is read by the built-in
        reader first, and only a file that opens but carries no UGRID topology falls back to a
        driver claiming that suffix. Either way a successful UGRID read triggers no driver
        discovery, so a ``.nc`` read still pays nothing.

        Resolution order:

        1. ``driver`` given — that driver, or an error. Never falls back.
        2. A ``/vsi`` path or a NetCDF suffix — the built-in UGRID reader. If a NetCDF-suffixed
           file opens but declares no UGRID topology, a registered driver claiming that suffix
           is tried before giving up, so 3Di, TUFLOW FV and other NetCDF-based formats are
           reachable; discovery runs only after the built-in read has already failed.
        3. Otherwise a registered driver claiming the suffix whose ``can_open`` accepts the file.
        4. Nothing claimed it — the built-in reader anyway, so a NetCDF file under an unusual
           suffix still opens exactly as it always did.
        5. That failed too — :class:`UnsupportedMeshFormatError`, naming what is installed.

        Args:
            path: Path to the file. A GDAL virtual-filesystem path
                (``/vsimem/``, ``/vsizip/``, ``/vsicurl/``, …) is passed straight to GDAL
                rather than checked on disk.
            mesh_name: Which topology to read, for a file declaring more than one (see
                :meth:`mesh_names`). ``None`` (the default) takes the first.
            driver: Name of a registered **mesh** driver to use, bypassing detection — the
                name a plugin declares, not a GDAL driver short name. ``None`` (the default)
                resolves as described above.
            lazy: Defer reading variable values until first access. Forwarded to the driver,
                which honours it when it declares ``lazy_data``. The built-in UGRID reader is
                always lazy, so it rejects ``lazy=False`` rather than ignoring it.

        Returns:
            UgridDataset instance.

        Raises:
            FileNotFoundError: If the file does not exist.
            DriverNotExistError: If ``driver`` names a driver that is not installed.
            UnsupportedMeshFormatError: If no installed driver reads the file, and the
                built-in UGRID reader cannot either.
            IsADirectoryError: The path is a directory that no driver claims.
            TypeError: ``lazy=False`` was asked of the built-in UGRID reader, which is always
                lazy and so cannot honour it.
            RuntimeError: GDAL cannot open a file that reached the built-in reader by its
                own suffix. A file whose suffix no driver claimed is reported as
                :class:`UnsupportedMeshFormatError` instead, so this is now the native
                route only.
            ValueError: If no UGRID topology is found in the file, or ``mesh_name`` names
                a topology the file does not declare.
        """
        selected, native = _resolve_read_driver(path, driver)
        if selected is not None:
            result = _read_through_driver(selected, path, mesh_name, cls, lazy)
        else:
            try:
                result = cls._read_ugrid_netcdf(
                    path, mesh_name, wrap_unreadable=not native, lazy=lazy
                )
            except NoMeshTopologyError:
                # A NetCDF file that opened but carries no UGRID topology -- a 3Di or
                # TUFLOW FV result. Only now, after the built-in read has failed, consult a
                # driver claiming the suffix; if none does, the original error stands. `lazy`
                # is forwarded to the driver, which may honour an eager read even though the
                # built-in reader cannot. A genuine UGRID file with `lazy=False` raised
                # TypeError inside `_read_ugrid_netcdf` instead, before reading any geometry.
                fallback = _netcdf_fallback_driver(path, driver)
                if fallback is None:
                    raise
                result = _read_through_driver(fallback, path, mesh_name, cls, lazy)
        return result

    @classmethod
    def _read_ugrid_netcdf(
        cls,
        path: str | Path,
        mesh_name: str | None,
        wrap_unreadable: bool = False,
        lazy: bool = True,
    ) -> UgridDataset:
        """Read a UGRID NetCDF file through GDAL's multidimensional API.

        The built-in reader, unchanged from before driver dispatch existed. It is reached both
        for a NetCDF suffix and as the fallback when no registered driver claims a file.

        Args:
            path: Path to the file, or a GDAL virtual-filesystem name.
            mesh_name: Which topology to read. ``None`` takes the first.
            wrap_unreadable: True when this call is step 4 of :meth:`read_file`'s resolution —
                a file whose suffix no driver claimed. A failure then reports the installed
                drivers rather than only GDAL's complaint, which would misleadingly suggest the
                file is corrupt rather than unsupported.
            lazy: Defer variable values to first access. The built-in reader is always lazy, so
                ``lazy=False`` is rejected with :class:`TypeError` once a topology is confirmed —
                before any geometry is read. Defaults to True.

        Returns:
            UgridDataset instance.

        Raises:
            FileNotFoundError: If the file does not exist.
            UnsupportedMeshFormatError: If GDAL cannot open a file that reached here as a
                fallback from driver detection.
            NoMeshTopologyError: If the file opens but declares no UGRID topology. Subclasses
                :class:`ValueError`, and is what lets a caller fall back to a driver.
            TypeError: If ``lazy=False``; the built-in reader has no eager mode.
            ValueError: If ``mesh_name`` names a topology the file does not declare.
        """
        # A GDAL virtual-filesystem path (/vsimem/, /vsizip/, /vsicurl/, …) is not an OS path:
        # `Path` would mangle the separators and `exists()` would always be False, so hand
        # those straight to GDAL and let it report. `from_bytes` reads through /vsimem/.
        target = str(path)
        if target.startswith("/vsi"):
            opened_name = target
        else:
            path = Path(path)
            if not path.exists():
                raise FileNotFoundError(
                    f"File not found: {Path(path)}"
                )  # pragma: no cover - `_require_existing_file` runs first on every public route
            opened_name = str(path)

        # GDAL exceptions are enabled package-wide (`Config` calls `gdal.UseExceptions()`),
        # so a file no driver recognises normally raises RuntimeError from inside the binding.
        # That is process-global state, though, and any dependency can call
        # `gdal.DontUseExceptions()` -- after which OpenEx answers None and this guard is the
        # difference between naming the file and an `AttributeError: 'NoneType' object has no
        # attribute 'GetRootGroup'`.
        ds = _open_multidim(opened_name, str(path), wrap_unreadable)
        rg = ds.GetRootGroup()
        if rg is None:  # pragma: no cover - no multidim driver answers None here
            # Kept as a clear message rather than letting `parse_ugrid_topology(None)`
            # raise an AttributeError, though every multidim driver that opens a file
            # does expose a root group.
            raise ValueError(f"Cannot get root group from: {path}")

        topologies = parse_ugrid_topology(rg)
        if not topologies:
            raise NoMeshTopologyError(f"No UGRID mesh topology found in: {path}")

        if not lazy:
            # The built-in reader always defers variable values to first `.data` access and has
            # no eager mode, so `lazy=False` cannot be honoured. Raise here -- after topology is
            # confirmed, before `from_gdal_group` reads any geometry -- so the rejection is cheap
            # and both `read_file` and `read_all` get it. A driver-backed file never reaches this:
            # it raised `NoMeshTopologyError` above, and `lazy` is forwarded to the driver instead.
            raise TypeError(
                "lazy=False is not supported for UGRID NetCDF: the built-in reader always defers "
                "variable values to first access. Call `.load_array()` on a variable to read it "
                "now."
            )

        # A file may declare several meshes. The default stays the first one — changing that
        # would silently alter every existing caller — but `mesh_name` picks any of them, and
        # `read_all` reads them all. Without either, a multi-mesh file quietly dropped the rest.
        if mesh_name is None:
            topo_info = topologies[0]
        else:
            available = [topo.mesh_name for topo in topologies]
            chosen = [topo for topo in topologies if topo.mesh_name == mesh_name]
            if not chosen:
                raise ValueError(
                    f"No UGRID topology named {mesh_name!r} in {path}; it declares "
                    f"{available}."
                )
            topo_info = chosen[0]
        mesh = Mesh2d.from_gdal_group(rg, topo_info)

        # Resolve to an absolute path before threading it into the lazy variable loaders:
        # data reads are deferred to first `.data` access (PERF-3), which re-opens the file.
        # A relative path would break that deferred open if the process changed directory in
        # the meantime; the old eager read was immune because it read while still in `read_file`.
        # A virtual-filesystem name is already absolute and has no OS path to resolve.
        source_name = (
            opened_name if opened_name.startswith("/vsi") else str(Path(path).resolve())
        )
        data_variables = _read_data_variables(rg, topo_info, source_name)
        # Re-attach each variable's active (wet/dry) mask from its `<var>_active` companion,
        # and rebuild the ragged volume topology from its companion arrays, if present — both
        # are additive, so a file carrying neither reads exactly as before (issues #1331/#1330).
        _attach_active_masks(rg, data_variables)
        volume_topology = parse_volume_topology(rg, topo_info.mesh_name)

        global_attrs = _read_attributes(rg)

        ds = None

        result = cls(
            mesh=mesh,
            data_variables=data_variables,
            global_attributes=global_attrs,
            topology_info=topo_info,
            crs_wkt=topo_info.crs_wkt,
            file_name=str(path),
            volume_topology=volume_topology,
        )
        return result

    @property
    def mesh(self) -> Mesh2d:
        """The mesh topology."""
        return self._mesh

    @property
    def mesh_name(self) -> str:
        """Name of the mesh topology variable."""
        result = (
            self._topology_info.mesh_name if self._topology_info else DEFAULT_MESH_NAME
        )
        return result

    @property
    def data_variable_names(self) -> list[str]:
        """Names of all data variables."""
        result = list(self._data_variables.keys())
        return result

    @property
    def crs(self) -> CRS | None:
        """CRS as a pyproj.CRS object, or None. Cached after first access."""
        if self._cached_crs is None and self._crs_wkt is not None:
            try:
                self._cached_crs = CRS.from_wkt(self._crs_wkt)
            except Exception:  # nosec B110 - best-effort CRS parse; falls back to None
                pass
        return cast("CRS | None", self._cached_crs)

    @property
    def epsg(self) -> int | None:
        """EPSG code of the CRS, or None."""
        crs = self.crs
        result = crs.to_epsg() if crs is not None else None
        return result

    @property
    def crs_wkt(self) -> str | None:
        """The mesh CRS as WKT, or `None` when there is none that parses.

        Gated on a successful parse rather than returning `_crs_wkt` verbatim.
        `_crs_wkt` is copied straight out of the file and :attr:`crs` tolerates
        an unparseable one on purpose, swallowing the error and reporting no
        CRS. Handing the raw string to a caller would move that parse out from
        under the handler, so a malformed WKT would start raising downstream
        instead of being reported as "no CRS" here.
        """
        return self._crs_wkt if self.crs is not None else None

    @property
    def bounds(self) -> tuple[float, float, float, float]:
        """Mesh bounding box as (xmin, ymin, xmax, ymax)."""
        return self._mesh.bounds

    @property
    def global_attributes(self) -> dict[str, Any]:
        """File-level NetCDF attributes."""
        return self._global_attributes

    @property
    def n_node(self) -> int:
        """Number of mesh nodes."""
        return self._mesh.n_node

    @property
    def n_face(self) -> int:
        """Number of mesh faces."""
        return self._mesh.n_face

    @property
    def n_edge(self) -> int:
        """Number of mesh edges."""
        return self._mesh.n_edge

    def get_data(self, variable_name: str) -> MeshVariable:
        """Get a data variable by name.

        Args:
            variable_name: Name of the data variable.

        Returns:
            MeshVariable instance.

        Raises:
            KeyError: If the variable name is not found.
        """
        if variable_name not in self._data_variables:
            raise KeyError(
                f"Variable '{variable_name}' not found. "
                f"Available: {self.data_variable_names}"
            )
        result = self._data_variables[variable_name]
        return result

    def __getitem__(self, key: str) -> MeshVariable:
        """Get a data variable by name using bracket notation.

        Args:
            key: The variable name.

        Returns:
            MeshVariable: The variable.

        Raises:
            KeyError: No variable is named ``key``; the message lists the available names.
        """
        return self.get_data(key)

    def __contains__(self, key: str) -> bool:
        """Whether ``key`` names a data variable of this dataset.

        Args:
            key: The variable name to test.

        Returns:
            bool: True when the dataset carries a variable of that name.
        """
        return key in self._data_variables

    def __len__(self) -> int:
        """The number of data variables.

        Returns:
            int: How many data variables the dataset carries. Note this counts *variables*,
            not elements -- :attr:`n_face` / :attr:`n_node` / :attr:`n_edge` are the mesh
            sizes.
        """
        return len(self._data_variables)

    def __iter__(self) -> Any:
        """Iterate over the data variable names.

        Returns:
            Iterator[str]: The variable names, in insertion order -- so ``list(dataset)``
            and ``dataset.keys()`` agree, and ``for name in dataset`` reads like a mapping.
        """
        return iter(self._data_variables)

    def keys(self) -> list[str]:
        """The data variable names (read-only mapping surface).

        Returns:
            list[str]: The names, in insertion order. A list rather than a view, so it does
            not track later edits -- every variable edit returns a new dataset anyway.
        """
        return list(self._data_variables.keys())

    def values(self) -> list[MeshVariable]:
        """The data variables (read-only mapping surface).

        Returns:
            list[MeshVariable]: The variables, in the same order as :meth:`keys`. The
            variables themselves are not copied, so reading ``.data`` on one still fires its
            lazy loader.
        """
        return list(self._data_variables.values())

    def items(self) -> list[tuple[str, MeshVariable]]:
        """The ``(name, variable)`` pairs (read-only mapping surface).

        Returns:
            list[tuple[str, MeshVariable]]: One pair per variable, in insertion order.
        """
        return list(self._data_variables.items())

    def get(self, key: str, default: Any = None) -> MeshVariable | Any:
        """The variable ``key``, or ``default`` when it is absent.

        Args:
            key: The variable name to look up.
            default: What to answer when no variable is named ``key``. Defaults to ``None``.

        Returns:
            MeshVariable | Any: The variable, or ``default``.
        """
        return self._data_variables.get(key, default)

    @property
    def data_vars(self) -> dict[str, MeshVariable]:
        """A copy of the ``{name: variable}`` mapping.

        A fresh dict, so mutating it never changes the dataset — variable edits go through
        :meth:`with_variable` / :meth:`drop_variables` / :meth:`rename_variable`, which each
        return a new dataset.
        """
        return dict(self._data_variables)

    def with_variable(
        self,
        name: str,
        data: np.ndarray,
        *,
        location: str = "face",
        nodata: float | None = None,
    ) -> UgridDataset:
        """Return a new dataset with ``name`` added or replaced.

        UGRID derivations are immutable — unlike ``NetCDF.set_variable`` this does not mutate
        in place but returns a fresh dataset. A 2-D ``data`` array is treated as temporal
        (leading axis = time); a 1-D array is a static per-element variable.

        Args:
            name: Variable name to add or replace.
            data: The values; ``(n_elements,)`` static or ``(n_time, n_elements)`` temporal.
            location: Mesh location — ``"face"``, ``"node"`` or ``"edge"``. Defaults to
                ``"face"``.
            nodata: Optional no-data value for the new variable.

        Returns:
            UgridDataset: A new dataset carrying the variable.
        """
        arr = np.asarray(data)
        variable = MeshVariable(
            name=name,
            location=location,
            mesh_name=self.mesh_name,
            shape=arr.shape,
            nodata=nodata,
            _data=arr,
        )
        new_vars = dict(self._data_variables)
        new_vars[name] = variable
        return self._rebuild(new_vars)

    def drop_variables(self, names: str | list[str]) -> UgridDataset:
        """Return a new dataset without the named variable(s).

        Args:
            names: A variable name, or a list of them, to drop.

        Returns:
            UgridDataset: A new dataset without those variables.

        Raises:
            KeyError: A named variable is not present.
        """
        drop = [names] if isinstance(names, str) else list(names)
        new_vars = dict(self._data_variables)
        for name in drop:
            if name not in new_vars:
                raise KeyError(
                    f"Variable {name!r} not found. Available: {self.data_variable_names}"
                )
            del new_vars[name]
        return self._rebuild(new_vars)

    def rename_variable(self, old: str, new: str) -> UgridDataset:
        """Return a new dataset with variable ``old`` renamed to ``new``.

        Args:
            old: The current variable name.
            new: The new name.

        Returns:
            UgridDataset: A new dataset with the variable renamed.

        Raises:
            KeyError: ``old`` is not present.
            ValueError: ``new`` already names a different variable.
        """
        if old not in self._data_variables:
            raise KeyError(
                f"Variable {old!r} not found. Available: {self.data_variable_names}"
            )
        if new in self._data_variables and new != old:
            raise ValueError(f"Variable {new!r} already exists.")
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if name == old:
                # `replace` keeps the lazy loader and every other field, only re-labelling
                # the variable, so a rename never forces a read.
                new_vars[new] = replace(var, name=new)
            else:
                new_vars[name] = var
        return self._rebuild(new_vars)

    @property
    def metadata(self) -> UgridMetadata:
        """Full metadata summary for this dataset."""
        topo_tuple = (self._topology_info,) if self._topology_info else ()
        data_vars = {name: var.location for name, var in self._data_variables.items()}
        conventions = self._global_attributes.get("Conventions")
        result = UgridMetadata(
            mesh_topologies=topo_tuple,
            data_variables=data_vars,
            global_attributes=self._global_attributes,
            conventions=conventions,
            n_nodes=self.n_node,
            n_faces=self.n_face,
            n_edges=self.n_edge,
        )
        return result

    def to_dataset(
        self,
        variable_name: str,
        cell_size: float,
        method: str = "nearest",
        bounds: tuple[float, float, float, float] | None = None,
        epsg: int | None = None,
        nodata: float = -9999.0,
    ) -> Dataset:
        """Convert a mesh variable to a regular-grid Dataset.

        Interpolates mesh data onto a regular grid and returns a
        standard pyramids Dataset. This is the bridge between
        unstructured (UGRID) and structured (raster) worlds.

        Args:
            variable_name: Name of the data variable to rasterize.
            cell_size: Target grid cell size in coordinate units.
            method: Interpolation method ("nearest" or "linear").
            bounds: Target (xmin, ymin, xmax, ymax). Defaults to mesh bounds.
            epsg: Target EPSG code. Defaults to mesh CRS.
            nodata: No-data value for the output raster.

        Returns:
            pyramids Dataset with the interpolated data.

        Raises:
            KeyError: No variable is named ``variable_name``.
            ValueError: The variable exists but holds no data (nothing was loaded and it
                carries no lazy loader), so there is nothing to interpolate.
        """
        var = self.get_data(variable_name)
        data = var.data
        if data is None:
            raise ValueError(f"Variable '{variable_name}' has no data loaded.")
        if var.has_time:
            # First step along the variable's own time axis (not blindly axis 0, which for
            # a trailing-time variable would take the first face) — see `_element_values`.
            data = np.take(np.asarray(data), 0, axis=cast("int", var.time_index))

        grid_array, geotransform = mesh_to_grid(
            mesh=self._mesh,
            data=data,
            location=var.location,
            cell_size=cell_size,
            method=method,
            bounds=bounds,
            nodata=nodata,
        )

        # `crs_spec` rather than `self.epsg or 4326`: a mesh carrying a projected
        # WKT with no EPSG code has `epsg is None`, and the old default stamped
        # EPSG:4326 onto metre coordinates. That is the case this resolves.
        #
        # The `or 4326` behind it keeps the *other* case as it was. A mesh with
        # no CRS at all -- no code and no WKT -- has always rasterised to WGS 84
        # here, and `to_dataset(...).to_file("out.tif")` writing a GeoTIFF with
        # no CRS instead would be a silent behaviour change for every caller
        # holding such a mesh. Narrowing that default is a decision for its own
        # change, not a side effect of teaching this line about WKT.
        target_epsg = (
            epsg if epsg is not None else crs_spec(self.epsg, self.crs_wkt) or 4326
        )
        result = Dataset.from_array(
            grid_array,
            no_data_value=nodata,
            geo_ref=GeoReference(
                geo=cast(
                    "tuple[float, float, float, float, float, float]", geotransform
                ),
                epsg=target_epsg,
            ),
        )
        return result

    def crop(
        self,
        mask: Any = None,
        touch: bool = True,
        *,
        bbox: tuple[float, float, float, float] | list[float] | None = None,
        epsg: int | None = None,
    ) -> UgridDataset:
        """Crop the mesh to a polygon mask or a bbox — the unstructured-mesh analogue of crop.

        The mesh equivalent of :meth:`pyramids.dataset.Dataset.crop` / :meth:`NetCDF.crop`. Rather
        than warping a raster, it selects the **faces** that fall inside the region (renumbering the
        node connectivity for the resulting sub-mesh) and keeps the data on the surviving elements.
        Delegates to :meth:`clip` for a polygon and :meth:`subset_by_bounds` for a bbox; this method
        exists so the spatial-subset call is named ``crop`` across the raster and mesh classes alike.

        Args:
            mask (Any):
                Polygon mask — a shapely geometry, ``GeoDataFrame``, or ``FeatureCollection``.
                Mutually exclusive with ``bbox``.
            touch (bool):
                Applies only to a polygon ``mask``: if ``True`` (default), keep faces that touch the
                mask boundary; if ``False``, keep only faces fully inside it. Ignored for ``bbox``,
                which always selects faces by its axis-aligned envelope (``subset_by_bounds``).
                Defaults to True.
            bbox (tuple or list of 4 floats, keyword-only):
                ``(west, south, east, north)`` in the mesh CRS, or in ``epsg`` when supplied. Accepts
                a tuple or a list. Selects faces by the axis-aligned envelope; not affected by
                ``touch``. Mutually exclusive with ``mask``.
            epsg (int, keyword-only):
                CRS of ``bbox``. When it differs from the mesh CRS the box is reprojected to the mesh
                CRS and subset by its envelope; when it equals the mesh CRS it is a no-op. Defaults to
                the mesh CRS.

        Returns:
            UgridDataset: A new sub-mesh — faces inside the region, connectivity renumbered, and data
                variables subset to the surviving elements.

        Raises:
            ValueError: If both ``mask`` and ``bbox`` are supplied, if ``bbox`` is not a 4-tuple, or
                if ``epsg`` is given for a ``bbox`` but the mesh has no CRS to reproject into.
            TypeError: If neither ``mask`` nor ``bbox`` is supplied.

        Examples:
            - Crop a mesh to a polygon (faces intersecting it survive):
                ```python
                >>> from shapely.geometry import Polygon
                >>> from pyramids.netcdf import UgridDataset
                >>> ug = UgridDataset.read_file("mesh.nc")                        # doctest: +SKIP
                >>> sub = ug.crop(Polygon([(-1, -1), (0, -1), (0, 1), (-1, 1)]))  # doctest: +SKIP
                >>> sub.n_face <= ug.n_face                                       # doctest: +SKIP
                True

                ```
            - Crop to a bounding box in the mesh's own CRS:
                ```python
                >>> sub = ug.crop(bbox=(-1.0, -1.0, 0.0, 1.0))                    # doctest: +SKIP

                ```

        See Also:
            clip: Polygon-mask subsetting that ``crop`` delegates to when ``mask`` is given.
            subset_by_bounds: Bounding-box subsetting that ``crop`` delegates to when ``bbox`` is
                given.
            pyramids.dataset.Dataset.crop: The raster equivalent on a gridded dataset.
        """
        if bbox is not None:
            if mask is not None:
                raise ValueError("crop accepts either `mask` or `bbox`, not both")
            if len(bbox) != 4:
                raise ValueError(
                    "bbox must be a 4-tuple of (west, south, east, north), "
                    f"got {len(bbox)} value(s)"
                )
            west, south, east, north = bbox
            if epsg is not None and (self.epsg is None or int(epsg) != int(self.epsg)):
                if self.epsg is None:
                    raise ValueError(
                        f"cannot reproject a bbox given in EPSG:{int(epsg)} into a mesh that has no "
                        "CRS; drop epsg to treat the bbox as native coordinates"
                    )
                # Reproject the bbox to the mesh CRS and subset by its envelope, so the bbox path
                # selects faces with the same rule (subset_by_bounds) regardless of the source CRS.
                west, south, east, north = (
                    gpd.GeoSeries([box(west, south, east, north)], crs=epsg)
                    .to_crs(self.epsg)
                    .total_bounds
                )
            result = self.subset_by_bounds(west, south, east, north)
        elif mask is not None:
            result = self.clip(mask, touch=touch)
        else:
            raise TypeError(
                "crop requires a `mask` (polygon) or a `bbox` (west, south, east, north) tuple"
            )
        return result

    def _wrap_subset(
        self, mesh: Mesh2d, data_variables: dict[str, MeshVariable]
    ) -> UgridDataset:
        """Wrap a ``(mesh, data_variables)`` pair from the spatial subsetters into a dataset.

        The spatial subsetting helpers (:func:`clip_mesh` / :func:`subset_by_bounds`) return
        the rebuilt mesh and data variables rather than a dataset (STR-3 — keeps
        ``ugrid.spatial`` independent of this module). This carries the source dataset's
        global attributes / topology info / CRS onto the subset.

        Args:
            mesh: The subset mesh.
            data_variables: The sliced data variables.

        Returns:
            UgridDataset: The wrapped subset.
        """
        return type(self)(
            mesh=mesh,
            data_variables=data_variables,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
        )

    def clip(self, mask: Any, touch: bool = True) -> UgridDataset:
        """Clip the mesh to a polygon mask.

        Selects faces that intersect (touch=True) or are fully
        contained within (touch=False) the mask polygon.

        Args:
            mask: Polygon mask (GeoDataFrame, FeatureCollection,
                or Shapely geometry).
            touch: If True, include faces touching the boundary.

        Returns:
            New UgridDataset with clipped mesh and data.
        """
        mesh, data_variables = clip_mesh(self, mask, touch=touch)
        return self._wrap_subset(mesh, data_variables)

    def subset_by_bounds(
        self,
        xmin: float,
        ymin: float,
        xmax: float,
        ymax: float,
    ) -> UgridDataset:
        """Subset mesh to faces within a bounding box.

        Args:
            xmin: Minimum x-coordinate.
            ymin: Minimum y-coordinate.
            xmax: Maximum x-coordinate.
            ymax: Maximum y-coordinate.

        Returns:
            New UgridDataset with subset mesh and data.
        """
        mesh, data_variables = subset_by_bounds(self, xmin, ymin, xmax, ymax)
        return self._wrap_subset(mesh, data_variables)

    def to_crs(self, to_epsg: int) -> UgridDataset:
        """Reproject all node coordinates to a new CRS.

        Uses pyproj.Transformer to reproject node coordinates.
        Face/edge center coordinates are recomputed after reprojection.
        Data values are preserved — only coordinates change.

        Args:
            to_epsg: Target EPSG code.

        Returns:
            New UgridDataset with reprojected coordinates.

        Raises:
            ValueError: The mesh declares neither an EPSG code nor a WKT, so there is no
                source CRS to reproject from.
        """
        # The mesh may describe its CRS by WKT alone, with no EPSG code to
        # resolve; `crs_spec` returns whichever of the two is usable, so a
        # projected mesh stops being refused for a CRS it plainly has.
        source_crs = crs_spec(self.epsg, self.crs_wkt)
        if source_crs is None:
            raise ValueError(
                "Cannot reproject: source CRS is unknown. "
                "Set CRS before calling to_crs()."
            )

        # Through `crs_from_user_input` so a mesh in a CRS whose code only GDAL's
        # PROJ database carries still reprojects (issue #943). The source goes in
        # as-is rather than as `EPSG:{...}`: `crs_spec` yields the mesh's WKT when
        # there is no usable code, and prefixing that builds the nonsense
        # `EPSG:PROJCS[...]`. The target is always a bare integer code.
        transformer = Transformer.from_crs(
            crs_from_user_input(source_crs),
            crs_from_user_input(f"EPSG:{to_epsg}"),
            always_xy=True,
        )
        new_node_x, new_node_y = transformer.transform(
            self._mesh.node_x,
            self._mesh.node_y,
        )

        new_face_x = None
        new_face_y = None
        if self._mesh.has_face_coords:
            new_face_x, new_face_y = transformer.transform(
                self._mesh.face_x,
                self._mesh.face_y,
            )

        new_edge_x = None
        new_edge_y = None
        if self._mesh.has_edge_coords:
            new_edge_x, new_edge_y = transformer.transform(
                self._mesh.edge_x,
                self._mesh.edge_y,
            )

        new_mesh = Mesh2d(
            node_x=new_node_x,
            node_y=new_node_y,
            face_node_connectivity=self._mesh.face_node_connectivity,
            edge_node_connectivity=self._mesh.edge_node_connectivity,
            face_edge_connectivity=self._mesh.face_edge_connectivity,
            face_face_connectivity=self._mesh.face_face_connectivity,
            edge_face_connectivity=self._mesh.edge_face_connectivity,
            face_x=new_face_x,
            face_y=new_face_y,
            edge_x=new_edge_x,
            edge_y=new_edge_y,
        )

        srs = sr_from_epsg(to_epsg)
        new_crs_wkt = srs.ExportToWkt()

        new_topo_info = None
        if self._topology_info is not None:
            new_topo_info = replace(self._topology_info, crs_wkt=new_crs_wkt)

        # `type(self)` and `file_name`, as every other derivation does: hard-coding
        # `UgridDataset(` silently downgraded a subclass, and dropping `file_name` made the
        # attribute unreliable rather than absent.
        result = type(self)(
            mesh=new_mesh,
            data_variables=self._data_variables,
            global_attributes=self._global_attributes,
            topology_info=new_topo_info,
            crs_wkt=new_crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )
        return result

    @property
    def time_values(self) -> list | None:
        """Parsed time coordinate values from the first temporal variable.

        Returns None if no variables have a time dimension.
        """
        result = None
        for var in self._data_variables.values():
            if var.has_time:
                time_attr = var.attributes.get("time_values")
                if time_attr is not None:
                    result = list(time_attr)
                else:
                    result = list(range(var.n_time_steps))
                break
        return result

    def sel_time(self, index: int) -> UgridDataset:
        """Select a single time step from all temporal variables.

        Non-temporal variables are kept unchanged.

        Args:
            index: Time step index.

        Returns:
            New UgridDataset with single time step data.
        """
        new_data_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if var.has_time:
                new_data_vars[name] = var.with_data(
                    var.sel_time(index), active=var._time_sliced_active(index, None)
                )
            else:
                new_data_vars[name] = var

        # See `to_crs`: `type(self)` keeps a subclass, and `file_name` is carried.
        result = type(self)(
            mesh=self._mesh,
            data_variables=new_data_vars,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )
        return result

    def sel_time_range(self, start: int, stop: int) -> UgridDataset:
        """Select a time range from all temporal variables.

        Args:
            start: Start index (inclusive).
            stop: Stop index (exclusive).

        Returns:
            New UgridDataset with the selected time range.
        """
        new_data_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if var.has_time:
                new_data_vars[name] = var.sel_time_range(start, stop)
            else:
                new_data_vars[name] = var

        # See `to_crs`: `type(self)` keeps a subclass, and `file_name` is carried.
        result = type(self)(
            mesh=self._mesh,
            data_variables=new_data_vars,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )
        return result

    def attach_data(
        self, path: str | Path, driver: str | None = None, lazy: bool = True
    ) -> UgridDataset:
        """Attach a results-only file's variables to this mesh.

        Some formats carry values but no topology — an SMS ``.dat``, an XMDF, an XDMF — and must
        be paired with a mesh read from elsewhere. Those drivers declare ``needs_mesh``, and
        :meth:`read_file` refuses them, pointing here.

            >>> mesh = UgridDataset.read_file("domain.2dm")       # doctest: +SKIP
            >>> result = mesh.attach_data("velocity.dat")         # doctest: +SKIP

        The driver returns **only the variables it read**; this method merges them onto the
        dataset. That division is deliberate — it makes a name the dataset already defines an
        unambiguous collision, and means a driver cannot drop the dataset's own variables by
        rebuilding the mapping.

        The driver checks the file's declared element count against this mesh. That check is the
        only safeguard there is: a bare results file carries nothing identifying the mesh it
        belongs to, so attaching it to the wrong one would otherwise produce a plausible and
        entirely wrong dataset.

        Args:
            path: The results file to read.
            driver: Name of a registered driver to use, bypassing detection. ``None`` (the
                default) picks the ``needs_mesh`` driver claiming the file's suffix.
            lazy: Defer reading variable values until first access. Defaults to True.

        Returns:
            A new dataset carrying this mesh plus the attached variables. This dataset is
            unchanged.

        Raises:
            FileNotFoundError: If the results file does not exist.
            DriverNotExistError: If ``driver`` names a driver that is not installed.
            UnsupportedMeshFormatError: If no installed driver can attach this file, the
                named driver does not declare ``needs_mesh``, or the driver returned
                something other than a mapping of name to :class:`MeshVariable`.
            ValueError: If the driver returns a variable whose name this dataset already
                uses, or reports an element-count mismatch against this mesh.
        """
        _require_existing_file(path)
        selected = _resolve_attach_driver(path, driver)
        produced = _read_data_through_driver(selected, path, self, lazy)
        collisions = sorted(set(produced) & set(self._data_variables))
        if collisions:
            raise ValueError(
                f"The {selected.name!r} driver read variable(s) {collisions} from "
                f"{str(path)!r}, which this dataset already defines. Rename one side before "
                f"attaching, or drop the existing variable first."
            )
        merged = dict(self._data_variables)
        merged.update(produced)
        return type(self)(
            mesh=self._mesh,
            data_variables=merged,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            volume_topology=self._volume_topology,
            file_name=self._file_name,
        )

    def to_file(
        self, path: str | Path, driver: str | None = None, **options: Any
    ) -> None:
        """Write the dataset to a file.

        Writes UGRID-compliant NetCDF by default: a topology variable, node coordinates,
        connectivity arrays, face/edge centres, data variables, and global attributes following
        the UGRID convention.

        With ``driver``, writes through a registered third-party driver instead — 2DM, Selafin,
        PLY and others are writable. The output format is never inferred from the suffix, so
        ``to_file("mesh.slf")`` still writes NetCDF; name the driver to change that.

        Args:
            path: Output file path.
            driver: Name of a registered **mesh** driver to write with, as declared by the
                plugin providing it (see :func:`~pyramids.netcdf.ugrid.mesh_drivers`) — not a
                GDAL driver short name, which is what ``driver`` means elsewhere in pyramids.
                ``None`` (the default) writes UGRID NetCDF through the built-in writer, and
                the output format is never inferred from ``path``'s suffix.
            **options: Format-specific settings for the driver, passed through uninterpreted
                (for example Selafin endianness, or PLY ascii versus binary). The built-in
                writer takes none, so passing any without a ``driver`` raises ``TypeError``
                rather than dropping them silently.

        Raises:
            DriverNotExistError: If ``driver`` names a driver that is not installed.
            UnsupportedMeshFormatError: If the named driver cannot write a mesh.
            TypeError: If ``options`` are given without a ``driver``.
        """
        if driver is not None:
            self._write_through_driver(path, driver, options)
        elif options:
            raise TypeError(
                f"to_file() got unexpected keyword argument(s) "
                f"{sorted(options)}; format options apply to a `driver=` write only."
            )
        else:
            self._write_ugrid_netcdf(Path(path))

    def _write_ugrid_netcdf(self, path: Path) -> None:
        """Write UGRID-compliant NetCDF through GDAL's multidimensional API.

        The built-in writer, unchanged from before driver dispatch existed.

        Args:
            path: Output file path.
        """
        drv = gdal.GetDriverByName("netCDF")
        ds = drv.CreateMultiDimensional(str(path))
        rg = ds.GetRootGroup()

        mesh_name = self.mesh_name
        dims = write_ugrid_topology(rg, self._mesh, mesh_name, self._crs_wkt)
        if self._volume_topology is not None:
            # Before the data variables, so the `nVolumes` dimension exists when a
            # `location="volume"` variable is written against it (issue #1330).
            write_volume_topology(rg, mesh_name, self._volume_topology, dims)

        for var in self._data_variables.values():
            if var.has_time and "time" not in dims:
                time_dim = rg.CreateDimension("time", None, None, var.n_time_steps)
                dims["time"] = time_dim
            # `load_array` reads each variable without memoising it on the shared dataset, so the
            # write streams one variable at a time instead of holding the whole cube resident (#982).
            write_ugrid_data_variable(
                rg, var.with_data(var.load_array()), mesh_name, dims
            )

        global_attrs = dict(self._global_attributes)
        if "Conventions" not in global_attrs:
            global_attrs["Conventions"] = UGRID_CONVENTIONS
        write_global_attributes(rg, global_attrs)

        ds = None

    def _write_through_driver(
        self, path: str | Path, driver: str, options: dict[str, Any]
    ) -> None:
        """Write through a registered third-party driver.

        Args:
            path: Output file path.
            driver: Name of the driver to write with.
            options: Format-specific settings, passed to the driver uninterpreted.

        Raises:
            DriverNotExistError: If the driver is not installed.
            UnsupportedMeshFormatError: If the driver cannot write a mesh, or raised.
        """
        if driver.strip().lower() in RESERVED_NAMES:
            self._write_ugrid_netcdf(Path(path))
            return
        installed = mesh_drivers()
        if driver not in installed:
            raise DriverNotExistError(_unknown_driver_message(driver))
        selected = installed[driver]
        if "write_mesh" not in selected.capabilities:
            can_do = ", ".join(sorted(selected.capabilities))
            raise UnsupportedMeshFormatError(
                f"The {driver!r} driver cannot write a mesh; it declares: {can_do}."
            )
        try:
            selected.write(self, str(path), **options)
        except (UnsupportedMeshFormatError, DriverNotExistError, OSError):
            raise
        except Exception as error:
            raise UnsupportedMeshFormatError(
                f"The {driver!r} driver failed to write {str(path)!r}: {error!r}"
            ) from error

        # Warned after the write, not before: a failed write previously still reported that
        # variables "will not be in" a file that was never created.
        if "write_data" not in selected.capabilities and self._data_variables:
            warnings.warn(
                f"The {driver!r} driver wrote a mesh frame but no data; "
                f"{len(self._data_variables)} variable(s) are not in {str(path)!r}.",
                RuntimeWarning,
                stacklevel=4,
            )

    def to_geodataframe(
        self,
        variable_name: str | None = None,
        location: str = "face",
    ) -> gpd.GeoDataFrame:
        """Convert mesh to a GeoDataFrame.

        For faces: each row is a Polygon with data columns.
        For nodes: each row is a Point.
        For edges: each row is a LineString.

        Args:
            variable_name: Optional data variable to include as a column.
            location: Mesh location ("face", "node", or "edge").

        Returns:
            geopandas GeoDataFrame.
        """
        geometries = self._build_geometries(location)

        data_dict: dict[str, Any] = {}
        if variable_name is not None:
            var = self.get_data(variable_name)
            if var.location == location:
                # For a temporal variable only the first step is tabulated; `sel_time(0)` reads just
                # that slab instead of loading every step to slice `[0]` (#982). `has_data_source`
                # avoids a temporal-specific `sel_time` error for a variable with no readable data
                # (checked without forcing a load — review L3).
                if var.has_time:
                    var_data = var.sel_time(0) if var.has_data_source else None
                else:
                    var_data = var.data
                # A variable with no readable data becomes a length-correct null column rather than
                # raising — pandas rejects a scalar `None` column ("must pass an index") (review L3).
                if var_data is None:
                    var_data = np.full(len(geometries), np.nan)
                data_dict[variable_name] = var_data

        gdf = gpd.GeoDataFrame(data_dict, geometry=geometries)
        if self.crs is not None:
            gdf = gdf.set_crs(self.crs)

        result = gdf
        return result

    def _build_geometries(self, location: str) -> list:
        """Build the geometry list for a mesh location.

        Args:
            location: Mesh location ("face", "node", or "edge").

        Returns:
            List of shapely geometries — Polygons for "face", Points for
            "node", LineStrings for "edge".

        Raises:
            ValueError: If `location` is unknown, or edge connectivity is
                unavailable for an edge conversion.
        """
        if location == "face":
            geometries = MeshSpatialIndex(self._mesh).face_polygons
        elif location == "node":
            # Vectorized point construction — meshes routinely have 1e5-1e7 nodes, so a per-node
            # Python `Point(...)` loop is a hot spot (ARC-59). `list(...)` keeps the return type
            # consistent with the face/edge branches (review N2).
            geometries = list(shapely.points(self._mesh.node_x, self._mesh.node_y))
        elif location == "edge":
            if self._mesh.edge_node_connectivity is None:
                raise ValueError("Edge connectivity not available.")
            geometries = self._edge_linestrings(self._mesh.edge_node_connectivity)
        else:
            raise ValueError(f"Unknown location: {location}")
        return geometries

    def _edge_linestrings(self, enc: Connectivity) -> Any:
        """Build one LineString per edge, vectorized for standard 2-node edges (ARC-59)."""
        node_idx = np.asarray(enc.data)
        # A `None` fill means no sentinels are present, so the fast path is valid without an
        # elementwise `node_idx != None` compare (which NumPy deprecates) (review N3).
        no_fill = enc.fill_value is None or bool(np.all(node_idx != enc.fill_value))
        if node_idx.ndim == 2 and node_idx.shape[1] == 2 and no_fill:
            xs = self._mesh.node_x[node_idx]
            ys = self._mesh.node_y[node_idx]
            return list(shapely.linestrings(np.stack([xs, ys], axis=-1)))
        # Rare ragged / filled edge connectivity: fall back to a per-edge build.
        # See `Network1d._edge_linestrings`: a row with fewer than two real nodes is no
        # edge, and GEOS refuses a one-point LineString, so such a row escaped as a raw
        # GEOSException. An empty geometry keeps one row per declared edge.
        return [
            LineString(
                [
                    (self._mesh.node_x[n], self._mesh.node_y[n])
                    for n in enc.get_element(i)
                ]
            )
            if len(enc.get_element(i)) >= 2
            else LineString()
            for i in range(enc.n_elements)
        ]

    def to_feature_collection(
        self,
        variable_name: str | None = None,
        location: str = "face",
    ) -> FeatureCollection:
        """Convert mesh to a pyramids FeatureCollection.

        Args:
            variable_name: Optional data variable to include.
            location: Mesh location ("face", "node", or "edge").

        Returns:
            pyramids FeatureCollection.
        """
        gdf = self.to_geodataframe(variable_name, location)
        result = FeatureCollection(gdf)
        return result

    @classmethod
    def from_arrays(
        cls,
        node_x: np.ndarray,
        node_y: np.ndarray,
        face_node_connectivity: np.ndarray,
        data: dict[str, np.ndarray] | None = None,
        data_locations: dict[str, str] | None = None,
        epsg: int = 4326,
        mesh_name: str = DEFAULT_MESH_NAME,
    ) -> UgridDataset:
        """Create a UgridDataset programmatically from arrays.

        Plural because an unstructured mesh is not one array: the topology
        needs node coordinates *and* a face-node connectivity table before any
        data can be attached. It therefore takes a flat `epsg` rather than the
        :class:`~pyramids.base.georeference.GeoReference` the gridded
        constructors take — a mesh carries its own coordinates, so there is no
        affine transform to describe.

        Args:
            node_x: Node x-coordinates.
            node_y: Node y-coordinates.
            face_node_connectivity: (n_faces, max_nodes) array of node
                indices. Use -1 as fill value for mixed meshes.
            data: Optional dict mapping variable name to data array.
            data_locations: Optional dict mapping variable name to
                location ("face", "node", "edge"). Defaults to "face".
            epsg: EPSG code for the CRS.
            mesh_name: Name for the topology variable.

        Returns:
            UgridDataset instance.

        Examples:
            - Build the smallest possible mesh — two triangles — and inspect
              its topology:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ... )
                >>> (mesh.n_node, mesh.n_face)
                (4, 2)
                >>> mesh.bounds
                (0.0, 0.0, 1.0, 1.0)

                ```
            - Attach a per-face variable and read it back:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"depth": np.array([1.5, 2.5])},
                ... )
                >>> mesh.data_variable_names
                ['depth']
                >>> mesh["depth"].location
                'face'
                >>> mesh["depth"].data.tolist()
                [1.5, 2.5]

                ```
        """
        fnc = Connectivity(
            data=np.asarray(face_node_connectivity, dtype=np.intp),
            fill_value=-1,
            cf_role="face_node_connectivity",
            original_start_index=0,
        )
        mesh = Mesh2d(
            node_x=np.asarray(node_x, dtype=np.float64),
            node_y=np.asarray(node_y, dtype=np.float64),
            face_node_connectivity=fnc,
        )

        data_variables: dict[str, MeshVariable] = {}
        topo_data_vars: dict[str, str] = {}
        if data is not None:
            if data_locations is None:
                data_locations = {}
            for name, arr in data.items():
                loc = data_locations.get(name, "face")
                topo_data_vars[name] = loc
                data_variables[name] = MeshVariable(
                    name=name,
                    location=loc,
                    mesh_name=mesh_name,
                    shape=arr.shape,
                    _data=arr,
                )

        srs = sr_from_epsg(epsg)
        crs_wkt = srs.ExportToWkt()

        topo_info = MeshTopologyInfo(
            mesh_name=mesh_name,
            topology_dimension=2,
            node_x_var=f"{mesh_name}_node_x",
            node_y_var=f"{mesh_name}_node_y",
            face_node_var=f"{mesh_name}_face_nodes",
            data_variables=topo_data_vars,
            crs_wkt=crs_wkt,
        )

        result = cls(
            mesh=mesh,
            data_variables=data_variables,
            global_attributes={"Conventions": UGRID_CONVENTIONS},
            topology_info=topo_info,
            crs_wkt=crs_wkt,
        )
        return result

    @classmethod
    def from_structured(
        cls,
        dataset: Any,
        band_names: list[str] | None = None,
        mesh_name: str = DEFAULT_MESH_NAME,
    ) -> UgridDataset:
        """Build a mesh from a raster, one quad face per cell.

        The bridge from the structured world: each cell becomes a quad over the corner
        lattice (shared corners deduplicated) and each band becomes a per-face variable, so a
        raster can be fed to the mesh operations.

        Args:
            dataset: The source raster (a pyramids ``Dataset`` / ``NetCDF`` variable).
            band_names: Names for the per-face variables, one per band. Defaults to
                ``band_1``, ``band_2``, ….
            mesh_name: The topology name to stamp. Defaults to the package default.

        Returns:
            UgridDataset: The quad mesh carrying the band values.
        """
        node_x, node_y, faces, data = _construct.from_structured(dataset, band_names)
        return cls.from_arrays(
            node_x,
            node_y,
            faces,
            data=data,
            epsg=dataset.epsg,
            mesh_name=mesh_name,
        )

    @classmethod
    def from_geodataframe(
        cls, gdf: Any, mesh_name: str = DEFAULT_MESH_NAME
    ) -> UgridDataset:
        """Build a mesh from polygon geometries, one face per polygon.

        Vertices are deduplicated into the node set and each ring is oriented
        counter-clockwise; the frame's attribute columns become per-face variables.

        Args:
            gdf: A GeoDataFrame of polygons. Its ``crs`` is carried onto the mesh verbatim,
                so a projected CRS stays projected even when it has no EPSG code.
            mesh_name: The topology name to stamp. Defaults to the package default.

        Returns:
            UgridDataset: The mesh built from the polygons.

        Raises:
            ValueError: The frame declares no CRS, so the mesh would carry no
                georeferencing.
        """
        node_x, node_y, faces, data = _construct.from_geodataframe(gdf)
        # Carry the frame's own CRS. Reading an EPSG code and defaulting to 4326 stamped degrees
        # on a projected frame whose CRS has no EPSG code, and on a frame with no CRS at all --
        # silent georeferencing corruption, since everything downstream trusts `epsg`.
        crs = getattr(gdf, "crs", None)
        if crs is None:
            raise ValueError(
                "from_geodataframe: the GeoDataFrame declares no CRS, so the mesh would have no "
                "georeferencing. Set `gdf.crs` (e.g. gdf.set_crs(4326)) before converting."
            )
        epsg = crs.to_epsg()
        built = cls.from_arrays(
            node_x,
            node_y,
            faces,
            data=data,
            epsg=epsg if epsg is not None else 4326,
            mesh_name=mesh_name,
        )
        if epsg is not None:
            return built
        # A CRS with no EPSG code (a projected one given as WKT, say) would otherwise be
        # reduced to the 4326 placeholder `from_arrays` needs, stamping degrees on metres.
        # Keep the frame's WKT verbatim instead.
        wkt = crs.to_wkt()
        return cls(
            mesh=built.mesh,
            data_variables=built.data_vars,
            global_attributes={"Conventions": UGRID_CONVENTIONS},
            topology_info=_topology_info_named(mesh_name, crs_wkt=wkt),
            crs_wkt=wkt,
            file_name=None,
        )

    @classmethod
    def from_xarray(cls, ds: Any) -> UgridDataset:
        """Read a mesh back from an ``xarray.Dataset`` in the UGRID-CF encoding.

        The inverse of :meth:`to_xarray`. xarray is a peer dependency, imported only inside
        the bridge, so nothing in this module imports it at module scope.

        Args:
            ds: The ``xarray.Dataset`` to read.

        Returns:
            UgridDataset: The reconstructed mesh and variables.

        Raises:
            OptionalPackageDoesNotExist: xarray is not installed.
        """
        mesh, data_variables = _interop.from_xarray(ds)
        mesh_name = _interop.read_mesh_name(ds)
        return cls(
            mesh=mesh,
            data_variables=data_variables,
            global_attributes={},
            # Keep the topology's own name: without it a mesh called anything other than
            # "mesh2d" came back renamed, and a re-export then wrote the wrong dimensions.
            topology_info=_topology_info_named(mesh_name),
            crs_wkt=_interop.read_crs_wkt(ds),
            file_name=None,
            # Rebuild the ragged volume topology from its companion arrays when present
            # (issue #1330); `None` for a 2-D / uniform mesh, leaving the dense path in force.
            volume_topology=_interop.volume_topology_from_xarray(ds),
        )

    @classmethod
    def from_zarr(cls, store: Any, **kwargs: Any) -> UgridDataset:
        """Read a mesh from a zarr store written by :meth:`to_zarr`.

        Args:
            store: The zarr store (a path or a mapping).
            **kwargs: Passed to ``xarray.open_zarr``.

        Returns:
            UgridDataset: The reconstructed mesh and variables.

        Raises:
            OptionalPackageDoesNotExist: xarray or zarr is not installed.
        """
        # One open, read twice: the mesh and the CRS both come off the same dataset, so the
        # store is not opened again just to recover the grid mapping.
        opened = _interop.open_zarr_dataset(store, **kwargs)
        mesh, data_variables = _interop.from_xarray(opened)
        return cls(
            mesh=mesh,
            data_variables=data_variables,
            global_attributes={},
            topology_info=_topology_info_named(_interop.read_mesh_name(opened)),
            crs_wkt=_interop.read_crs_wkt(opened),
            file_name=None,
        )

    @staticmethod
    def mesh_names(path: str | Path, driver: str | None = None) -> list[str]:
        """The names of every topology a file declares.

        A mesh file may carry several topologies; :meth:`read_file` takes the first unless told
        otherwise, so this is how a caller discovers what else is in there.

        Dispatch mirrors :meth:`read_file` exactly, including the short-circuit that keeps a
        NetCDF suffix from triggering driver discovery. Every driver is asked, not only those
        declaring ``multi_mesh``: a single-mesh format answers with a one-item list, which is
        the right answer rather than an error.

        Args:
            path: Path to the file.
            driver: Name of a registered driver to ask, bypassing detection. ``None`` (the
                default) resolves as described above.

        Returns:
            list[str]: One name per declared topology, in file order.

        Raises:
            FileNotFoundError: The file does not exist.
            DriverNotExistError: ``driver`` names a driver that is not installed.
            UnsupportedMeshFormatError: No installed driver claims the suffix and the
                built-in reader cannot open it either, or a driver failed or returned
                something other than a list of topology names.
            IsADirectoryError: The path is a directory that no driver claims.
            RuntimeError: GDAL cannot open a file that reached the built-in reader by its own
                suffix. GDAL exceptions are enabled package-wide, so the open failure
                surfaces as GDAL's own error.
            ValueError: The file declares no UGRID topology.
        """
        selected, native = _resolve_read_driver(path, driver)
        if selected is not None:
            result = _mesh_names_through_driver(selected, path)
        else:
            try:
                result = UgridDataset._ugrid_netcdf_mesh_names(
                    path, wrap_unreadable=not native
                )
            except NoMeshTopologyError:
                fallback = _netcdf_fallback_driver(path, driver)
                if fallback is None:
                    raise
                result = _mesh_names_through_driver(fallback, path)
        return result

    @staticmethod
    def _ugrid_netcdf_mesh_names(
        path: str | Path, wrap_unreadable: bool = False
    ) -> list[str]:
        """List the topology names in a UGRID NetCDF file.

        Args:
            path: Path to the ``.nc`` file.
            wrap_unreadable: True when this call is the fallback for a suffix no driver
                claimed, so a failure reports the installed drivers rather than GDAL's
                "not recognized" — the same treatment :meth:`read_file` gives it.

        Returns:
            list[str]: One name per declared topology, in file order.

        Raises:
            FileNotFoundError: The file does not exist.
            UnsupportedMeshFormatError: GDAL cannot open a file that reached here as a
                fallback from driver detection.
            ValueError: The file declares no UGRID topology.
        """
        target = str(path)
        if not target.startswith("/vsi"):
            resolved = Path(path)
            if not resolved.exists():
                raise FileNotFoundError(
                    f"File not found: {Path(resolved)}"
                )  # pragma: no cover - `_require_existing_file` runs first on every public route
            target = str(resolved)
        # See `read_file`: with GDAL exceptions on an unopenable file raises RuntimeError,
        # but that setting is process-global and reversible, so the guard stays.
        try:
            dataset = gdal.OpenEx(
                target, gdal.OF_MULTIDIM_RASTER | gdal.OF_VERBOSE_ERROR
            )
        except RuntimeError as error:
            if not wrap_unreadable:
                raise
            if not target.startswith("/vsi") and Path(target).is_dir():
                raise IsADirectoryError(
                    f"Not a mesh file: {target} is a directory."
                ) from error
            suffix = "" if target.startswith("/vsi") else Path(target).suffix
            raise UnsupportedMeshFormatError(
                unsupported_format_message(str(path), suffix)
            ) from error
        if (
            dataset is None
        ):  # pragma: no cover - needs gdal.DontUseExceptions() process-wide
            raise ValueError(f"GDAL cannot open file: {path}")
        rg = dataset.GetRootGroup()
        if rg is None:  # pragma: no cover - no multidim driver answers None here
            # Kept as a clear message rather than letting `parse_ugrid_topology(None)`
            # raise an AttributeError, though every multidim driver that opens a file
            # does expose a root group.
            raise ValueError(f"Cannot get root group from: {path}")
        topologies = parse_ugrid_topology(rg)
        if not topologies:
            raise NoMeshTopologyError(f"No UGRID mesh topology found in: {path}")
        return [topo.mesh_name for topo in topologies]

    @classmethod
    def read_all(
        cls, path: str | Path, driver: str | None = None, lazy: bool = True
    ) -> dict[str, UgridDataset]:
        """Read **every** topology in a file, one dataset each.

        :meth:`read_file` answers a single mesh (the first, or the one named). This answers
        all of them, so nothing in a multi-mesh file is silently dropped. Dispatch follows
        :meth:`mesh_names` and :meth:`read_file`, so a third-party format works here too.

        Args:
            path: Path to the file.
            driver: Name of a registered **mesh** driver to use, bypassing detection.
                ``None`` (the default) detects.
            lazy: Defer reading variable values until first access. Forwarded to a driver,
                which may honour an eager read; the built-in UGRID reader has no eager mode, so
                ``lazy=False`` on a native read raises :class:`TypeError`, exactly as
                :meth:`read_file` does. Without it a multi-topology third-party format could
                only ever be read lazily.

        Returns:
            dict[str, UgridDataset]: Each topology name mapped to its dataset.

        Raises:
            FileNotFoundError: The file does not exist.
            DriverNotExistError: ``driver`` names a driver that is not installed.
            UnsupportedMeshFormatError: No installed driver reads the file and the built-in
                reader cannot either — raised from :meth:`mesh_names`, which this delegates
                to, so every error that method raises surfaces here too.
            IsADirectoryError: The path is a directory that no driver claims.
            RuntimeError: GDAL cannot open a file that reached the built-in reader by its own
                suffix. GDAL exceptions are enabled package-wide, so the open failure
                surfaces as GDAL's own error.
            TypeError: ``lazy=False`` on a native (built-in UGRID) read, which has no eager
                mode -- the same rejection :meth:`read_file` makes.
            ValueError: The file declares no UGRID topology.
        """
        # Resolve once, for real. Passing a driver *name* through only short-circuited the
        # route where detection had found something: when nothing claimed the suffix the name
        # was None, so `mesh_names` and every per-topology read re-ran full detection anyway
        # -- measured at two `can_open` calls for one `read_all`, growing per topology.
        # Threading the resolved object through covers the fallback route too.
        selected, native = _resolve_read_driver(path, driver)
        # Set in the native branch below; stays empty whenever a driver handles the file --
        # resolved up front, or picked by the fallback -- in which case it is never read.
        native_names: list[str] = []
        if selected is None:
            try:
                native_names = cls._ugrid_netcdf_mesh_names(
                    path, wrap_unreadable=not native
                )
            except NoMeshTopologyError:
                # The file opened as NetCDF but declares no UGRID topology; fall back to a
                # driver claiming the suffix, as read_file does. If none claims it, the
                # original error stands.
                selected = _netcdf_fallback_driver(path, driver)
                if selected is None:
                    raise
        if selected is None:
            result = {
                name: cls._read_ugrid_netcdf(
                    path, name, wrap_unreadable=not native, lazy=lazy
                )
                for name in native_names
            }
        else:
            names = _mesh_names_through_driver(selected, path)
            result = {
                name: _read_through_driver(selected, path, name, cls, lazy)
                for name in names
            }
        return result

    def _layer_axis(self, var: MeshVariable) -> int | None:
        """The axis holding the layer dimension, or ``None`` when the variable has none.

        A mesh variable's axes read ``[time?, layer?, element]`` — the element axis is the
        last and time is whichever :attr:`MeshVariable.time_index` names, so a remaining
        middle axis is the layer. A 2-D variable whose dimension names mark it non-temporal
        is therefore ``(layer, element)`` with layer axis 0.

        Two cases answer ``None`` rather than guessing. A variable carrying no dimension
        names at all (an array-built one) falls back to "the leading axis of a >1-D array is
        time", so its 2-D form reads as ``(time, element)``; name the dimensions to have it
        read as layers. And a variable whose time dimension is *last* contradicts the layout
        above — the element axis is supposed to be last — so no axis can be claimed as the
        layer axis without mistaking the element axis for it.

        Args:
            var: The variable to inspect.

        Returns:
            int | None: The layer axis, or ``None``.
        """
        if not var.has_data_source:
            return None
        # The declared shape, not `var.data`: touching the data fires the lazy loader and
        # pulls every variable off disk just to learn how many axes it has -- the same defect
        # `info()` carried.
        rank = len(var.shape)
        if rank < 2:
            return None
        if var.time_index == rank - 1:
            # Time sits where the element axis belongs, so the layout is not one we can read.
            return None
        axes = [axis for axis in range(rank - 1) if axis != var.time_index]
        return axes[0] if axes else None

    @property
    def volume_topology(self) -> VolumeTopology | None:
        """The ragged (per-column) vertical layering, or ``None`` for a uniform / 2-D mesh.

        Present only for a genuinely 3-D mesh whose columns carry different layer counts
        (issue #1330): it maps each face to its slice of the flat ``volume`` axis that a
        ``location="volume"`` variable is indexed through. ``None`` means the dense
        ``(layer, element)`` path applies — the uniform-layer behaviour is unchanged.
        """
        return self._volume_topology

    @property
    def n_volume(self) -> int | None:
        """The total number of volumes, or ``None`` when the mesh has no volume topology.

        The flat ``volume``-axis length of a ragged 3-D mesh (the sum of every column's layer
        count). ``None`` for a 2-D or uniform-layer mesh, where :attr:`n_layers` applies
        instead.
        """
        return None if self._volume_topology is None else self._volume_topology.n_volume

    @property
    def n_layers(self) -> int | None:
        """The number of layers, or ``None`` when no variable is layered.

        A 3-D mesh output puts some variables on the ``n`` layer centres and others on the
        ``n + 1`` layer interfaces, so two counts differing by one is normal rather than a
        contradiction; this reports ``n``, the layer count proper. Any other disagreement is
        a real inconsistency and raises. :meth:`isel_layer` is unaffected either way — it
        indexes each variable on its own layer axis.

        Reporting ``n`` means the ``n + 1`` is not visible here, so a caller iterating
        ``range(n_layers)`` over an interface variable would skip its last interface. Read
        :attr:`n_layer_interfaces` for that count rather than inferring it: it answers ``None``
        unless a variable really sits on the interfaces.

        For a **ragged** 3-D mesh (:attr:`volume_topology` set) there is no single layer count:
        this reports the **maximum** over the columns — the number of layer *slots* — so
        ``range(n_layers)`` covers the deepest column, and :meth:`isel_layer` makes the absent
        shallow-column layers explicit (NaN) rather than padding them silently.

        Returns:
            int | None: The layer-centre count (the max, for a ragged mesh), or ``None`` when
            no variable is layered.

        Raises:
            ValueError: The layered variables carry counts that are not a layer-centre /
                layer-interface pair.
        """
        if self._volume_topology is not None:
            return self._volume_topology.max_layers
        counts = self._layer_counts()
        return min(counts) if counts else None

    @property
    def n_layer_interfaces(self) -> int | None:
        """The number of layer interfaces, or ``None`` when nothing sits on them.

        The companion to :attr:`n_layers`. A 3-D output's vertical velocities and fluxes live on
        the ``n + 1`` interfaces between the ``n`` layer centres, and ``n_layers`` deliberately
        reports the centre count -- so this is where the other half of the pair is readable.

        It is **not** simply ``n_layers + 1``, which is always computable: that would assert
        that something in the dataset is interface-located. Only a variable actually carrying
        that count makes it true, so a centres-only dataset answers ``None``.

        Returns:
            int | None: The layer-interface count, or ``None`` when no variable is layered or
            every layered variable sits on the centres.

        Raises:
            ValueError: The layered variables carry counts that are not a layer-centre /
                layer-interface pair -- the same guard :attr:`n_layers` applies, so the two
                properties agree about what is readable.
        """
        counts = self._layer_counts()
        if not counts:
            return None
        interfaces = min(counts) + 1
        return interfaces if interfaces in counts else None

    def _layer_counts(self) -> set[int]:
        """The distinct layer counts across the layered variables, validated.

        Returns:
            set[int]: One entry per distinct count, empty when no variable is layered.

        Raises:
            ValueError: The counts are not a single value or a layer-centre / layer-interface
                pair. The test is ``counts - {smallest, smallest + 1}`` rather than "every gap
                is 1", so ``{20, 21, 22}`` is refused too: a mesh cannot have both 21 and 22
                interfaces for 20 centres.
        """
        counts = set()
        for var in self._data_variables.values():
            axis = self._layer_axis(var)
            if axis is not None:
                counts.add(int(var.shape[axis]))
        smallest = min(counts) if counts else 0
        if counts - {smallest, smallest + 1}:
            raise ValueError(
                f"layered variables disagree on their layer count: {sorted(counts)}."
            )
        return counts

    @property
    def layer_values(self) -> list | None:
        """The layer coordinate values, when a layered variable carries them.

        Read from a layered variable's ``layer_values`` attribute. The UGRID reader does not
        populate that attribute -- as with :attr:`time_values`, the coordinate is supplied by
        whoever builds or transforms the dataset -- so this is normally ``None`` for a mesh
        read straight from a file, and :meth:`sel_layer` then selects by layer position.
        """
        for var in self._data_variables.values():
            if self._layer_axis(var) is not None:
                stamps = var.attributes.get("layer_values")
                if stamps is not None:
                    return list(stamps)
        return None

    def isel_layer(self, index: int | slice | list[int] | np.ndarray) -> UgridDataset:
        """Select along the layer axis by position.

        A layered variable — ``(layer, element)``, or ``(time, layer, element)`` — is indexed
        on its layer axis. An integer collapses that axis (the variable loses its layer
        dimension); a slice or sequence keeps it, trimmed. Variables with no layer axis are
        carried through untouched, as the time selectors carry static variables.

        For a **ragged** 3-D mesh (:attr:`volume_topology` set) a ``location="volume"``
        variable has no rectangular layer axis to index; the selection is per column instead.
        ``index`` must be an ``int`` ``k``: each column yields its ``k``-th layer where it has
        one, and a column shallower than ``k + 1`` layers yields NaN — the absent layers are
        made explicit, never padded silently. The volume variable becomes a plain
        ``location="face"`` field; other variables carry through as on the uniform path.

        Args:
            index: An ``int`` layer, a ``slice``, or a sequence of layer positions. A ragged
                mesh accepts only an ``int``.

        Returns:
            UgridDataset: A new dataset with the layer selection applied.

        Raises:
            ValueError: No variable has a layer dimension, or a non-``int`` ``index`` was given
                for a ragged mesh.
        """
        if self._volume_topology is not None:
            return self._isel_layer_ragged(index)
        new_vars: dict[str, MeshVariable] = {}
        layered = False
        for name, var in self._data_variables.items():
            axis = self._layer_axis(var)
            if axis is None:
                new_vars[name] = var
                continue
            layered = True
            data = np.asarray(var.data)
            if isinstance(index, (int, np.integer)):
                picked = np.take(data, int(index), axis=axis)
                moved = var.with_data(picked)
                moved.dimensions = tuple(
                    dim
                    for position, dim in enumerate(var.dimensions)
                    if position != axis
                )
            else:
                positions = (
                    np.arange(data.shape[axis])[index]
                    if isinstance(index, slice)
                    else np.asarray(index, dtype=np.intp)
                )
                moved = var.with_data(np.take(data, positions, axis=axis))
            new_vars[name] = moved
        if not layered:
            raise ValueError(
                "isel_layer needs at least one variable with a layer dimension; this "
                "dataset has none."
            )
        return self._rebuild(new_vars)

    def sel_layer(self, value: Any) -> UgridDataset:
        """Select along the layer axis by coordinate value.

        Matches ``value`` against :attr:`layer_values`, falling back to layer positions when
        no layer coordinate is present -- which is the usual case, since the reader does not
        populate `layer_values` (see :attr:`layer_values`). So on a mesh read from a file,
        ``sel_layer(1)`` selects layer position 1.

        Args:
            value: The layer coordinate value to select.

        Returns:
            UgridDataset: A new dataset with that layer selected.

        Raises:
            ValueError: No layer dimension, or the value is not among the layer coordinates.
        """
        count = self.n_layers
        if count is None:
            raise ValueError(
                "sel_layer needs at least one variable with a layer dimension; this "
                "dataset has none."
            )
        # Read the property once: it walks every variable, so evaluating it twice walked
        # them twice.
        stored = self.layer_values
        coords = stored if stored is not None else list(range(count))
        if value not in coords:
            raise ValueError(
                f"sel_layer: layer value {value!r} not found in the layer coordinate "
                f"{coords!r}."
            )
        return self.isel_layer(coords.index(value))

    def _isel_layer_ragged(
        self, index: int | slice | list[int] | np.ndarray
    ) -> UgridDataset:
        """Select the ``index``-th layer per column for a ragged 3-D mesh (issue #1330).

        The per-column counterpart of :meth:`isel_layer`: a ``location="volume"`` variable's
        flat ``(time?, n_volume)`` data is gathered to a ``(time?, n_face)`` face field, each
        column contributing its ``index``-th volume where it has one and NaN where it does not,
        so the shallow columns' absent layers are explicit rather than padded. Non-volume
        variables are carried through untouched.

        Args:
            index: The 0-based layer position to take from every column. Only an ``int`` is
                meaningful for a ragged mesh.

        Returns:
            UgridDataset: A new dataset with the volume variables collapsed to face fields.

        Raises:
            ValueError: ``index`` is not an ``int``, or is negative.
        """
        if not isinstance(index, (int, np.integer)):
            raise ValueError(
                "isel_layer on a ragged (volume) mesh needs an int layer position; a slice "
                "or sequence is ambiguous when columns have different layer counts. Select "
                "one layer at a time."
            )
        if int(index) < 0:
            # A negative layer would make every column "valid" (`layer_count > -1`) and index
            # `volume_offset + layer` with a negative offset, wrapping into the previous
            # column's volumes — garbage, not a per-column top layer. Reject it, matching
            # VolumeTopology.volume_index, which guards negatives too (review L1).
            raise ValueError(
                f"isel_layer on a ragged (volume) mesh needs a non-negative layer position; "
                f"got {int(index)}. A column's top layer varies by column, so there is no "
                f"well-defined negative index."
            )
        topology = cast("VolumeTopology", self._volume_topology)
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if var.location != "volume":
                new_vars[name] = var
                continue
            new_vars[name] = self._gather_layer_from_volume(var, int(index), topology)
        return self._rebuild_without_volume(new_vars)

    def _gather_layer_from_volume(
        self, var: MeshVariable, layer: int, topology: VolumeTopology
    ) -> MeshVariable:
        """Collapse one ragged ``volume`` variable to a ``(time?, n_face)`` face field.

        Face ``f`` takes volume ``volume_offset[f] + layer`` when ``layer < layer_count[f]``,
        else NaN. Vectorised: the valid faces gather in one fancy-index, the rest are left NaN.

        Args:
            var: The ``location="volume"`` variable (flat ``(time?, n_volume)`` data).
            layer: The 0-based layer position to take from each column.
            topology: The mesh's volume topology.

        Returns:
            MeshVariable: A ``location="face"`` variable of shape ``(time?, n_face)``.
        """
        data = np.asarray(var.data, dtype="float64")
        n_face = topology.n_face
        valid = topology.layer_count > layer
        volume_positions = topology.volume_offset[:-1][valid] + layer
        out_shape = (*data.shape[:-1], n_face)
        out = np.full(out_shape, np.nan, dtype="float64")
        out[..., valid] = data[..., volume_positions]
        # Drop the flat `volume` dimension name for the face dimension; keep a leading time
        # dimension name when the variable had one.
        if var.dimensions:
            new_dims = (*var.dimensions[:-1], f"{var.mesh_name}_nFaces")
        else:
            new_dims = ()
        return MeshVariable(
            name=var.name,
            location="face",
            mesh_name=var.mesh_name,
            shape=out.shape,
            attributes=dict(var.attributes),
            nodata=var.nodata,
            units=var.units,
            standard_name=var.standard_name,
            dimensions=new_dims,
            _data=out,
        )

    def _rebuild_without_volume(
        self, data_variables: dict[str, MeshVariable]
    ) -> UgridDataset:
        """A :meth:`_rebuild` that drops the volume topology — the layer axis is gone.

        Used by the ragged layer selectors: once a ``volume`` variable has been gathered to a
        face field, the result is an ordinary 2-D dataset and must not keep a volume topology
        that no variable is indexed through any more.
        """
        return type(self)(
            mesh=self._mesh,
            data_variables=data_variables,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
        )

    def collapse_layer(self, layer: int, *, location: str = "face") -> UgridDataset:
        """Collapse a ragged 3-D mesh's ``volume`` variables to a 2-D field (issue #1330).

        The explicit 2-D collapse of a ragged mesh: each column's ``layer``-th value becomes a
        plain ``(n_face,)`` (or ``(time, n_face)``) face field, absent layers NaN. It is
        :meth:`isel_layer` under a name that says "give me a 2-D slice", so a caller can ask for
        the surface/bed layer as a flat field without reaching for the per-column internals.
        (Named ``collapse_layer`` rather than ``to_location`` because the latter already moves a
        variable *between* mesh locations by graph interpolation — a different operation.)

        Args:
            layer: The 0-based layer position to take from each column.
            location: The target mesh location. Only ``"face"`` is supported — a ragged volume
                collapses onto its columns' faces. Defaults to ``"face"``.

        Returns:
            UgridDataset: A new dataset whose volume variables are 2-D face fields.

        Raises:
            ValueError: ``location`` is not ``"face"``, or the dataset has no volume topology
                to collapse.
        """
        if self._volume_topology is None:
            raise ValueError(
                "collapse_layer needs a ragged volume topology; this dataset has none (it is "
                "2-D or uniformly layered). Use isel_layer for a uniform mesh."
            )
        if location != "face":
            raise ValueError(
                f"collapse_layer supports only 'face' for a ragged volume mesh, not "
                f"{location!r}."
            )
        return self._isel_layer_ragged(int(layer))

    def to_dataframe(
        self, variables: Sequence[str] | None = None, *, dropna: bool = False
    ) -> pd.DataFrame:
        """A tidy frame of the mesh values, one row per element (and step).

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.to_dataframe`, in the **tidy**
        shape rather than the raster ``(y, x)`` MultiIndex one: an unstructured element has no
        row/column address, so each row carries the element index, its centroid coordinates,
        the time step where the variable has one, and the value.

        Args:
            variables: Which data variables to include. ``None`` (the default) takes them all.
            dropna: Drop rows whose value is no-data. Defaults to False.

        Returns:
            pandas.DataFrame: Columns ``variable``, ``element``, ``x``, ``y``, ``layer``,
            ``time`` and ``value``. ``element`` indexes the facet the variable sits on and
            ``x``/``y`` are that facet's coordinates, so a node variable is addressed by node.
            ``layer`` is the layer coordinate, or None for an unlayered variable, and ``time``
            the step coordinate, or None for a static one.

        Raises:
            KeyError: A requested variable is not a data variable of this dataset.
        """
        names = list(self._data_variables) if variables is None else list(variables)
        frames: list[pd.DataFrame] = []
        for name in names:
            if name not in self._data_variables:
                raise KeyError(name)
            var = self._data_variables[name]
            if var.data is None:
                continue
            frames.extend(self._variable_frames(name, var))
        if not frames:
            # Match the populated frame's dtypes, so a caller that concatenates or
            # dtype-checks sees one schema either way.
            return pd.DataFrame(
                {
                    "variable": pd.Series(dtype="str"),
                    "element": pd.Series(dtype="int64"),
                    "x": pd.Series(dtype="float64"),
                    "y": pd.Series(dtype="float64"),
                    "layer": pd.Series(dtype="object"),
                    "time": pd.Series(dtype="object"),
                    "value": pd.Series(dtype="float64"),
                }
            )
        frame = pd.concat(frames, ignore_index=True)
        if dropna:
            frame = frame[~frame["value"].isna()].reset_index(drop=True)
        return frame

    def _variable_frames(self, name: str, var: MeshVariable) -> list[pd.DataFrame]:
        """One tidy block per (time step, layer) of a single variable.

        Args:
            name: The variable's name, carried into the ``variable`` column.
            var: The variable to lay out.

        Returns:
            list: The blocks, in time-then-layer order.
        """
        if var.location == "volume" and self._volume_topology is not None:
            # A ragged volume variable is flat `(time?, n_volume)`: lay it out one row per
            # volume, carrying the column (`element`) and the layer position within it rather
            # than fabricating rows for the absent layers of shallow columns (issue #1330).
            return self._volume_variable_frames(name, var)
        raw = np.asarray(var.data)
        # Gap-masking casts to float64, which a classified / label field cannot survive --
        # and such a field carries no no-data value to mask anyway.
        values = (
            gaps_as_nan(raw, var.nodata) if np.issubdtype(raw.dtype, np.number) else raw
        )
        # Address the rows with the coordinates of the facet the variable actually sits on. Using
        # face centroids for every variable left a node- or edge-located one with a NaN
        # coordinate on every row, since its value count is n_node / n_edge.
        coords = self._element_coords(var.location)
        arranged, steps, layer_labels, temporal, layered = self._arrange_for_rows(
            var, values
        )
        blocks: list[pd.DataFrame] = []
        for step_position, step in enumerate(steps):
            per_step = arranged[step_position] if temporal else arranged
            for layer_position, layer in enumerate(layer_labels):
                slab = per_step[layer_position] if layered else per_step
                blocks.append(
                    self._tidy_rows(name, np.asarray(slab), coords, step, layer)
                )
        return blocks

    def _volume_variable_frames(
        self, name: str, var: MeshVariable
    ) -> list[pd.DataFrame]:
        """Tidy blocks for a ragged ``volume`` variable — one row per volume (issue #1330).

        Each row carries the column (``element`` = the face the volume belongs to), the
        0-based ``layer`` within that column, the face's ``x``/``y`` centroid, and the value.
        A temporal variable emits one block per step; a static one a single block. No row is
        emitted for an absent layer, since the flat volume axis holds only the layers that
        exist.

        Args:
            name: The variable name, carried into the ``variable`` column.
            var: The ``location="volume"`` variable.

        Returns:
            list: The per-step blocks.
        """
        topology = cast("VolumeTopology", self._volume_topology)
        raw = np.asarray(var.data)
        values = (
            gaps_as_nan(raw, var.nodata) if np.issubdtype(raw.dtype, np.number) else raw
        )
        face_of_volume = topology.face_of_volume()
        layer_of_volume = topology.layer_of_volume()
        face_coords = self._element_coords("face")
        have_coords = face_coords.shape[0] == topology.n_face
        coords = (
            face_coords[face_of_volume]
            if have_coords
            else np.full((topology.n_volume, 2), np.nan, dtype="float64")
        )
        if var.has_time:
            time_axis = cast("int", var.time_index)
            stamps = var.attributes.get("time_values")
            steps = (
                list(stamps)
                if stamps is not None
                else list(range(values.shape[time_axis]))
            )
            per_step = [np.take(values, i, axis=time_axis) for i in range(len(steps))]
        else:
            steps = [None]
            per_step = [values]
        blocks: list[pd.DataFrame] = []
        for step, slab in zip(steps, per_step):
            flat = np.asarray(slab).ravel()
            blocks.append(
                pd.DataFrame(
                    {
                        "variable": np.repeat(name, flat.shape[0]),
                        "element": face_of_volume,
                        "x": coords[:, 0],
                        "y": coords[:, 1],
                        "layer": layer_of_volume,
                        "time": np.repeat(step, flat.shape[0]),
                        "value": (
                            flat.astype("float64")
                            if np.issubdtype(flat.dtype, np.number)
                            else flat
                        ),
                    }
                )
            )
        return blocks

    def _element_axis(self, var: MeshVariable, values: np.typing.NDArray) -> int:
        """The axis of ``values`` that runs over the mesh elements.

        The element axis is conventionally last, and assuming so is what broke a variable whose
        dimension names put it first: the last axis was read as the elements, so a
        ``(n_faces, time)`` variable emitted one row per *time step*, dropping the values past
        the element count and leaving their coordinates NaN.

        The variable's declared element count decides it. The last axis is preferred when more
        than one matches -- a square variable is genuinely ambiguous, and last is the convention.

        Args:
            var: The variable, for its location and dimension names.
            values: Its values.

        Returns:
            int: The element axis. The last axis when nothing matches the element count, which
            keeps the previous behaviour for a variable whose shape says nothing.
        """
        if values.ndim < 2:
            return values.ndim - 1
        counts = {"face": self.n_face, "node": self.n_node, "edge": self.n_edge}
        expected = counts.get(var.location)
        matching = (
            [axis for axis in range(values.ndim) if values.shape[axis] == expected]
            if expected
            else []
        )
        return matching[-1] if matching else values.ndim - 1

    def _row_axes(
        self, var: MeshVariable, values: np.typing.NDArray
    ) -> tuple[int, int | None, int | None]:
        """The element, time and layer axes the tidy frame will walk.

        Resolves the three against each other and refuses anything left over, so
        :meth:`_arrange_for_rows` is left with the transpose and the labels.

        An axis can be claimed twice: `var.time_index` may name the axis that holds the
        elements. :meth:`_element_axis` settles that by the declared element count, and the
        element reading wins -- so such an axis is *not* walked as a step, and the caller is
        told which axes were.

        Args:
            var: The variable being laid out.
            values: Its values.

        Returns:
            tuple: ``(element_axis, time_axis, layer_axis)``, the last two ``None`` when there
            is no such axis to walk.

        Raises:
            ValueError: An axis is neither walked nor addressed. The tidy layout is one row per
                element with a ``time`` and a ``layer`` column, so a fourth dimension has
                nowhere to go -- and folding it into the element slot emitted ``element``
                indices past the element count with NaN coordinates, a frame a caller joins on.
        """
        element_axis = self._element_axis(var, values)
        time_axis = cast("int | None", var.time_index) if var.has_time else None
        time_axis = None if time_axis == element_axis else time_axis
        layer_axis = self._layer_axis(var)
        layer_axis = None if layer_axis == element_axis else layer_axis

        walked = {element_axis, *(a for a in (time_axis, layer_axis) if a is not None)}
        unaddressed = [axis for axis in range(values.ndim) if axis not in walked]
        if unaddressed:
            names = var.dimensions or ()
            labelled = [
                names[axis] if axis < len(names) else f"axis {axis}"
                for axis in unaddressed
            ]
            subject = "has" if len(labelled) == 1 else "have"
            pronoun = "it" if len(labelled) == 1 else "them"
            raise ValueError(
                f"to_dataframe cannot address variable {var.name!r}: the tidy frame carries one "
                f"row per element with a 'time' and a 'layer' column, and {labelled} "
                f"{subject} no column. Select {pronoun} away first, e.g. with isel()."
            )
        return element_axis, time_axis, layer_axis

    def _arrange_for_rows(
        self, var: MeshVariable, values: np.typing.NDArray
    ) -> tuple[np.typing.NDArray, list[Any], list[Any], bool, bool]:
        """Reorder a variable's axes to ``(time?, layer?, element)`` and label the leading ones.

        The tidy frame walks one row per element, so the axes above the element axis have to be
        walked in a known order with each position labelled. Flattening instead ran the element
        index past the element count and lost which layer a row came from.

        A missing axis is reported as the single label ``[None]``, so the caller's loop runs once
        over it and the frame still carries the column.

        Args:
            var: The variable whose axes are being arranged.
            values: Its values, already gap-masked.

        Returns:
            tuple: ``(arranged, steps, layer_labels, walked_time, walked_layer)`` -- the
            transposed array, one label per time step, one per layer, and whether a time and a
            layer axis were actually walked. The last two matter because they can disagree with
            `var.has_time` / `_layer_axis`: when the dimension names put time (or layer) in the
            element slot, the element reading wins and that axis is *not* walked. A caller that
            trusted `var.has_time` instead indexed `arranged[0]`, slicing the variable down to
            its first element and emitting one row for a two-face mesh.

        Raises:
            ValueError: The variable carries an axis the frame cannot address. The tidy layout is
                one row per element with a ``time`` and a ``layer`` column, so a fourth dimension
                has nowhere to go. Making the transpose a permutation stopped numpy raising, but
                the consumer then folded that axis into the element slot and emitted ``element``
                indices past the element count with NaN coordinates -- a frame a caller joins on,
                which is worse than the exception. Select the extra dimension away first.
        """
        element_axis, time_axis, layer_axis = self._row_axes(var, values)
        leading = [axis for axis in (time_axis, layer_axis) if axis is not None]
        order = [*leading, element_axis]
        arranged = np.transpose(values, order) if values.ndim > 1 else values

        steps: list[Any] = [None]
        if time_axis is not None:
            stamps = var.attributes.get("time_values")
            steps = (
                list(stamps) if stamps is not None else list(range(arranged.shape[0]))
            )

        layer_labels: list[Any] = [None]
        if layer_axis is not None:
            depth = arranged.shape[1 if time_axis is not None else 0]
            stored = var.attributes.get("layer_values")
            layer_labels = list(stored) if stored is not None else list(range(depth))

        return (
            arranged,
            steps,
            layer_labels,
            time_axis is not None,
            layer_axis is not None,
        )

    def _element_coords(self, location: str) -> np.typing.NDArray:
        """The ``(n_elements, 2)`` coordinate table of a mesh facet.

        Args:
            location: ``"face"``, ``"node"`` or ``"edge"``.

        Returns:
            np.ndarray: One ``(x, y)`` row per element, or an empty table when the facet's
            coordinates are not available (an edge-located variable on a mesh carrying no edge
            coordinates), which leaves those rows with a NaN coordinate.
        """
        try:
            table = _location_coords(self._mesh, location)
        except ValueError:
            table = np.empty((0, 2), dtype="float64")
        return np.asarray(table, dtype="float64")

    def _tidy_rows(
        self,
        name: str,
        values: np.typing.NDArray,
        element_coords: np.typing.NDArray,
        step: Any,
        layer: Any = None,
    ) -> pd.DataFrame:
        """One tidy block: every element of ``values`` as a row."""
        flat = np.asarray(values).ravel()
        count = flat.shape[0]
        coords = (
            element_coords
            if element_coords.shape[0] == count
            else np.full((count, 2), np.nan, dtype="float64")
        )
        return pd.DataFrame(
            {
                "variable": np.repeat(name, count),
                "element": np.arange(count),
                "x": coords[:, 0],
                "y": coords[:, 1],
                "layer": np.repeat(layer, count),
                "time": np.repeat(step, count),
                # Keep a non-numeric variable as it is. Forcing float64 raised
                # "could not convert string to float" on exactly the classified /
                # label field `polygonize` recommends.
                "value": (
                    flat.astype("float64")
                    if np.issubdtype(flat.dtype, np.number)
                    else flat
                ),
            }
        )

    def copy(self) -> UgridDataset:
        """An independent deep copy of this dataset.

        Every variable's values are copied, so writing into the copy never shows in the
        original — the counterpart of :meth:`pyramids.netcdf.NetCDF.copy`.

        Returns:
            UgridDataset: The copy.
        """
        copied: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            copied[name] = (
                var
                if var.data is None
                else var.with_data(np.array(var.data, copy=True))
            )
        return type(self)(
            mesh=self._mesh,
            data_variables=copied,
            global_attributes=dict(self._global_attributes),
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )

    @classmethod
    def from_bytes(cls, data: bytes, *, suffix: str = ".nc") -> UgridDataset:
        """Read a UGRID mesh from an in-memory buffer.

        Writes ``data`` into GDAL's in-memory filesystem and reads it as a file, mirroring
        :meth:`pyramids.netcdf.NetCDF.from_bytes` — for a mesh fetched over the network or
        held in a database blob. The values are materialised before the buffer is unlinked, so
        the result does not depend on it.

        Args:
            data: The bytes of a UGRID NetCDF file.
            suffix: The extension to present to GDAL. Defaults to ``".nc"``.

        Returns:
            UgridDataset: The mesh read from the buffer.

        Raises:
            RuntimeError: GDAL cannot read the buffer at all. It is written to ``/vsimem`` and
                read through :meth:`read_file`, so GDAL's own error comes back -- naming the
                in-memory path rather than the caller's bytes.
            ValueError: The buffer reads as a file but declares no UGRID topology.
        """
        name = f"/vsimem/pyramids_ugrid_{uuid4().hex}{suffix}"
        gdal.FileFromMemBuffer(name, data)
        try:
            result = cls.read_file(name).copy()
        finally:
            gdal.Unlink(name)
        return result

    def info(self, buf: Any = None, drivers: bool = False) -> None:
        """Print a summary of the mesh, its variables and its attributes.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.info`: one readable block naming
        the topology, the element counts, the CRS, and every data variable with its location,
        dtype and shape.

        Args:
            buf: Where to write. Defaults to ``sys.stdout``.
            drivers: Also list the mesh file-format drivers available in this interpreter —
                the built-in UGRID reader plus any installed plugin. Off by default because
                resolving them walks every installed distribution, which a summary of one
                dataset should not pay for.
        """
        stream = sys.stdout if buf is None else buf
        lines = [
            f"UgridDataset: {self.mesh_name}",
            f"  nodes: {self.n_node}  faces: {self.n_face}  edges: {self.n_edge}",
            f"  epsg: {self.epsg}",
            f"  variables ({len(self._data_variables)}):",
        ]
        for name, var in self._data_variables.items():
            # Metadata only: `var.data` would fire the lazy loader and pull every variable's
            # full array off disk, which is the opposite of what a summary is for. The declared
            # shape and `has_data_source` answer the same questions without a read.
            shape = str(tuple(var.shape)) if var.has_data_source else "unloaded"
            temporal = "temporal" if var.has_time else "static"
            lines.append(
                f"    {name}: location={var.location} dtype={var.dtype} "
                f"shape={shape} ({temporal})"
            )
        if self._global_attributes:
            lines.append(f"  global attributes ({len(self._global_attributes)}):")
            lines.extend(
                f"    {key}: {value}" for key, value in self._global_attributes.items()
            )
        if drivers:
            driver_lines = describe_drivers()
            # Count the plugins, not the lines: reporting "mesh drivers (1)" on a bare install
            # counted the built-in reader as a driver, while DriverNotExistError described the
            # same state as having none installed.
            plugins = len(driver_lines) - 1
            suffix = "" if plugins == 1 else "s"
            lines.append(f"  mesh drivers (built-in + {plugins} plugin{suffix}):")
            lines.extend(f"    {line}" for line in driver_lines)
        stream.write("\n".join(lines) + "\n")

    def set_global_attribute(self, name: str, value: Any) -> UgridDataset:
        """Return a new dataset carrying one more global attribute.

        A UGRID derivation is immutable here (as :meth:`with_variable` is), so this answers a
        new dataset rather than writing in place; :meth:`to_file` then persists the attributes
        through the shared CF writer.

        Args:
            name: The attribute name.
            value: The attribute value.

        Returns:
            UgridDataset: A new dataset whose global attributes include ``name``.
        """
        return type(self)(
            mesh=self._mesh,
            data_variables=dict(self._data_variables),
            global_attributes={**self._global_attributes, name: value},
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )

    def delete_global_attribute(self, name: str) -> UgridDataset:
        """Return a new dataset without the global attribute ``name``.

        Args:
            name: The attribute to drop. Dropping an absent attribute is not an error,
                mirroring the raster ``delete_global_attribute``.

        Returns:
            UgridDataset: A new dataset without that attribute.
        """
        remaining = {
            key: value for key, value in self._global_attributes.items() if key != name
        }
        return type(self)(
            mesh=self._mesh,
            data_variables=dict(self._data_variables),
            global_attributes=remaining,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )

    def active_mask(self, variable_name: str) -> np.typing.NDArray | None:
        """The variable's per-element active (wet/dry) mask, or ``None`` when it has none.

        The first-class companion of the data (issue #1331): ``True`` marks an active/wet
        element, ``False`` a dry one, kept separate from ``nodata``/NaN so a consumer can tell
        a dry element from a missing value. The mask is element-indexed — ``(n_element,)``
        when it is static or ``(n_time, n_element)`` when it varies per step — and is returned
        as read, never applied to the values. Use :meth:`masked_values` to get the values with
        it applied.

        Args:
            variable_name: Name of the data variable.

        Returns:
            np.ndarray | None: The boolean mask, or ``None`` when the variable carries none.

        Raises:
            KeyError: ``variable_name`` is not a data variable of this dataset.

        Examples:
            - A variable with a mask reports it; one without answers ``None``:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> ds = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"depth": np.array([2.0, 4.0])},
                ... )
                >>> ds["depth"].active = np.array([True, False])
                >>> ds.active_mask("depth").tolist()
                [True, False]
                >>> ds.active_mask("depth") is not None
                True

                ```
        """
        var = self.get_data(variable_name)
        return None if var.active is None else np.asarray(var.active, dtype=bool)

    def masked_values(self, variable_name: str) -> np.typing.NDArray:
        """The variable's values with both ``nodata`` **and** the active mask blanked to NaN.

        The **opt-in** counterpart of a bare ``.data`` read (issue #1331): dry elements (where
        :meth:`active_mask` is ``False``) become NaN on top of the usual ``nodata`` blanking,
        so a caller that wants "treat dry as missing" gets it explicitly — the values are
        never masked by default. Keeps the variable's full shape (every time step and layer),
        unlike the single-step :meth:`sample` / :meth:`weighted` path. When the variable has no
        mask the result is just the ``nodata``-blanked values.

        Args:
            variable_name: Name of the data variable.

        Returns:
            np.ndarray: The float64 values, dry and missing elements alike as NaN.

        Raises:
            KeyError: ``variable_name`` is not a data variable of this dataset.
            ValueError: The variable has no loaded data.

        Examples:
            - Dry elements become NaN, so a mean over the wet cells is straightforward:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> ds = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"depth": np.array([2.0, 4.0])},
                ... )
                >>> ds["depth"].active = np.array([True, False])
                >>> ds.masked_values("depth").tolist()
                [2.0, nan]
                >>> float(np.nanmean(ds.masked_values("depth")))
                2.0

                ```
        """
        var = self.get_data(variable_name)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {variable_name!r} has no loaded data.")
        values = gaps_as_nan(np.asarray(data), var.nodata)
        if var.active is not None:
            values = _apply_active_mask(var, values)
        return cast("np.typing.NDArray", values)

    def _element_values(
        self, variable_name: str, time_index: int = 0, *, apply_active: bool = False
    ) -> tuple[MeshVariable, np.typing.NDArray]:
        """The per-element values of a variable, no-data blanked to NaN.

        Shared by the analysis members (:meth:`sample`, :meth:`weighted`,
        :meth:`zonal_stats`). A temporal variable is reduced to one
        step (``time_index``, the first by default) so the returned array is
        ``(n_elements,)``, matching the single-step contract :meth:`plot` and
        :meth:`to_dataset` already use. The variable's declared no-data value is turned
        into NaN through the shared :func:`pyramids.base._reductions.gaps_as_nan`, so
        every statistic leaves gaps out.

        Args:
            variable_name: Name of the data variable.
            time_index: Which step to take for a temporal variable. Defaults to 0.
            apply_active: Also blank dry elements (:attr:`MeshVariable.active` ``False``) to
                NaN, the mask reduced to the same step. Defaults to ``False`` — the mask is
                never applied unless a caller asks, so "dry" stays distinct from "missing"
                (issue #1331).

        Returns:
            tuple: The :class:`MeshVariable` and its ``(n_elements,)`` float64 values.

        Raises:
            ValueError: The variable has no loaded data, or is not per-element after the
                time collapse (e.g. a layered ``(n_layers, n_face)`` non-temporal variable).
        """
        var = self.get_data(variable_name)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {variable_name!r} has no loaded data.")
        if var.has_time:
            # Take the step along the variable's own time axis, not blindly axis 0 — a
            # mesh variable may store time as a trailing axis (e.g. (nFaces, time)), and
            # the rest of this module keys off var.time_index for exactly that reason.
            data = np.take(
                np.asarray(data), time_index, axis=cast("int", var.time_index)
            )
        result = gaps_as_nan(np.asarray(data), var.nodata)
        if apply_active and var.active is not None and result.ndim == 1:
            # Reduce a per-step mask to the same step, then blank dry elements. A static mask
            # is used as-is. Only meaningful once `result` is the 1-D per-element vector.
            mask = np.asarray(var.active, dtype=bool)
            if mask.ndim >= 2:
                mask = mask[time_index]
            result = np.where(mask, result, np.nan)
        if result.ndim != 1:
            # A non-temporal multi-dimensional variable (e.g. layered (n_layers, n_face))
            # has no single per-element vector; reducing it over an arbitrary axis would
            # silently return a per-column scalar. Refuse rather than mislead.
            raise ValueError(
                f"Variable {variable_name!r} is not per-element after the time collapse "
                f"(shape {result.shape}); the analysis members need a 1-D variable. A "
                "layered/multi-dimensional non-temporal variable is not supported here."
            )
        return var, result

    def summary(
        self,
        variables: Sequence[str] | None = None,
        *,
        metrics: Sequence[str] = DEFAULT_METRICS,
        skipna: bool = True,
        ddof: int = 0,
    ) -> pd.DataFrame:
        """A per-variable summary table over the whole mesh.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.summary`, built on the same shared
        assembly (:func:`pyramids.base._summary.variable_summary`) so the two classes answer in an
        identical shape. Each variable is reduced over **all** of its samples — every element and,
        for a temporal variable, every step — with the declared no-data value left out.

        Args:
            variables: Which data variables to summarise, in output-row order. ``None`` (the
                default) takes every data variable. An unknown name raises ``KeyError``.
            metrics: Which statistics to report, in column order. Defaults to
                ``("count", "min", "max", "mean", "std")``; ``median`` / ``var`` / ``sum`` /
                ``prod`` are available too.
            skipna: Leave no-data out of the statistics (the default) or let it propagate.
            ddof: Delta degrees of freedom for ``std`` / ``var``. Defaults to 0.

        Returns:
            pandas.DataFrame: One row per variable (index name ``"variable"``), one column per
            metric. ``count`` is ``int64``; the rest are ``float64``. A variable with no valid
            element yields NaN statistics and a ``count`` of 0.

        Raises:
            KeyError: A requested variable is not a data variable of this dataset.
            TypeError: A requested variable is non-numeric.
            ValueError: A requested variable has no loaded data, or an unknown metric.

        Examples:
            - Per-variable summary of a two-triangle mesh:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"depth": np.array([2.0, 4.0])},
                ... )
                >>> mesh.summary().loc["depth", ["count", "min", "max", "mean"]].tolist()
                [2.0, 2.0, 4.0, 3.0]

                ```
        """
        names = list(self._data_variables) if variables is None else list(variables)
        arrays: dict[str, np.typing.NDArray] = {}
        for name in names:
            if name not in self._data_variables:
                raise KeyError(name)
            var = self._data_variables[name]
            if var.data is None:
                raise ValueError(f"Variable {name!r} has no loaded data.")
            data = np.asarray(var.data)
            if not np.issubdtype(data.dtype, np.number):
                if variables is None:
                    warnings.warn(
                        f"skipping non-numeric variable {name!r} in summary",
                        stacklevel=2,
                    )
                    continue
                raise TypeError(f"variable {name!r} is non-numeric; cannot summarise")
            arrays[name] = gaps_as_nan(data, var.nodata)
        return variable_summary(arrays, metrics=metrics, skipna=skipna, ddof=ddof)

    def sample(
        self,
        variable_name: str,
        x: float | np.ndarray,
        y: float | np.ndarray,
        *,
        method: str = "contains",
        time_index: int = 0,
    ) -> np.typing.NDArray:
        """Sample a mesh variable at point locations.

        The mesh counterpart of :meth:`pyramids.dataset.Dataset.sample`. Where the raster
        version inverts an affine geotransform to find the pixel under each point, a mesh
        resolves the point to an element through :class:`MeshSpatialIndex`: a face-located
        variable by which face contains the point (``method="contains"``) or the nearest
        face centroid (``method="nearest"``); a node-located variable by nearest node.

        Args:
            variable_name: Name of the data variable.
            x: Query x-coordinate(s), in the mesh CRS.
            y: Query y-coordinate(s), in the mesh CRS.
            method: ``"contains"`` (point-in-face, face variables only) or ``"nearest"``.
                Defaults to ``"contains"``.
            time_index: Step for a temporal variable. Defaults to 0.

        Returns:
            numpy.ndarray: One value per query point; NaN where a ``"contains"`` query
            falls outside every face.

        Raises:
            ValueError: An unknown ``method``, ``"contains"`` on a non-face variable, or a
                variable located on edges (unsupported).
        """
        var, arr = self._element_values(variable_name, time_index=time_index)
        xs = np.atleast_1d(np.asarray(x, dtype="float64"))
        ys = np.atleast_1d(np.asarray(y, dtype="float64"))
        index = MeshSpatialIndex(self._mesh)
        if var.location == "face":
            if method == "contains":
                idx = index.locate_faces(xs, ys)
            elif method == "nearest":
                idx = np.atleast_1d(index.locate_nearest_face(xs, ys)).ravel()
            else:
                raise ValueError(
                    f"sample method must be 'contains' or 'nearest', got {method!r}"
                )
        elif var.location == "node":
            # A node is a point, so "contains" has no meaning here — only "nearest"
            # applies. Reject "contains" rather than silently doing a nearest lookup, as
            # the docstring promises.
            if method == "contains":
                raise ValueError(
                    "sample method='contains' is only valid for a face-located variable; "
                    f"{variable_name!r} is on 'node' — use method='nearest'."
                )
            if method != "nearest":
                raise ValueError(
                    f"sample method must be 'contains' or 'nearest', got {method!r}"
                )
            idx = np.atleast_1d(index.locate_nearest_node(xs, ys)).ravel()
        else:
            raise ValueError(
                f"sample supports 'face' and 'node' variables; {variable_name!r} is on "
                f"{var.location!r}."
            )
        out = np.full(len(xs), np.nan, dtype="float64")
        inside = idx >= 0
        out[inside] = arr[idx[inside]]
        return out

    def weighted(
        self,
        variable_name: str,
        *,
        how: str = "mean",
        time_index: int = 0,
        q: float | None = None,
    ) -> float:
        """Area-weighted statistic of a face variable over the mesh.

        An irregular mesh has faces of different sizes, so the meaningful aggregate weights
        each face by its area. Delegates to the shared
        :func:`pyramids.base._reductions.weighted_statistic` with :attr:`Mesh2d.face_areas`
        as the weights — the same kernel the raster ``weighted`` reduction uses.

        Args:
            variable_name: Name of a **face-located** data variable.
            how: One of ``"mean"``, ``"sum"``, ``"sum_of_weights"``, ``"std"``, ``"var"``,
                ``"quantile"`` (which needs ``q``). Defaults to ``"mean"``.
            time_index: Step for a temporal variable. Defaults to 0.
            q: The quantile in ``[0, 1]`` when ``how="quantile"`` (the area-weighted quantile
                uses the Hazen convention and does not match ``reduce(how="quantile")``); must be
                ``None`` otherwise.

        Returns:
            float: The area-weighted statistic over all faces.

        Raises:
            ValueError: ``how`` is not one of ``WEIGHTED_HOWS``; the variable is not
                face-located; or its length does not match the number of faces.

        Examples:
            - Area-weighted mean: the larger face (area 4, value 10) outweighs the
              smaller (area 2, value 20), pulling the mean below the unweighted 15.0:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"v": np.array([10.0, 20.0])},
                ... )
                >>> round(mesh.weighted("v"), 3)
                13.333

                ```
            - The area-weighted quantile clamps to the data range at the extremes:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"v": np.array([10.0, 20.0])},
                ... )
                >>> mesh.weighted("v", how="quantile", q=0.0)
                10.0
                >>> mesh.weighted("v", how="quantile", q=1.0)
                20.0

                ```
        """
        if how not in WEIGHTED_HOWS:
            # Validate up front, mirroring the raster weighted path: the kernel's catch-all
            # else-branch would otherwise return the standard deviation for any unsupported
            # `how`, a silent wrong answer.
            raise ValueError(
                f"weighted: how must be one of {sorted(WEIGHTED_HOWS)}, got {how!r}."
            )
        if how == "quantile":
            if q is None or isinstance(q, bool) or not 0.0 <= float(q) <= 1.0:
                raise ValueError(
                    f"weighted(how='quantile') needs q in [0, 1], got {q!r}."
                )
        elif q is not None:
            raise ValueError(
                f"weighted(q=...) is only valid with how='quantile', not {how!r}."
            )
        var, arr = self._element_values(variable_name, time_index=time_index)
        if var.location != "face":
            raise ValueError(
                "area-weighted statistics need a face-located variable; "
                f"{variable_name!r} is on {var.location!r}."
            )
        areas = self._mesh.face_areas
        if arr.shape[-1] != areas.shape[0]:
            raise ValueError(
                f"variable {variable_name!r} has {arr.shape[-1]} face value(s) but the "
                f"mesh has {areas.shape[0]} face(s)."
            )
        # `arr` already carries NaN for gaps, so `ndv=None` and `skipna=True` reduce over
        # the single face axis; the kernel keeps the axis as length 1, squeezed off here.
        result = weighted_statistic(arr, areas, (0,), how, None, True, q)
        return float(np.asarray(result).ravel()[0])

    def zonal_stats(
        self,
        zones: Any,
        *,
        variable_name: str,
        stats: tuple[str, ...] = ("mean",),
        weighted: bool = True,
        time_index: int = 0,
    ) -> pd.DataFrame:
        """Aggregate a face variable within polygons, weighted by face area.

        The mesh counterpart of :func:`pyramids.dataset.ops._zonal.zonal_stats`. Each face
        is assigned to the zone whose polygon contains its centroid (the mesh analogue of a
        raster pixel's cell centre), then the shared
        :func:`pyramids.base._reductions.reduce_by_label` reduces the face values per zone.
        With ``weighted`` (the default) the **averaging** statistics (``mean`` / ``std`` /
        ``var``) are area-weighted, so a zone's mean is area-exact — a capability the raster
        path does not yet have. ``sum`` is always the plain Σ value (never an area integral),
        and ``count`` / ``min`` / ``max`` are weight-invariant.

        Args:
            zones: Polygons, as a :class:`~pyramids.feature.FeatureCollection` (or any
                object exposing ``geometry``, ``index`` and ``len``). CRS must match the
                mesh; reproject first if not.
            variable_name: Name of a **face-located** data variable.
            stats: Statistics per zone, any of ``"mean"``, ``"sum"``, ``"min"``, ``"max"``,
                ``"std"``, ``"var"``, ``"count"``. Defaults to ``("mean",)``.
            weighted: Area-weight the averaging statistics (``mean`` / ``std`` / ``var``).
                Defaults to True. ``sum`` / ``count`` / ``min`` / ``max`` are unaffected.
            time_index: Step for a temporal variable. Defaults to 0.

        Returns:
            pandas.DataFrame: Indexed like ``zones``; one column per statistic. A zone
            containing no face centroid is NaN (``count`` ``0.0``).

        Raises:
            ValueError: The variable is not face-located, or the zones' CRS disagrees with
                the mesh CRS.
        """
        var, arr = self._element_values(variable_name, time_index=time_index)
        if var.location != "face":
            raise ValueError(
                "mesh zonal_stats needs a face-located variable; "
                f"{variable_name!r} is on {var.location!r}."
            )
        zone_crs = getattr(zones, "crs", None)
        if zone_crs is not None and self.epsg is not None:
            zone_epsg = zone_crs.to_epsg()
            if zone_epsg is not None and zone_epsg != self.epsg:
                raise ValueError(
                    f"zonal_stats: zones CRS (EPSG:{zone_epsg}) does not match mesh CRS "
                    f"(EPSG:{self.epsg}). Reproject the zones to the mesh CRS first."
                )
        geometries = list(zones.geometry)
        cx, cy = self._mesh.face_centroids
        centroids = shapely.points(cx, cy)
        tree = shapely.STRtree(geometries)
        # Face centroid within a zone polygon -> that zone. A centroid in more than one
        # (overlapping zones) keeps the first, mirroring a raster pixel landing in one cell.
        centroid_idx, zone_idx = tree.query(centroids, predicate="within")
        labels = np.full(self._mesh.n_face, -1, dtype=np.intp)
        for face_i, zone_i in zip(centroid_idx, zone_idx):
            if labels[face_i] == -1:
                labels[face_i] = zone_i
        n_zones = len(geometries)
        columns: dict[str, np.typing.NDArray] = {}
        # Area weighting applies only to the averaging statistics. `sum` stays the plain
        # Σ value (a weighted sum would be an area integral — a different quantity, and a
        # silent surprise); `count` / `min` / `max` are weight-invariant anyway.
        averaging = [s for s in stats if s in ("mean", "std", "var")]
        plain = [s for s in stats if s not in ("mean", "std", "var")]
        if weighted and averaging:
            columns.update(
                reduce_by_label(
                    arr, labels, n_zones, averaging, weights=self._mesh.face_areas
                )
            )
            if plain:
                columns.update(reduce_by_label(arr, labels, n_zones, plain))
        else:
            columns.update(reduce_by_label(arr, labels, n_zones, list(stats)))
        return pd.DataFrame({stat: columns[stat] for stat in stats}, index=zones.index)

    def _temporal_names(self) -> list[str]:
        """Names of the variables that carry a real time dimension."""
        return [name for name, var in self._data_variables.items() if var.has_time]

    def _rebuild(self, data_variables: dict[str, MeshVariable]) -> UgridDataset:
        """A new dataset with the same mesh/metadata but different data variables.

        Every along-time member below is immutable — it derives a fresh dataset rather than
        mutating in place — mirroring :meth:`sel_time` / :meth:`crop`. The mesh topology,
        global attributes, topology info, CRS and source file name all carry over unchanged,
        and the result is of the receiver's own class so a subclass is not downgraded.
        """
        return type(self)(
            mesh=self._mesh,
            data_variables=data_variables,
            global_attributes=self._global_attributes,
            topology_info=self._topology_info,
            crs_wkt=self._crs_wkt,
            file_name=self._file_name,
            volume_topology=self._volume_topology,
        )

    def _require_temporal(self, operation: str) -> list[str]:
        """The temporal variable names, or a clear error when there are none."""
        temporal = self._temporal_names()
        if not temporal:
            raise ValueError(
                f"{operation} needs at least one variable with a time dimension; this "
                f"dataset has none."
            )
        return temporal

    def _check_time_dim(self, operation: str, dim: str | None) -> None:
        """Refuse a `dim` that is not the time axis.

        The along-time members take `dim` for signature parity with the raster side, where a
        cube has several non-spatial dimensions to choose from. A mesh has exactly one: the
        element axis is the mesh's own, and carrying a value along it is meaningless. Accept
        ``"time"`` (or ``None``) and refuse anything else rather than silently using time.

        Args:
            operation: The calling method's name, for the message.
            dim: The dimension the caller named, or ``None``.

        Raises:
            ValueError: ``dim`` is neither ``None`` nor ``"time"``.
        """
        if dim is not None and dim != "time":
            raise ValueError(
                f"{operation}: a mesh has one along-able dimension, 'time'; got dim={dim!r}. "
                "The element axis is the mesh's own axis, which carries no ordering to run "
                "along."
            )

    def reduce(
        self,
        how: str,
        *,
        skipna: bool = True,
        q: float | None = None,
        groupby: Any = None,
    ) -> UgridDataset:
        """Reduce the time dimension of every temporal variable, whole or in groups.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.reduce` over time. Each
        temporal variable is reduced along its time axis through the shared
        :func:`pyramids.base._reductions.reduce_axis` — the very kernel the raster reductions
        use. Non-temporal variables are kept unchanged.

        Without ``groupby`` the whole axis collapses to one statistic and the variable becomes
        static. With ``groupby`` each group is reduced on its own slab and the results are
        stacked back along the time axis, so the variable stays temporal with one step per
        group — the same shape contract the raster grouped reduce has. For a ``(4, 2)``
        variable over four monthly steps, ``reduce("mean")`` answers shape ``(2,)`` with
        ``has_time`` False, while ``reduce("mean", groupby="MS")`` answers shape ``(2, 2)``
        with ``has_time`` still True.

        Args:
            how: One of ``"mean"``, ``"sum"``, ``"min"``, ``"max"``, ``"std"``, ``"var"``,
                ``"median"``, ``"prod"``, ``"quantile"``, ``"count"``, ``"all"`` or ``"any"``.
            skipna: Skip the declared no-data value and NaN. Defaults to True.
            q: The quantile for ``how="quantile"``; must stay ``None`` otherwise.
            groupby: Reduce within groups of steps instead of over the whole axis. Two
                spellings, matching the raster grouped reduce: a **sequence of per-step
                labels** the same length as the time axis (grouped in first-appearance order,
                so the caller controls the order), or a **pandas offset alias** (``"MS"``,
                ``"YS"``, …) binned against the dataset's date-like time coordinates. Each
                group's label becomes the result's ``time_values`` entry, and empty bins are
                dropped. ``None`` reduces the whole axis. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset. Without ``groupby`` each temporal variable is
            collapsed over time into a static per-element variable; with ``groupby`` each one
            keeps its time dimension, at one step per group.

        Raises:
            ValueError: ``how`` is not a known reduction; ``q`` is given for a non-quantile
                ``how`` or missing for ``how="quantile"``; no variable has a time dimension;
                a temporal variable has no loaded data; a ``groupby`` label sequence's length
                is not the time length; or a ``groupby`` offset alias is asked of time
                coordinates that are step positions, or that pandas cannot read as dates.

        Examples:
            - Mean over two time steps of a per-face variable:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0]])},
                ...     data_locations={"d": "face"},
                ... )
                >>> mesh["d"].has_time
                True
                >>> reduced = mesh.reduce("mean")
                >>> reduced["d"].data.tolist()
                [2.0, 3.0]
                >>> reduced["d"].has_time
                False

                ```
        """
        _check_reduce_how(how, q)
        self._require_temporal("reduce")
        # all/any collapse to a uint8 flag band whose all-gap value is FLAG_NO_DATA (255);
        # the collapsed variable must declare that so a consumer does not read 255 as data.
        result_nodata = FLAG_NO_DATA if how in ("all", "any") else None
        grouped = (
            _group_positions(self._time_coords("reduce"), groupby)
            if groupby is not None
            else None
        )
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to reduce.")
            arr = np.asarray(data)
            nodata = result_nodata if result_nodata is not None else var.nodata
            if grouped is None:
                reduced = reduce_axis(arr, axis, how, skipna, var.nodata, q)
                new_vars[name] = _static_from(
                    var, np.asarray(reduced), axis, nodata=nodata
                )
                continue
            labels, positions = grouped
            # Each group is reduced on its own slab and the results stacked back along the
            # time axis, so a grouped reduce keeps the dimension (shorter) instead of
            # collapsing it — the same shape contract the raster grouped reduce has.
            parts = [
                np.asarray(
                    reduce_axis(
                        np.take(arr, group, axis=axis), axis, how, skipna, var.nodata, q
                    )
                )
                for group in positions
            ]
            # A grouped reduce re-bins the time axis, so a per-step mask no longer aligns —
            # even when n_groups == n_time keeps the shape, which would slip past the
            # length-only implicit guard (review L3). Drop it; a static mask is step-
            # independent and kept.
            new_var = var.with_data(
                np.stack(parts, axis=axis),
                active=None if var.active_has_time else _KEEP_ACTIVE,
            )
            new_var.nodata = nodata
            new_var.attributes = {**var.attributes, "time_values": list(labels)}
            new_vars[name] = new_var
        return self._rebuild(new_vars)

    def resample(
        self,
        freq: str,
        how: str = "mean",
        *,
        skipna: bool = True,
        q: float | None = None,
    ) -> UgridDataset:
        """Bin the time axis by a calendar frequency and reduce each bin.

        Sugar over ``reduce(how, groupby=freq)``: the mesh counterpart of the raster
        ``resample``. The time coordinates must be date-like, since the bins come from a pandas
        offset alias (``"1MS"``, ``"YS"``, ``"7D"``, …). The time dimension is kept, one step
        per non-empty bin, stamped with **pandas' own label for that bin** — which is the bin's
        start for a start-anchored alias and its end for an end-anchored one. Over steps in
        January, February and March 2020, ``"MS"`` labels the bins 01-01 / 02-01 / 03-01 while
        ``"ME"`` labels them 01-31 / 02-29 / 03-31, and ``"W"`` labels each week by its Sunday.

        Args:
            freq: A pandas offset alias naming the bin width.
            how: The statistic each bin is reduced with. Defaults to ``"mean"``.
            skipna: Skip the declared no-data value and NaN. Defaults to True.
            q: The quantile, required when ``how="quantile"``.

        Returns:
            UgridDataset: A new dataset with one time step per bin.

        Raises:
            ValueError: No variable has a time dimension, the coordinates are not date-like,
                or ``how``/``q`` is invalid.
        """
        return self.reduce(how, skipna=skipna, q=q, groupby=freq)

    def mean(self, *, skipna: bool = True) -> UgridDataset:
        """Mean over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time mean.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("mean", skipna=skipna)

    def sum(self, *, skipna: bool = True) -> UgridDataset:
        """Sum over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time sum.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("sum", skipna=skipna)

    def min(self, *, skipna: bool = True) -> UgridDataset:
        """Minimum over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time minimum.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("min", skipna=skipna)

    def max(self, *, skipna: bool = True) -> UgridDataset:
        """Maximum over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time maximum.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("max", skipna=skipna)

    def std(self, *, skipna: bool = True) -> UgridDataset:
        """Std-dev over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time std.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("std", skipna=skipna)

    def var(self, *, skipna: bool = True) -> UgridDataset:
        """Variance over time of every temporal variable (thin wrapper over :meth:`reduce`).

        Args:
            skipna: Skip the declared no-data value and NaN. Defaults to True.

        Returns:
            UgridDataset: A new dataset; each temporal variable collapsed to its time variance.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self.reduce("var", skipna=skipna)

    def _transform_time(
        self, operation: str, fn: Any, active_fn: Any = None
    ) -> UgridDataset:
        """Apply a shape-preserving ``fn(float_data, axis)`` to each temporal variable.

        ``fn`` receives the variable's values as float64 with no-data blanked to NaN, and the
        integer time axis, and returns an array of the same shape. Static variables are kept
        unchanged. Shared by the cumulative, shift, fill and rolling members.

        ``active_fn`` transforms a **per-step** active mask alongside the data. It is needed
        only for an op that *relocates* snapshots along time (``shift``): the implicit mask
        carry keeps a shape-preserving mask as-is, which is right for the value-preserving
        per-step ops (``cumsum``/``cumprod``, ``ffill``/``bfill``, ``rolling`` — step ``t``
        still describes step ``t``) but wrong for ``shift``, where row ``t`` now holds another
        step's snapshot. When given, ``active_fn(mask)`` is applied to a per-step mask (time on
        axis 0) so it moves with the data; a static mask is step-independent and left unchanged.
        """
        self._require_temporal(operation)
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(
                    f"Variable {name!r} has no loaded data for {operation}."
                )
            transformed = fn(gaps_as_nan(np.asarray(data), var.nodata), axis)
            if active_fn is not None and var.active_has_time and var.active is not None:
                moved = var.with_data(
                    np.asarray(transformed),
                    active=active_fn(np.asarray(var.active)),
                )
            else:
                moved = var.with_data(np.asarray(transformed))
            new_vars[name] = moved
        return self._rebuild(new_vars)

    def cumsum(self) -> UgridDataset:
        """Cumulative sum along time for every temporal variable.

        Returns:
            UgridDataset: A new dataset in which each temporal variable holds the running
            sum of its steps along the time axis; static variables are unchanged.

        Raises:
            ValueError: No variable has a time dimension, or a temporal variable has no
                loaded data.

        Examples:
            - Running sum over three time steps of a per-face variable:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0], [5.0, 6.0]])},
                ...     data_locations={"d": "face"},
                ... )
                >>> mesh.cumsum()["d"].data.tolist()
                [[1.0, 2.0], [4.0, 6.0], [9.0, 12.0]]

                ```
        """
        return self._transform_time("cumsum", lambda d, axis: np.cumsum(d, axis=axis))

    def cumprod(self) -> UgridDataset:
        """Cumulative product along time for every temporal variable.

        Returns:
            UgridDataset: A new dataset in which each temporal variable holds the running
            product of its steps along the time axis; static variables are unchanged.

        Raises:
            ValueError: No variable has a time dimension, or a temporal variable has no
                loaded data.
        """
        return self._transform_time("cumprod", lambda d, axis: np.cumprod(d, axis=axis))

    def shift(self, periods: int = 1) -> UgridDataset:
        """Shift each temporal variable ``periods`` steps along time, vacated steps NaN.

        Args:
            periods: Steps to shift; negative shifts towards the start. Defaults to 1.

        Returns:
            UgridDataset: A new dataset with each temporal variable shifted along time, the
            vacated steps filled with NaN; static variables are unchanged.

        Raises:
            ValueError: No variable has a time dimension, or a temporal variable has no
                loaded data.
        """
        return self._transform_time(
            "shift",
            lambda d, axis: shifted(d, axis, periods, np.nan),
            # Move a per-step mask with the snapshots (time on axis 0), vacated steps inactive —
            # a shifted-in step holds NaN data, so False is right (review H1).
            active_fn=lambda m: shifted(m, 0, periods, False),
        )

    def ffill(
        self, *, limit: int | None = None, dim: str | None = None
    ) -> UgridDataset:
        """Forward-fill gaps along time from the last valid step.

        Args:
            limit: Maximum consecutive gaps one valid step may fill, or ``None`` for no limit.
            dim: The dimension to fill along. A mesh fills along ``"time"`` only, so any other
                name is refused; ``None`` means time. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset with each temporal variable's gaps carried forward
            from the previous valid step; static variables are unchanged.

        Raises:
            ValueError: ``dim`` names anything but ``"time"``, no variable has a time
                dimension, or a temporal variable has no loaded data.
        """
        self._check_time_dim("ffill", dim)
        return self._transform_time(
            "ffill", lambda d, axis: pushed(d, axis, limit, False)
        )

    def bfill(
        self, *, limit: int | None = None, dim: str | None = None
    ) -> UgridDataset:
        """Back-fill gaps along time from the next valid step.

        Args:
            limit: Maximum consecutive gaps one valid step may fill, or ``None`` for no limit.
            dim: The dimension to fill along. A mesh fills along ``"time"`` only, so any other
                name is refused; ``None`` means time. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset with each temporal variable's gaps carried backward
            from the next valid step; static variables are unchanged.

        Raises:
            ValueError: ``dim`` names anything but ``"time"``, no variable has a time
                dimension, or a temporal variable has no loaded data.
        """
        self._check_time_dim("bfill", dim)
        return self._transform_time(
            "bfill", lambda d, axis: pushed(d, axis, limit, True)
        )

    def interpolate_na(
        self,
        *,
        method: str = "linear",
        limit: int | None = None,
        use_coordinate: bool = True,
        dim: str | None = None,
    ) -> UgridDataset:
        """Fill each interior time gap by interpolating between the steps on either side.

        Distance is measured along the time coordinate by default, so unevenly spaced steps
        weight their neighbours accordingly; pass ``use_coordinate=False`` to measure by step
        position instead. A leading or trailing gap has only one neighbour and is left alone.

        Args:
            method: One of ``"linear"`` (distance-weighted), ``"nearest"``, or the scipy spline
                kinds ``"slinear"`` / ``"quadratic"`` / ``"cubic"`` (each falling back to linear on
                a variable with too few valid steps). Defaults to ``"linear"``.
            limit: Maximum consecutive gaps a run may fill, or ``None`` for no limit.
            use_coordinate: Measure distance along the time coordinate rather than by step
                position. With time values ``[0, 1, 10]`` and a gap at the middle step, the
                coordinate reading fills ``1.0`` and the positional reading ``5.0``. Date-like
                coordinates count as distances too (converted to nanoseconds), so unevenly
                spaced dates weight their neighbours; a coordinate with no measurable spacing
                (plain labels) falls back to step positions. Defaults to ``True``.
            dim: The dimension to interpolate along. A mesh interpolates along ``"time"``
                only, so any other name is refused; ``None`` means time. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset with each temporal variable's interior gaps
            interpolated along time; static variables are unchanged.

        Raises:
            ValueError: ``method`` is not one of the accepted interpolations; no variable has a
                time dimension; or a temporal variable has no loaded data.
        """
        if method not in INTERP_METHODS:
            # Validate up front, before the time check, so a bogus method is reported as such
            # even on a mesh with no temporal variable — matching the raster interpolate_na.
            raise ValueError(
                f"interpolate_na() takes method="
                f"{' or '.join(repr(one) for one in INTERP_METHODS)}, got {method!r}."
            )

        self._check_time_dim("interpolate_na", dim)
        stamps = self.time_values if use_coordinate else None
        axis_values = None
        if stamps is not None:
            axis_values = _axis_distances(stamps)

        def _fill(data: np.ndarray, axis: int) -> np.ndarray:
            size = data.shape[axis]
            if axis_values is not None and axis_values.shape[0] == size:
                positions = axis_values
            else:
                positions = np.arange(size, dtype="float64")
            return np.asarray(interpolated(data, axis, positions, method, limit))

        return self._transform_time("interpolate_na", _fill)

    def rolling(
        self,
        window: int,
        how: str = "mean",
        *,
        center: bool = False,
        min_periods: int | None = None,
        q: float | None = None,
        dim: str | None = None,
    ) -> UgridDataset:
        """Rolling-window reduction along time for every temporal variable.

        Each time step becomes a statistic of the window of steps it owns (ending at it, or
        centred on it), cut where the window runs off the axis. Gaps are skipped; a step whose
        window holds fewer than ``min_periods`` valid cells is NaN. The time dimension keeps
        its length.

        Args:
            window: Steps per window.
            how: The reduction over each window (``"mean"``, ``"sum"``, ``"min"``, ``"max"``,
                ``"std"``, ``"var"``, ``"median"``, ``"quantile"``). Defaults to ``"mean"``.
            center: Centre each window on its step rather than ending at it. Defaults to False.
            min_periods: Valid cells a window needs before its step holds a value. ``None``
                means the full ``window``, matching the raster `rolling`, so a step whose
                window runs off the axis is NaN rather than a partial statistic. Defaults to
                ``None``.
            q: The quantile in ``[0, 1]``, required when ``how="quantile"`` and rejected
                otherwise. Defaults to ``None``.
            dim: The dimension to roll along. A mesh rolls along ``"time"`` only, so any
                other name is refused; ``None`` means time. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset with each temporal variable replaced by its rolling
            statistic along time; static variables are unchanged.

        Raises:
            ValueError: ``how`` is not a known reduction; ``q`` is given for a non-quantile
                ``how`` or missing for ``how="quantile"``; ``window`` is below 1;
                ``min_periods`` falls outside ``[1, window]``; ``dim`` names anything but
                ``"time"``; no variable has a time dimension; or a temporal variable has no
                loaded data.
        """
        if how not in _ROLLING_HOWS:
            # Restrict to the documented statistics. The counting reductions
            # (count/all/any) are excluded on purpose: a window's all/any flag carries the
            # 255 FLAG_NO_DATA sentinel, which would surface as a literal data value here.
            raise ValueError(
                f"rolling how must be one of {sorted(_ROLLING_HOWS)}, got {how!r}."
            )
        # Same guard as every other along-time member, through the shared check rather than
        # a second copy of it with its own wording.
        self._check_time_dim("rolling", dim)
        if how == "quantile" and q is None:
            raise ValueError("rolling(how='quantile') needs q in [0, 1].")
        if how != "quantile" and q is not None:
            raise ValueError(
                f"rolling(q=...) is only valid with how='quantile', not {how!r}."
            )
        if window < 1:
            raise ValueError(f"rolling: window must be >= 1, got {window!r}.")
        # `None` means the full window, as the raster rolling defaults — a step whose window
        # runs off the axis then has no statistic rather than a partial one.
        needed = window if min_periods is None else int(min_periods)
        if not 1 <= needed <= window:
            raise ValueError(
                f"rolling: min_periods must be between 1 and window ({window}), got "
                f"{min_periods!r}."
            )

        def _roll(data: np.ndarray, axis: int) -> np.ndarray:
            moved = np.moveaxis(data, axis, 0)
            size = moved.shape[0]
            out = np.full_like(moved, np.nan, dtype="float64")
            for step in range(size):
                members = window_members(step, size, window, center)
                block = moved[members]
                valid = np.sum(~np.isnan(block), axis=0)
                reduced = reduce_axis(block, 0, how, True, None, q)
                out[step] = np.where(valid >= needed, reduced, np.nan)
            return np.moveaxis(out, 0, axis)

        return self._transform_time("rolling", _roll)

    def _time_length(self, operation: str) -> int:
        """The shared time-axis length across all temporal variables.

        The positional/label selectors build one index set and apply it to every temporal
        variable, so they require a single shared length. Validate that here rather than let
        a mismatch leak a raw numpy ``IndexError`` (or silently trim against the wrong axis).

        Raises:
            ValueError: No variable has a time dimension, or temporal variables disagree on
                their time length.
        """
        temporal = self._require_temporal(operation)
        lengths = {name: self._data_variables[name].n_time_steps for name in temporal}
        distinct = set(lengths.values())
        if len(distinct) > 1:
            raise ValueError(
                f"{operation} needs every temporal variable to share one time length, but "
                f"they differ: {lengths}. Select per variable, or align them first."
            )
        return distinct.pop()

    def _select_steps(self, indices: np.typing.NDArray) -> UgridDataset:
        """Keep the time steps at ``indices`` (in order) for every temporal variable.

        Trims each temporal variable's data along its time axis and, when the variable
        carries a ``time_values`` coordinate attribute, trims that to match. Static variables
        are kept unchanged. The time dimension is retained (shorter), so the result stays
        temporal.
        """
        indices = np.asarray(indices, dtype=np.intp)
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to select.")
            new_var = var.with_data(
                np.take(np.asarray(data), indices, axis=axis),
                active=var._time_indexed_active(indices),
            )
            times = var.attributes.get("time_values")
            if times is not None:
                new_var.attributes = {
                    **var.attributes,
                    "time_values": [times[int(i)] for i in indices],
                }
            new_vars[name] = new_var
        return self._rebuild(new_vars)

    def _select_elements(
        self, element: int | slice | list[int] | np.ndarray
    ) -> UgridDataset:
        """Keep the faces at ``element`` positions, renumbering the mesh around them.

        The element axis is the mesh's own axis, so selecting along it is a *topology* change,
        not a slice: the kept faces' nodes are renumbered compactly and the edges filtered, the
        way :meth:`clip` and :meth:`subset_by_bounds` already do. This shares their machinery
        (:func:`pyramids.netcdf.ugrid.spatial.subset_mesh_by_faces`) rather than re-deriving it.

        Args:
            element: An ``int`` face index, a ``slice``, or a sequence of face indices.

        Returns:
            UgridDataset: A new dataset whose mesh holds only the selected faces.

        Raises:
            IndexError: A face index is out of range for this mesh. A negative index counts
                from the end, so ``-1`` is the last face.
        """
        count = self._mesh.n_face
        if isinstance(element, slice):
            faces = [int(i) for i in np.arange(count)[element]]
        else:
            asked = [int(i) for i in np.atleast_1d(np.asarray(element, dtype=np.intp))]
            # A negative index counts from the end, as `isel(time=-1)` already does through
            # np.take. Refusing it here left one method with two index conventions.
            faces = [i + count if -count <= i < 0 else i for i in asked]
        out_of_range = [i for i in faces if i < 0 or i >= count]
        if out_of_range:
            raise IndexError(
                f"isel: face index/indices {out_of_range} out of range for a mesh with "
                f"{count} face(s)."
            )
        mesh, data_variables = subset_mesh_by_faces(self, faces)
        return self._wrap_subset(mesh, data_variables)

    def isel(
        self,
        time: int | slice | list[int] | np.ndarray | None = None,
        *,
        element: int | slice | list[int] | np.ndarray | None = None,
    ) -> UgridDataset:
        """Select by position along the time axis, the element axis, or both.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.isel`. Over **time** an integer
        selects a single step and collapses the time dimension (the variables become static);
        a slice or sequence keeps the time dimension, trimmed to the chosen steps. Over
        **element** the selection is a topology change: the chosen faces are kept and the mesh
        is renumbered around them (as :meth:`clip` does), so the result is a smaller mesh.
        Giving both applies the element selection first, then the time selection.

        Args:
            time: An ``int`` step, a ``slice``, or a sequence of integer step positions.
                ``None`` (the default) leaves the time axis alone.
            element: An ``int`` face index, a ``slice``, or a sequence of face indices.
                ``None`` (the default) leaves the mesh alone.

        Returns:
            UgridDataset: A new dataset with the selection applied.

        Raises:
            ValueError: Neither ``time`` nor ``element`` was given, or ``time`` was asked of a
                dataset with no time dimension.
            IndexError: A face index is out of range for this mesh.

        Examples:
            - Keep one face, renumbering the mesh around it:
                ```python
                >>> import numpy as np
                >>> from pyramids.netcdf.ugrid import UgridDataset
                >>> mesh = UgridDataset.from_arrays(
                ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
                ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
                ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
                ...     data={"depth": np.array([2.0, 4.0])},
                ... )
                >>> one = mesh.isel(element=0)
                >>> one.n_face, one.n_node
                (1, 3)
                >>> one["depth"].data.tolist()
                [2.0]

                ```
        """
        if time is None and element is None:
            raise ValueError("isel needs `time`, `element`, or both.")
        if element is not None:
            selected = self._select_elements(element)
            return selected if time is None else selected.isel(time)
        n = self._time_length("isel")
        if isinstance(time, (int, np.integer)):
            return self._collapse_to_step(int(time))
        if isinstance(time, slice):
            indices = np.arange(n)[time]
        else:
            indices = np.asarray(time, dtype=np.intp)
        return self._select_steps(indices)

    def _collapse_to_step(self, index: int) -> UgridDataset:
        """Take one time step from every temporal variable, dropping the time dimension.

        The single-step half of :meth:`isel`: each temporal variable becomes static, as
        :meth:`reduce` produces, while a static variable is carried through untouched.

        Args:
            index: The time position to take.

        Returns:
            UgridDataset: A new dataset whose temporal variables hold that step.

        Raises:
            ValueError: A temporal variable has no loaded data.
        """
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to select.")
            step = np.take(np.asarray(data), index, axis=axis)
            new_vars[name] = _static_from(
                var,
                np.asarray(step),
                axis,
                active=var._time_sliced_active(index, None),
            )
        return self._rebuild(new_vars)

    def head(self, n: int = 5) -> UgridDataset:
        """Keep the first ``n`` time steps (fewer if the axis is shorter).

        Args:
            n: Number of leading steps to keep. Defaults to 5.

        Returns:
            UgridDataset: A new dataset trimmed to the first ``n`` steps.

        Raises:
            ValueError: No variable has a time dimension.
        """
        length = self._time_length("head")
        return self._select_steps(np.arange(min(n, length)))

    def tail(self, n: int = 5) -> UgridDataset:
        """Keep the last ``n`` time steps (fewer if the axis is shorter).

        Args:
            n: Number of trailing steps to keep. Defaults to 5.

        Returns:
            UgridDataset: A new dataset trimmed to the last ``n`` steps.

        Raises:
            ValueError: No variable has a time dimension.
        """
        length = self._time_length("tail")
        return self._select_steps(np.arange(max(length - n, 0), length))

    def thin(self, step: int) -> UgridDataset:
        """Keep every ``step``-th time step.

        Args:
            step: Stride; must be >= 1.

        Returns:
            UgridDataset: A new dataset keeping every ``step``-th step.

        Raises:
            ValueError: ``step`` is less than 1, or no variable is temporal.
        """
        if step < 1:
            raise ValueError(f"thin step must be >= 1, got {step}.")
        length = self._time_length("thin")
        return self._select_steps(np.arange(0, length, step))

    def drop_isel(self, indices: int | list[int] | np.ndarray) -> UgridDataset:
        """Drop the time steps at ``indices`` by position, keeping the rest.

        Args:
            indices: A step position, or a sequence of them, to drop (negative indices
                count from the end).

        Returns:
            UgridDataset: A new dataset without the dropped steps.

        Raises:
            ValueError: No variable has a time dimension.
        """
        length = self._time_length("drop_isel")
        drop = {int(i) % length for i in np.atleast_1d(np.asarray(indices))}
        keep = np.array([i for i in range(length) if i not in drop], dtype=np.intp)
        return self._select_steps(keep)

    def squeeze(self) -> UgridDataset:
        """Drop the time dimension when it has length 1; otherwise return unchanged.

        Mirrors :meth:`pyramids.netcdf.NetCDF.squeeze` for the time axis: a single-step
        temporal variable becomes static, a multi-step one is left alone.

        Returns:
            UgridDataset: A new dataset with single-step temporal variables collapsed to
            static, or ``self`` when there is nothing to squeeze.

        Raises:
            ValueError: A single-step temporal variable has no loaded data.
        """
        # Decide per variable, not from the first temporal one: with mixed time lengths a
        # later single-step variable must still be squeezed even if the first has many steps.
        if not any(
            var.has_time and var.n_time_steps == 1
            for var in self._data_variables.values()
        ):
            return self
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time or var.n_time_steps != 1:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to squeeze.")
            # Squeezing the size-1 time axis is taking its single step, so collapse a per-step
            # mask to that step (`mask[0]`) rather than letting the implicit carry drop it as a
            # now-rank-mismatched 2-D mask (review L3). A static mask passes straight through.
            new_vars[name] = _static_from(
                var,
                np.squeeze(np.asarray(data), axis=axis),
                axis,
                active=var._time_sliced_active(0, None),
            )
        return self._rebuild(new_vars)

    def diff(self, n: int = 1) -> UgridDataset:
        """Discrete difference along time (``out[i] = x[i] - x[i-1]``), ``n`` times.

        The time dimension shortens by ``n``; a ``time_values`` coordinate, if present, drops
        its first ``n`` entries to stay aligned with the result.

        Args:
            n: The number of successive differences. Defaults to 1.

        Returns:
            UgridDataset: A new dataset with each temporal variable differenced along time
            (its time axis shorter by ``n``); static variables are unchanged.

        Raises:
            ValueError: No variable has a time dimension, or a temporal variable has no
                loaded data.
        """
        self._require_temporal("diff")
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to diff.")
            diffed = np.diff(gaps_as_nan(np.asarray(data), var.nodata), n=n, axis=axis)
            new_var = var.with_data(diffed)
            times = var.attributes.get("time_values")
            if times is not None:
                new_var.attributes = {**var.attributes, "time_values": list(times[n:])}
            new_vars[name] = new_var
        return self._rebuild(new_vars)

    def _extremum_along_time(
        self, operation: str, *, largest: bool, as_coordinate: bool
    ) -> UgridDataset:
        """Locate each element's extreme time step, as a position or as a coordinate.

        The time axis is dropped — the answer is one value per element — so the result is a
        static variable, as :meth:`reduce` produces. Gaps are left out: the search runs over the
        no-data-masked values, and an element with no valid step at all has no answer, which the
        position form declares as ``-1`` and the coordinate form as NaN (mirroring the raster
        ``argmin`` / ``idxmin``).

        Args:
            operation: The calling method's name, for error messages.
            largest: Search for the maximum rather than the minimum.
            as_coordinate: Answer with the time coordinate value instead of the step position.

        Returns:
            UgridDataset: A new dataset whose temporal variables became static; static
            variables are carried over unchanged.

        Raises:
            ValueError: No variable has a time dimension, a temporal variable has no loaded
                data, or the coordinate form was asked of date-like time coordinates, which
                cannot be expressed as the float64 the answer carries.
        """
        self._require_temporal(operation)
        coords = self._time_coords(operation) if as_coordinate else None
        if coords is not None and not all(
            isinstance(stamp, (int, float, np.integer, np.floating))
            and not isinstance(stamp, bool)
            for stamp in coords
        ):
            # The coordinate form answers float64, so a date-like coordinate cannot be
            # expressed. Say so, rather than letting the float() cast surface a raw
            # TypeError about Timestamp from inside numpy.
            raise ValueError(
                f"{operation}() answers the time coordinate as a float, so it needs numeric "
                "time coordinates; this dataset's are date-like. Use argmin()/argmax() for "
                "the step position instead."
            )
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                new_vars[name] = var
                continue
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data.")
            values, nodata = _extremum_values(
                gaps_as_nan(np.asarray(data), var.nodata),
                axis,
                largest=largest,
                coords=coords if as_coordinate else None,
            )
            new_vars[name] = _static_from(var, values, axis, nodata=nodata)
        return self._rebuild(new_vars)

    def argmin(self) -> UgridDataset:
        """The time-step position of each element's minimum.

        Returns:
            UgridDataset: Static ``int64`` variables holding the step index of the minimum,
            declaring ``-1`` for an element with no valid step.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self._extremum_along_time("argmin", largest=False, as_coordinate=False)

    def argmax(self) -> UgridDataset:
        """The time-step position of each element's maximum.

        Returns:
            UgridDataset: Static ``int64`` variables holding the step index of the maximum,
            declaring ``-1`` for an element with no valid step.

        Raises:
            ValueError: No variable has a time dimension.
        """
        return self._extremum_along_time("argmax", largest=True, as_coordinate=False)

    def idxmin(self) -> UgridDataset:
        """The time **coordinate** of each element's minimum.

        Where :meth:`argmin` answers a step position, this answers the value of the time
        coordinate at that step, so it needs numeric time coordinates.

        Returns:
            UgridDataset: Static ``float64`` variables holding the time coordinate of the
            minimum, NaN where an element has no valid step.

        Raises:
            ValueError: No variable has a time dimension, or the time coordinates are
                date-like rather than numeric (use `argmin` / `argmax` for the step
                position instead).
        """
        return self._extremum_along_time("idxmin", largest=False, as_coordinate=True)

    def idxmax(self) -> UgridDataset:
        """The time **coordinate** of each element's maximum.

        Returns:
            UgridDataset: Static ``float64`` variables holding the time coordinate of the
            maximum, NaN where an element has no valid step.

        Raises:
            ValueError: No variable has a time dimension, or the time coordinates are
                date-like rather than numeric (use `argmin` / `argmax` for the step
                position instead).
        """
        return self._extremum_along_time("idxmax", largest=True, as_coordinate=True)

    def _time_coords(self, operation: str) -> list:
        """The time coordinate values, or a clear error when the dataset has no time."""
        coords = self.time_values
        if coords is None:
            raise ValueError(
                f"{operation} needs a time dimension; this dataset has none."
            )
        return list(coords)

    def sel(
        self,
        time: Any = None,
        *,
        method: str | None = None,
        tolerance: float | None = None,
    ) -> UgridDataset:
        """Select time steps by coordinate value, exactly or by nearest.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.sel` over time. A scalar selects
        one step and collapses the time dimension; a sequence keeps it. Values are matched
        against the dataset's :attr:`time_values` (which fall back to step positions when the
        file carries no explicit time coordinate). ``method="nearest"`` snaps each value to the
        closest coordinate instead of requiring an exact hit, and ``tolerance`` caps how far it
        may snap — the same contract the raster ``sel`` uses, including raising ``KeyError`` (not
        ``ValueError``) when a snap would exceed ``tolerance``.

        Args:
            time: A coordinate value, or a sequence of them.
            method: ``None`` (the default) matches exactly; ``"nearest"`` snaps to the closest
                coordinate, which needs numeric coordinates.
            tolerance: The furthest a ``method="nearest"`` snap may travel. Only valid with
                ``"nearest"``, and must not be negative.

        Returns:
            UgridDataset: A new dataset with the selected steps.

        Raises:
            ValueError: No time dimension; an unknown ``method``; ``tolerance`` without
                ``method="nearest"`` or negative; a value is not among the coordinates (exact
                match); or ``"nearest"`` was asked of non-numeric coordinates.
            KeyError: A ``"nearest"`` snap would exceed ``tolerance``.
        """
        if time is None:
            # `time` is optional in the signature for parity with the raster `sel`, but a
            # missing value has no meaning here. Say so, as `isel` does, instead of
            # reporting that the coordinate [None] was not found.
            raise ValueError(
                "sel needs a time value to select; pass one, or use isel() to select by "
                "position."
            )
        coords = self._time_coords("sel")
        if method not in (None, "nearest"):
            raise ValueError(f"sel: method must be None or 'nearest', got {method!r}.")
        if tolerance is not None:
            if method != "nearest":
                raise ValueError("sel: tolerance is only valid with method='nearest'.")
            if float(tolerance) < 0:
                raise ValueError(
                    f"sel: tolerance must not be negative, got {tolerance!r}."
                )
        wanted = list(time) if isinstance(time, (list, tuple, np.ndarray)) else [time]
        positions = _resolve_time_positions(coords, wanted, method, tolerance)
        if isinstance(time, (list, tuple, np.ndarray)):
            result = self._select_steps(np.asarray(positions, dtype=np.intp))
        else:
            result = self.isel(positions[0])
        return result

    def drop_sel(self, values: Any) -> UgridDataset:
        """Drop the time steps whose coordinate is in ``values``, keeping the rest.

        Args:
            values: A coordinate value, or a sequence of them, to drop.

        Returns:
            UgridDataset: A new dataset without the steps whose coordinate is in ``values``.

        Raises:
            ValueError: No variable has a time dimension.
        """
        coords = self._time_coords("drop_sel")
        drop = (
            set(values)
            if isinstance(values, (list, tuple, set))
            else {*np.atleast_1d(np.asarray(values)).tolist()}
        )
        keep = [i for i, coord in enumerate(coords) if coord not in drop]
        return self._select_steps(np.asarray(keep, dtype=np.intp))

    def sortby(self) -> UgridDataset:
        """Sort the time steps by their coordinate value (stable).

        Returns:
            UgridDataset: A new dataset with steps ordered by ascending time coordinate.

        Raises:
            ValueError: No variable has a time dimension.
        """
        coords = self._time_coords("sortby")
        order = np.argsort(np.asarray(coords), kind="stable")
        return self._select_steps(order)

    def drop_duplicates(self) -> UgridDataset:
        """Keep the first step of each distinct time coordinate value.

        Returns:
            UgridDataset: A new dataset with duplicate-coordinate steps removed, first kept.

        Raises:
            ValueError: No variable has a time dimension.
        """
        coords = self._time_coords("drop_duplicates")
        seen: set = set()
        keep: list[int] = []
        for i, coord in enumerate(coords):
            if coord not in seen:
                seen.add(coord)
                keep.append(i)
        return self._select_steps(np.asarray(keep, dtype=np.intp))

    def dropna(
        self, how: str = "any", thresh: int | None = None, *, dim: str | None = None
    ) -> UgridDataset:
        """Drop time steps that do not hold enough valid cells.

        A step is kept only when **every** temporal variable meets the threshold at that
        step, so the variables stay time-aligned. ``how="any"`` requires every cell of a
        variable to be valid (a single gap drops the step); ``how="all"`` requires at least
        one; ``thresh`` overrides ``how`` with an explicit valid-cell count per variable.

        Args:
            how: ``"any"`` or ``"all"``. Defaults to ``"any"``.
            thresh: Explicit minimum valid-cell count per variable, overriding ``how``.
            dim: The dimension to drop along. A mesh drops along ``"time"`` only, so any other
                name is refused; ``None`` means time. Defaults to ``None``.

        Returns:
            UgridDataset: A new dataset with the surviving steps.

        Raises:
            ValueError: ``how`` is neither ``"any"`` nor ``"all"``; ``dim`` names anything but
                ``"time"``; no variable has a time dimension; a temporal variable has no
                loaded data; or the threshold would drop **every** step, which would leave a
                temporal dataset with no steps for anything downstream to read.
        """
        if how not in ("any", "all"):
            raise ValueError(f"dropna how must be 'any' or 'all', got {how!r}.")
        self._check_time_dim("dropna", dim)
        temporal = self._require_temporal("dropna")
        length = self._time_length("dropna")
        keep = np.ones(length, dtype=bool)
        for name in temporal:
            var = self._data_variables[name]
            axis = cast("int", var.time_index)
            data = var.data
            if data is None:
                raise ValueError(f"Variable {name!r} has no loaded data to dropna.")
            moved = np.moveaxis(gaps_as_nan(np.asarray(data), var.nodata), axis, 0)
            flat = moved.reshape(moved.shape[0], -1)
            per_step_valid = np.sum(~np.isnan(flat), axis=1)
            needed = flat.shape[1] if how == "any" else 1
            if thresh is not None:
                needed = thresh
            keep &= per_step_valid >= needed
        if not keep.any():
            # Returning a zero-length time axis would leave a "temporal" dataset with no
            # steps, which nothing downstream can read; the raster dropna refuses too.
            raise ValueError(
                f"dropna(how={how!r}, thresh={thresh!r}) would drop every one of the "
                f"{length} time step(s); nothing would be left."
            )
        return self._select_steps(np.flatnonzero(keep))

    def _assert_same_topology(self, other: UgridDataset, operation: str) -> None:
        """Refuse ``operation`` unless ``other`` sits on the same mesh as ``self``.

        Same mesh means the same node/face/edge counts and the same node coordinates and
        face-node connectivity — combining data across different meshes is meaningless.
        """
        mine, theirs = self._mesh, other._mesh
        same = (
            mine.n_node == theirs.n_node
            and mine.n_face == theirs.n_face
            and mine.n_edge == theirs.n_edge
            and np.array_equal(mine.node_x, theirs.node_x)
            and np.array_equal(mine.node_y, theirs.node_y)
            and np.array_equal(
                np.asarray(mine.face_node_connectivity.data),
                np.asarray(theirs.face_node_connectivity.data),
            )
        )
        if not same:
            raise AlignmentError(
                f"{operation} requires datasets on the same mesh topology "
                f"(matching node/face/edge counts, coordinates and connectivity)."
            )

    @_joins_meshes
    def concat(self, others: UgridDataset | list[UgridDataset]) -> UgridDataset:
        """Concatenate same-topology datasets along time.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.concat`. Every dataset must sit
        on the same mesh. Each temporal variable's data is concatenated along its time axis
        (and its ``time_values`` coordinate, when present); a static variable is taken from
        ``self`` unchanged. The variable set must match across datasets.

        A **per-step** active mask (:attr:`MeshVariable.active` of shape ``(n_time, n_element)``)
        is **dropped** on concatenation — the parts' masks are not joined along time, and the
        concatenated time length no longer matches ``self``'s mask, so the implicit carry
        discards it (issue #1331). A **static** ``(n_element,)`` mask survives, since it applies
        to every step. Re-attach a per-step mask after concatenating if you need one.

        Args:
            others: One dataset, or a list of them, to append after ``self`` in order.

        Returns:
            UgridDataset: A new dataset spanning the concatenated time axis.

        Raises:
            AlignmentError: A dataset sits on a different mesh. Note this is **not** a
                `ValueError` subclass, so a caller catching `ValueError` around `concat` has
                to catch `AlignmentError` instead.
            ValueError: A dataset carries a different variable set, or a variable has no
                loaded data.
        """
        parts = [
            self,
            *([others] if isinstance(others, UgridDataset) else list(others)),
        ]
        for other in parts[1:]:
            self._assert_same_topology(other, "concat")
            if set(other._data_variables) != set(self._data_variables):
                raise ValueError(
                    "concat requires the same variables in every dataset; got "
                    f"{sorted(self._data_variables)} vs {sorted(other._data_variables)}."
                )
        new_vars: dict[str, MeshVariable] = {}
        for name, var in self._data_variables.items():
            if not var.has_time:
                # Static in self: refuse if any other part has it temporal, rather than
                # silently drop that part's time series (the mirror of the temporal-in-self
                # / static-in-other case _concat_temporal_variable rejects).
                if any(part._data_variables[name].has_time for part in parts[1:]):
                    raise ValueError(
                        f"concat: variable {name!r} is static in one dataset but temporal "
                        "in another; they cannot be joined along time."
                    )
                new_vars[name] = var  # a static variable is taken from self, as-is
            else:
                new_vars[name] = _concat_temporal_variable(name, var, parts)
        return self._rebuild(new_vars)

    @_joins_meshes
    def merge(
        self,
        others: UgridDataset | list[UgridDataset],
        *,
        compat: str = "no_conflicts",
    ) -> UgridDataset:
        """Merge the variables of same-topology datasets into one.

        The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.merge`. Every dataset must sit
        on the same mesh; the union of their variables is returned. A variable name present
        in more than one dataset is a conflict and is refused.

        Args:
            others: One dataset, or a list of them, whose variables join ``self``'s. Under
                the class spelling ``UgridDataset.merge([a, b, c])`` this one argument is the
                whole join: its first element becomes ``self`` and the rest are merged onto it.
            compat: How to handle a variable name present in more than one dataset --
                ``"no_conflicts"`` refuses the collision, ``"override"`` keeps the value from
                the **first dataset in join order that carries the name**: ``self``, then each
                of ``others`` in turn. That is ``self``'s only when ``self`` has the variable;
                when it does not, the first of ``others`` carrying it wins. (Under the class
                spelling ``UgridDataset.merge([a, b, c])``, ``self`` is ``a``.) Defaults to
                ``"no_conflicts"``.

        Returns:
            UgridDataset: A new dataset carrying every variable.

        Raises:
            AlignmentError: A dataset sits on a different mesh.
            ValueError: An unknown ``compat``, or a variable name collides under
                ``compat="no_conflicts"``.
        """
        if compat not in ("no_conflicts", "override"):
            raise ValueError(
                f"merge: compat must be 'no_conflicts' or 'override', got {compat!r}."
            )
        parts = [others] if isinstance(others, UgridDataset) else list(others)
        new_vars: dict[str, MeshVariable] = dict(self._data_variables)
        for other in parts:
            self._assert_same_topology(other, "merge")
            for name, var in other._data_variables.items():
                if name in new_vars:
                    if compat == "no_conflicts":
                        raise ValueError(
                            f"merge: variable {name!r} is present in more than one dataset; "
                            "rename it first, or pass compat='override'."
                        )
                    # compat="override": the first dataset that carries the name wins, which
                    # is what already sits in new_vars, so the duplicate is simply skipped.
                    continue
                new_vars[name] = var
        return self._rebuild(new_vars)

    def plot(
        self,
        variable_name: str,
        ax: Any = None,
        cmap: str = "viridis",
        title: str | None = None,
        basemap: bool | str | None = None,
        colorbar: bool | ColorBar | None = None,
        points: np.ndarray | PointOverlay | None = None,
        kind: str = "auto",
        color: ColorScaling | None = None,
        contour: Contour | None = None,
        data_style: DataStyle | None = None,
        **kwargs: Any,
    ) -> Any:
        """Plot a mesh data variable.

        N-6 — this facade now goes through the same module-level
        helper as the raster path. The mesh-specific dispatch lives in
        :func:`pyramids.dataset._plot_helpers.mesh_render`; both
        ``Dataset.plot``/``NetCDF.plot`` and ``UgridDataset.plot`` share
        the "resolve data, hand to a single helper" contract so the
        two formats no longer maintain independent plotting code paths.

        Args:
            variable_name: Name of the data variable to plot.
            ax: matplotlib Axes. Created if None.
            cmap: Colormap name.
            title: Plot title. Defaults to variable name.
            basemap: If True, add an OpenStreetMap basemap. If a string,
                use it as the tile provider name (e.g. "CartoDB.Positron").
                Default is None (no basemap). Requires the [viz] extra.
            colorbar (bool or ColorBar, optional): Colour-bar spec, part of the
                shared plot signature. A ``pyramids.plot.ColorBar(label=…, …)``
                configures the bar; ``False`` hides it and ``None`` (default) uses
                cleopatra's default (a bar is drawn). Only forwarded when set.
            points (np.ndarray or PointOverlay, optional): Accepted for signature
                symmetry with the raster plot family, but a **no-op here** — a mesh
                has no point-overlay layer (the mesh geometry is the data). Ignored.
            kind (str, optional): Accepted for signature symmetry with the raster
                plot family, but a **no-op here** — ``kind`` selects a raster
                renderer (``imshow``/``pcolormesh``); the mesh always renders via
                ``tripcolor``/``tricontour``. Ignored.
            color (ColorScaling, optional): Colour-scale spec
                ``pyramids.plot.ColorScaling`` (linear / power / sym-log / boundary /
                midpoint norm), e.g. ``ColorScaling.power(gamma=0.7)``. Default ``None``.
            contour (Contour, optional): Contour-line spec
                ``pyramids.plot.Contour(levels=…, label_kw=…)``. Default ``None``.
            data_style (DataStyle, optional): Data-style / relief spec
                ``pyramids.plot.DataStyle(style=…, hillshade=…)``. (A mesh has no
                cell-value overlay, so there is no ``cells`` param here.) Default ``None``.
            **kwargs: Additional arguments passed to mesh_render
                (forwarded to plot_mesh_data). Notably ``colorbar``
                (``bool``, default ``True``): pass ``colorbar=False`` to
                suppress the per-mesh colorbar when you want to attach a
                custom or shared one to ``glyph.ax``. Also ``style`` (name of
                a cleopatra ``DATA_STYLES`` preset, e.g. ``"flow_accumulation"``)
                and ``hillshade`` (``True`` or a params dict) to colour / relief-
                shade the mesh; both require cleopatra >= 0.24 (``hillshade``
                needs node-centered data). Distinct from
                :meth:`pyramids.dataset.Dataset.hillshade`, which *returns* a
                shaded-relief array.

        Returns:
            cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the plot
                rendered. Use the returned object to access the matplotlib
                handles and the mappable:

                - ``glyph.fig`` / ``glyph.ax`` — Figure and Axes.
                - ``glyph.im`` — the mesh mappable (the
                  ``tripcolor``/``tricontour(f)`` artist) set by ``plot()``;
                  use it for a custom colorbar or ``glyph.im.set_clim(...)``.
                  It is ``None`` after :meth:`plot_outline` (an outline
                  carries no scalar mapping).
                - ``glyph.apply_style(style)`` (cleopatra >= 0.25) — re-apply a
                  ``DATA_STYLES`` preset by name in place, without re-plotting.

        Raises:
            ValueError: If the selected variable has no loaded data, or if
                `basemap` is requested while the dataset has no CRS (`epsg`).
        """
        var = self.get_data(variable_name)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {variable_name!r} has no loaded data to plot.")
        if var.has_time:
            # First step along the variable's own time axis (see `_element_values`).
            data = np.take(np.asarray(data), 0, axis=cast("int", var.time_index))
        if title is None:
            title = variable_name
        if basemap and self.epsg is None:
            raise ValueError("UgridDataset must have a CRS (epsg) to use basemap.")
        # ``points`` / ``kind`` are part of the shared raster-family plot signature
        # but have no meaning for a mesh (no point overlay; the renderer is fixed to
        # tripcolor/tricontour), so they are accepted and ignored. ``colorbar`` and the
        # typed render groups (``color`` / ``contour`` / ``data_style``) map onto the mesh
        # backend and are forwarded only when set (so cleopatra's backend defaults are
        # preserved otherwise).
        if colorbar is not None:
            kwargs["colorbar"] = colorbar
        kwargs.update(
            _nonnull_group_kwargs(color=color, contour=contour, data_style=data_style)
        )
        result = _mesh_render(
            mesh=self._mesh,
            data=data,
            location=var.location,
            ax=ax,
            cmap=cmap,
            title=title,
            basemap=basemap,
            basemap_epsg=self.epsg,
            **kwargs,
        )
        return result

    def plot_outline(self, ax: Any = None, **kwargs: Any) -> Any:
        """Plot mesh wireframe.

        Args:
            ax: matplotlib Axes. Created if None.
            **kwargs: Additional arguments passed to plot_mesh_outline.

        Returns:
            cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the wireframe
                rendered. ``glyph.fig`` / ``glyph.ax`` are the matplotlib
                handles; ``glyph.im`` is ``None`` (an outline carries no
                scalar mapping, so no mappable is produced).
        """
        from pyramids.netcdf.ugrid.plot import plot_mesh_outline

        result = plot_mesh_outline(self._mesh, ax=ax, **kwargs)
        return result

    def __str__(self) -> str:
        """Human-readable summary of the dataset."""
        lines = [
            f"UgridDataset: {self._file_name or '(in-memory)'}",
            f"  Mesh: {self.mesh_name}",
            f"  Nodes: {self.n_node}, Faces: {self.n_face}, Edges: {self.n_edge}",
            f"  Bounds: {self.bounds}",
            # A mesh may carry a projected WKT with no EPSG code; reporting
            # 'unknown' for a CRS it plainly has was misleading.
            f"  CRS: {self.epsg or (self.crs.name if self.crs is not None else 'unknown')}",
            f"  Data variables ({len(self._data_variables)}):",
        ]
        for name, var in self._data_variables.items():
            lines.append(f"    {name}: location={var.location}, shape={var.shape}")
        result = "\n".join(lines)
        return result

    def __repr__(self) -> str:
        """Repr string for the dataset."""
        result = (
            f"UgridDataset(mesh='{self.mesh_name}', "
            f"n_node={self.n_node}, n_face={self.n_face}, n_edge={self.n_edge}, "
            f"variables={self.data_variable_names})"
        )
        return result

mesh property

The mesh topology.

mesh_name property

Name of the mesh topology variable.

data_variable_names property

Names of all data variables.

crs property

CRS as a pyproj.CRS object, or None. Cached after first access.

epsg property

EPSG code of the CRS, or None.

crs_wkt property

The mesh CRS as WKT, or None when there is none that parses.

Gated on a successful parse rather than returning _crs_wkt verbatim. _crs_wkt is copied straight out of the file and :attr:crs tolerates an unparseable one on purpose, swallowing the error and reporting no CRS. Handing the raw string to a caller would move that parse out from under the handler, so a malformed WKT would start raising downstream instead of being reported as "no CRS" here.

bounds property

Mesh bounding box as (xmin, ymin, xmax, ymax).

global_attributes property

File-level NetCDF attributes.

n_node property

Number of mesh nodes.

n_face property

Number of mesh faces.

n_edge property

Number of mesh edges.

data_vars property

A copy of the {name: variable} mapping.

A fresh dict, so mutating it never changes the dataset — variable edits go through :meth:with_variable / :meth:drop_variables / :meth:rename_variable, which each return a new dataset.

metadata property

Full metadata summary for this dataset.

time_values property

Parsed time coordinate values from the first temporal variable.

Returns None if no variables have a time dimension.

volume_topology property

The ragged (per-column) vertical layering, or None for a uniform / 2-D mesh.

Present only for a genuinely 3-D mesh whose columns carry different layer counts (issue #1330): it maps each face to its slice of the flat volume axis that a location="volume" variable is indexed through. None means the dense (layer, element) path applies — the uniform-layer behaviour is unchanged.

n_volume property

The total number of volumes, or None when the mesh has no volume topology.

The flat volume-axis length of a ragged 3-D mesh (the sum of every column's layer count). None for a 2-D or uniform-layer mesh, where :attr:n_layers applies instead.

n_layers property

The number of layers, or None when no variable is layered.

A 3-D mesh output puts some variables on the n layer centres and others on the n + 1 layer interfaces, so two counts differing by one is normal rather than a contradiction; this reports n, the layer count proper. Any other disagreement is a real inconsistency and raises. :meth:isel_layer is unaffected either way — it indexes each variable on its own layer axis.

Reporting n means the n + 1 is not visible here, so a caller iterating range(n_layers) over an interface variable would skip its last interface. Read :attr:n_layer_interfaces for that count rather than inferring it: it answers None unless a variable really sits on the interfaces.

For a ragged 3-D mesh (:attr:volume_topology set) there is no single layer count: this reports the maximum over the columns — the number of layer slots — so range(n_layers) covers the deepest column, and :meth:isel_layer makes the absent shallow-column layers explicit (NaN) rather than padding them silently.

Returns:

Type	Description
int | None	int | None: The layer-centre count (the max, for a ragged mesh), or None when
int | None	no variable is layered.

Raises:

Type	Description
ValueError	The layered variables carry counts that are not a layer-centre / layer-interface pair.

n_layer_interfaces property

The number of layer interfaces, or None when nothing sits on them.

The companion to :attr:n_layers. A 3-D output's vertical velocities and fluxes live on the n + 1 interfaces between the n layer centres, and n_layers deliberately reports the centre count -- so this is where the other half of the pair is readable.

It is not simply n_layers + 1, which is always computable: that would assert that something in the dataset is interface-located. Only a variable actually carrying that count makes it true, so a centres-only dataset answers None.

Returns:

Type	Description
int | None	int | None: The layer-interface count, or None when no variable is layered or
int | None	every layered variable sits on the centres.

Raises:

Type	Description
ValueError	The layered variables carry counts that are not a layer-centre / layer-interface pair -- the same guard :attr:n_layers applies, so the two properties agree about what is readable.

layer_values property

The layer coordinate values, when a layered variable carries them.

Read from a layered variable's layer_values attribute. The UGRID reader does not populate that attribute -- as with :attr:time_values, the coordinate is supplied by whoever builds or transforms the dataset -- so this is normally None for a mesh read straight from a file, and :meth:sel_layer then selects by layer position.

connected_components()

Label the mesh's connected face components.

Returns:

Type	Description
ndarray	numpy.ndarray: One component label per face.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def connected_components(self) -> np.ndarray:
    """Label the mesh's connected face components.

    Returns:
        numpy.ndarray: One component label per face.
    """
    return _graph.connected_components(self._mesh)  # type: ignore[attr-defined]

exterior_edges()

The indices of the edges on the mesh boundary.

Returns:

Type	Description
ndarray	numpy.ndarray: Boundary edge indices.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def exterior_edges(self) -> np.ndarray:
    """The indices of the edges on the mesh boundary.

    Returns:
        numpy.ndarray: Boundary edge indices.
    """
    return _graph.exterior_edges(self._mesh)  # type: ignore[attr-defined]

exterior_faces()

The indices of the faces touching the mesh boundary.

Returns:

Type	Description
ndarray	numpy.ndarray: Boundary face indices.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def exterior_faces(self) -> np.ndarray:
    """The indices of the faces touching the mesh boundary.

    Returns:
        numpy.ndarray: Boundary face indices.
    """
    return _graph.exterior_faces(self._mesh)  # type: ignore[attr-defined]

bounding_polygon()

The mesh footprint as one dissolved polygon, holes preserved.

Returns:

Type	Description
Any	shapely.geometry.base.BaseGeometry: The (multi)polygon of the mesh outline.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def bounding_polygon(self) -> Any:
    """The mesh footprint as one dissolved polygon, holes preserved.

    Returns:
        shapely.geometry.base.BaseGeometry: The (multi)polygon of the mesh outline.
    """
    return _graph.bounding_polygon(self._mesh)  # type: ignore[attr-defined]

circumcenters()

The circumcentre of every face (triangular meshes only).

Returns:

Type	Description
ndarray	numpy.ndarray: (n_face, 2) circumcentre coordinates.

Raises:

Type	Description
ValueError	The mesh is not triangular.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def circumcenters(self) -> np.ndarray:
    """The circumcentre of every face (triangular meshes only).

    Returns:
        numpy.ndarray: ``(n_face, 2)`` circumcentre coordinates.

    Raises:
        ValueError: The mesh is not triangular.
    """
    return _graph.circumcenters(self._mesh)  # type: ignore[attr-defined]

voronoi()

The Voronoi tesselation of the mesh nodes (the mesh dual).

Returns:

Type	Description
Any	scipy.spatial.Voronoi: The tesselation.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def voronoi(self) -> Any:
    """The Voronoi tesselation of the mesh nodes (the mesh dual).

    Returns:
        scipy.spatial.Voronoi: The tesselation.
    """
    return _graph.voronoi(self._mesh)  # type: ignore[attr-defined]

sparse_adjacency(kind='face')

The mesh adjacency as a sparse matrix.

Parameters:

Name	Type	Description	Default
kind	str	"face" (faces sharing an edge) or "node" (nodes sharing an edge).	'face'

Returns:

Type	Description
Any	scipy.sparse.csr_matrix: The symmetric adjacency.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def sparse_adjacency(self, kind: str = "face") -> Any:
    """The mesh adjacency as a sparse matrix.

    Args:
        kind: ``"face"`` (faces sharing an edge) or ``"node"`` (nodes sharing an edge).

    Returns:
        scipy.sparse.csr_matrix: The symmetric adjacency.
    """
    return _graph.sparse_adjacency(self._mesh, kind)  # type: ignore[attr-defined]

binary_dilation(variable_name=None, *, iterations=1, border_value=False, fixed=None)

Grow a boolean face mask over its neighbours.

Parameters:

Name	Type	Description	Default
variable_name	str | None	The boolean face variable to dilate, or None for the only one.	None
iterations	int	How many times to grow. Defaults to 1.	1
border_value	bool	What lies outside the mesh. Defaults to False.	False
fixed	ndarray | None	A boolean per-face mask of the cells the result must preserve, whatever the dilation would do to them — scipy's mask argument. None leaves every face free to change. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset with the dilated mask.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def binary_dilation(
    self,
    variable_name: str | None = None,
    *,
    iterations: int = 1,
    border_value: bool = False,
    fixed: np.ndarray | None = None,
) -> Any:
    """Grow a boolean face mask over its neighbours.

    Args:
        variable_name: The boolean face variable to dilate, or ``None`` for the only one.
        iterations: How many times to grow. Defaults to 1.
        border_value: What lies outside the mesh. Defaults to False.
        fixed: A boolean per-face mask of the cells the result must preserve, whatever the
            dilation would do to them — scipy's ``mask`` argument. ``None`` leaves every
            face free to change. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with the dilated mask.
    """
    var = self._facade_variable("binary_dilation", variable_name)
    grown = _graph.binary_dilation(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data).astype(bool),
        iterations=iterations,
        border_value=border_value,
        fixed=fixed,
    )
    return self._rewrap(var, grown)

binary_erosion(variable_name=None, *, iterations=1, border_value=False, fixed=None)

Shrink a boolean face mask away from its boundary.

Parameters:

Name	Type	Description	Default
variable_name	str | None	The boolean face variable to erode, or None for the only one.	None
iterations	int	How many times to shrink. Defaults to 1.	1
border_value	bool	What lies outside the mesh. Defaults to False, which erodes the boundary; pass True to erode only the interior.	False
fixed	ndarray | None	A boolean per-face mask of the cells the result must preserve, whatever the erosion would do to them — scipy's mask argument. None leaves every face free to change. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset with the eroded mask.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def binary_erosion(
    self,
    variable_name: str | None = None,
    *,
    iterations: int = 1,
    border_value: bool = False,
    fixed: np.ndarray | None = None,
) -> Any:
    """Shrink a boolean face mask away from its boundary.

    Args:
        variable_name: The boolean face variable to erode, or ``None`` for the only one.
        iterations: How many times to shrink. Defaults to 1.
        border_value: What lies outside the mesh. Defaults to False, which erodes the
            boundary; pass True to erode only the interior.
        fixed: A boolean per-face mask of the cells the result must preserve, whatever the
            erosion would do to them — scipy's ``mask`` argument. ``None`` leaves every
            face free to change. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with the eroded mask.
    """
    var = self._facade_variable("binary_erosion", variable_name)
    shrunk = _graph.binary_erosion(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data).astype(bool),
        iterations=iterations,
        border_value=border_value,
        fixed=fixed,
    )
    return self._rewrap(var, shrunk)

to_location(location, variable_name=None)

Remap a variable onto another facet of the mesh.

Averages the values of the contributing elements, so a face field asked for on the nodes becomes each node's mean of the faces around it.

Parameters:

Name	Type	Description	Default
location	str	The facet to remap onto — "node", "edge" or "face".	required
variable_name	str | None	The variable to remap, or None for the only one.	None

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset whose variable sits on location.

Raises:

Type	Description
ValueError	An unknown location, or a remap the kernel does not offer.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def to_location(self, location: str, variable_name: str | None = None) -> Any:
    """Remap a variable onto another facet of the mesh.

    Averages the values of the contributing elements, so a face field asked for on the
    nodes becomes each node's mean of the faces around it.

    Args:
        location: The facet to remap onto — ``"node"``, ``"edge"`` or ``"face"``.
        variable_name: The variable to remap, or ``None`` for the only one.

    Returns:
        UgridDataset: A new dataset whose variable sits on ``location``.

    Raises:
        ValueError: An unknown ``location``, or a remap the kernel does not offer.
    """
    var = self._facade_variable("to_location", variable_name)
    remap = {"node": _graph.to_node, "edge": _graph.to_edge, "face": _graph.to_face}
    if location not in remap:
        raise ValueError(
            f"to_location: location must be one of {sorted(remap)}, got {location!r}."
        )
    if var.location == location:
        # Already there. The averaging kernels only accept the two *other* facets as a
        # source, so without this an identity remap failed with "source must be 'node' or
        # 'edge'" -- a confusing answer to a request that is trivially already satisfied.
        return self._rebuild(dict(self._data_variables))  # type: ignore[attr-defined]
    values = remap[location](
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        source=var.location,
    )
    moved = var.with_data(np.asarray(values))
    moved.location = location
    # An element with no contributor (an orphan node, say) averages to NaN, so the remapped
    # variable has to declare NaN as its no-data value -- the same reasoning the regrid facade
    # already applies. Otherwise `to_file` writes those NaNs under the old `_FillValue`.
    # Through the same check `_rewrap` applies, rather than declared unconditionally.
    # `graph._average_over` answers float64 today, so NaN is always representable -- but a
    # future integer-preserving remap would reintroduce exactly the defect `_rewrap` guards.
    moved_values = np.asarray(moved.data)
    if _representable(float("nan"), moved_values.dtype):
        moved.nodata = float("nan")
    if moved.dimensions:
        # The element axis is the last dimension, by the mesh-variable invariant. Renaming it
        # to the new facet matters because a file-read variable carries real dimension names
        # and the xarray bridge prefers them: leaving the old facet's name on an axis that now
        # has the new facet's length makes the export fail on conflicting dimension sizes.
        mesh_name = self.mesh_name  # type: ignore[attr-defined]
        moved.dimensions = moved.dimensions[:-1] + (
            f"{mesh_name}_n{location.capitalize()}s",
        )
    new_vars = dict(self._data_variables)  # type: ignore[attr-defined]
    new_vars[var.name] = moved
    return self._rebuild(new_vars)  # type: ignore[attr-defined]

fill_na_spatial(variable_name=None, *, max_distance=None)

Fill each gap from the nearest valid element in space.

Where :meth:interpolate_na fills along time, this fills across the mesh: a gap takes the value of the closest element that has one.

Parameters:

Name	Type	Description	Default
variable_name	str | None	The variable to fill, or None for the only one.	None
max_distance	float | None	Leave a gap unfilled when no valid element is this close. None (the default) is unbounded.	None

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset with the gaps filled.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def fill_na_spatial(
    self, variable_name: str | None = None, *, max_distance: float | None = None
) -> Any:
    """Fill each gap from the nearest valid element in space.

    Where :meth:`interpolate_na` fills along time, this fills across the mesh: a gap takes
    the value of the closest element that has one.

    Args:
        variable_name: The variable to fill, or ``None`` for the only one.
        max_distance: Leave a gap unfilled when no valid element is this close. ``None``
            (the default) is unbounded.

    Returns:
        UgridDataset: A new dataset with the gaps filled.
    """
    var = self._facade_variable("fill_na_spatial", variable_name)
    filled = _fill.fill_na_spatial(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        var.location,
        max_distance=max_distance,
    )
    return self._rewrap(var, filled)

laplace_interpolate(variable_name=None, *, use_weights=True, direct_solve=False)

Fill the interior gaps by solving Laplace's equation over the mesh.

The valid values are the boundary condition and each gap becomes the smooth solution between them — a better hole-filler than nearest when the field varies smoothly.

Parameters:

Name	Type	Description	Default
variable_name	str | None	The variable to fill, or None for the only one.	None
use_weights	bool	Weight the graph Laplacian by inverse distance. Defaults to True.	True
direct_solve	bool	Solve directly rather than iteratively. Defaults to False.	False

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset with the interior gaps filled.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def laplace_interpolate(
    self,
    variable_name: str | None = None,
    *,
    use_weights: bool = True,
    direct_solve: bool = False,
) -> Any:
    """Fill the interior gaps by solving Laplace's equation over the mesh.

    The valid values are the boundary condition and each gap becomes the smooth solution
    between them — a better hole-filler than nearest when the field varies smoothly.

    Args:
        variable_name: The variable to fill, or ``None`` for the only one.
        use_weights: Weight the graph Laplacian by inverse distance. Defaults to True.
        direct_solve: Solve directly rather than iteratively. Defaults to False.

    Returns:
        UgridDataset: A new dataset with the interior gaps filled.
    """
    var = self._facade_variable("laplace_interpolate", variable_name)
    filled = _fill.laplace_interpolate(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        var.location,
        use_weights=use_weights,
        direct_solve=direct_solve,
    )
    return self._rewrap(var, filled)

polygonize(variable_name=None, *, decimals=None)

Dissolve adjacent faces of equal value into polygons.

Parameters:

Name	Type	Description	Default
variable_name	str | None	The face variable to dissolve by, or None for the only one.	None
decimals	int | None	Round to this many decimals before grouping, for approximate equality.	None

Returns:

Type	Description
Any	geopandas.GeoDataFrame: One row per merged region, with its value and geometry.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def polygonize(
    self, variable_name: str | None = None, *, decimals: int | None = None
) -> Any:
    """Dissolve adjacent faces of equal value into polygons.

    Args:
        variable_name: The face variable to dissolve by, or ``None`` for the only one.
        decimals: Round to this many decimals before grouping, for approximate equality.

    Returns:
        geopandas.GeoDataFrame: One row per merged region, with its value and geometry.
    """
    var = self._facade_variable("polygonize", variable_name)
    return _vector_ops.polygonize(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        decimals=decimals,
        crs=self.crs,  # type: ignore[attr-defined]
    )

sample_line(line, variable_name=None)

Sample a variable along a line, ordered by distance along it.

One sample per crossing, at the midpoint of each part of the line-face intersection. A line that leaves a face and re-enters it is sampled once per entry, so face_indices can repeat -- see :func:pyramids.netcdf.ugrid.vector_ops.sample_line.

Parameters:

Name	Type	Description	Default
line	Any	A shapely LineString, or a sequence of coordinates.	required
variable_name	str | None	The variable to sample, or None for the only one.	None

Returns:

Name	Type	Description
tuple	Any	(distances, values, face_indices), one entry per crossing in
	Any	along-line order. face_indices is not unique when the line re-enters a face.

Raises:

Type	Description
ValueError	variable_name is None while the dataset holds no variable or more than one, or the variable's last axis does not match n_face.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def sample_line(self, line: Any, variable_name: str | None = None) -> Any:
    """Sample a variable along a line, ordered by distance along it.

    One sample per *crossing*, at the midpoint of each part of the line-face intersection.
    A line that leaves a face and re-enters it is sampled once per entry, so
    ``face_indices`` can repeat -- see :func:`pyramids.netcdf.ugrid.vector_ops.sample_line`.

    Args:
        line: A shapely ``LineString``, or a sequence of coordinates.
        variable_name: The variable to sample, or ``None`` for the only one.

    Returns:
        tuple: ``(distances, values, face_indices)``, one entry per crossing in
        along-line order. ``face_indices`` is not unique when the line re-enters a face.

    Raises:
        ValueError: ``variable_name`` is ``None`` while the dataset holds no variable or
            more than one, or the variable's last axis does not match ``n_face``.
    """
    var = self._facade_variable("sample_line", variable_name)
    return _vector_ops.sample_line(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        line,
    )

burn(geometry, value, *, name='burned', fill=float('nan'), all_touched=True)

Burn vector geometry onto the faces as a new variable.

The mesh analogue of rasterising a vector layer: every face the geometry covers takes value, the rest take fill.

Parameters:

Name	Type	Description	Default
geometry	Any	A shapely geometry, GeoDataFrame, FeatureCollection, or iterable of them.	required
value	float	The value burned into the covered faces.	required
name	str	The new variable's name. Defaults to "burned".	'burned'
fill	float	The value for the faces the geometry misses. Defaults to NaN.	float('nan')
all_touched	bool	Burn every face the geometry touches, rather than only those whose centroid falls inside. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset carrying the burned variable.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def burn(
    self,
    geometry: Any,
    value: float,
    *,
    name: str = "burned",
    fill: float = float("nan"),
    all_touched: bool = True,
) -> Any:
    """Burn vector geometry onto the faces as a new variable.

    The mesh analogue of rasterising a vector layer: every face the geometry covers takes
    ``value``, the rest take ``fill``.

    Args:
        geometry: A shapely geometry, GeoDataFrame, FeatureCollection, or iterable of them.
        value: The value burned into the covered faces.
        name: The new variable's name. Defaults to ``"burned"``.
        fill: The value for the faces the geometry misses. Defaults to NaN.
        all_touched: Burn every face the geometry touches, rather than only those whose
            centroid falls inside. Defaults to True.

    Returns:
        UgridDataset: A new dataset carrying the burned variable.
    """
    values = _vector_ops.burn(
        self._mesh,  # type: ignore[attr-defined]
        geometry,
        value,
        fill=fill,
        all_touched=all_touched,
    )
    # Declare the fill as no-data only when it is NaN. A NaN fill means "untouched", so
    # naming it is what makes an untouched face distinguishable once written. An explicit
    # numeric fill is a value the caller chose, not a sentinel: `burn(geom, 1.0, fill=0.0)`
    # is the 0/1 mask idiom, and declaring 0.0 as missing made `dropna` and every `skipna`
    # reduction discard the unburned faces.
    burned = np.asarray(values)
    declared = float(fill) if np.isnan(fill) else None
    return self.with_variable(  # type: ignore[attr-defined]
        name, burned, location="face", nodata=declared
    )

rasterize_like(reference, variable_name=None, *, method='nearest', nodata=-9999.0, max_distance=None)

Interpolate a variable onto an existing raster's grid.

Where :meth:to_dataset builds its own grid from a cell size, this snaps to the geotransform, shape and CRS of a raster you already have, so the result is co-registered with it.

Parameters:

Name	Type	Description	Default
reference	Any	The raster whose grid to match (a pyramids Dataset / NetCDF).	required
variable_name	str | None	The variable to interpolate, or None for the only one.	None
method	str	"nearest", "linear" or "idw". Defaults to "nearest".	'nearest'
nodata	float	The value for cells the mesh does not cover.	-9999.0
max_distance	float | None	How far a "nearest" / "idw" sample may reach. None uses twice the longer of the reference's two cell edges. Defaults to None.	None

Returns:

Name	Type	Description
Dataset	Any	A raster on reference's grid.

Raises:

Type	Description
ValueError	An unknown method, or the mesh declares an EPSG while reference declares no CRS to reproject into.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def rasterize_like(
    self,
    reference: Any,
    variable_name: str | None = None,
    *,
    method: str = "nearest",
    nodata: float = -9999.0,
    max_distance: float | None = None,
) -> Any:
    """Interpolate a variable onto an existing raster's grid.

    Where :meth:`to_dataset` builds its own grid from a cell size, this snaps to the
    geotransform, shape and CRS of a raster you already have, so the result is
    co-registered with it.

    Args:
        reference: The raster whose grid to match (a pyramids ``Dataset`` / ``NetCDF``).
        variable_name: The variable to interpolate, or ``None`` for the only one.
        method: ``"nearest"``, ``"linear"`` or ``"idw"``. Defaults to ``"nearest"``.
        nodata: The value for cells the mesh does not cover.
        max_distance: How far a ``"nearest"`` / ``"idw"`` sample may reach. ``None`` uses
            twice the longer of the reference's two cell edges. Defaults to ``None``.

    Returns:
        Dataset: A raster on ``reference``'s grid.

    Raises:
        ValueError: An unknown ``method``, or the mesh declares an EPSG while
            ``reference`` declares no CRS to reproject into.
    """
    var = self._facade_variable("rasterize_like", variable_name)
    return _regrid.rasterize_like(
        self._mesh,  # type: ignore[attr-defined]
        np.asarray(var.data),
        reference,
        location=var.location,
        method=method,
        nodata=nodata,
        source_epsg=self.epsg,  # type: ignore[attr-defined]
        max_distance=max_distance,
    )

interp_time(new_times, *, method='linear', bounds='nan')

Interpolate every temporal variable onto new time coordinates.

Distinct from :meth:interpolate_na, which fills gaps on the existing axis: this relabels the axis, resampling the series onto the times you ask for.

Parameters:

Name	Type	Description	Default
new_times	Any	The time coordinates to interpolate onto.	required
method	str	"linear" or "nearest". Defaults to "linear". (Not every scipy.interpolate.interp1d kind: the kernel accepts these two.)	'linear'
bounds	str	What a label outside the existing axis answers -- "nan" for NaN, or "clamp" to hold the first/last step's value. Defaults to "nan".	'nan'

Returns:

Name	Type	Description
UgridDataset	Any	A new dataset on the requested time coordinates.

Raises:

Type	Description
ValueError	No variable has a time dimension; method is neither "linear" nor "nearest"; bounds is neither "nan" nor "clamp"; or the time coordinates are not numeric.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def interp_time(
    self, new_times: Any, *, method: str = "linear", bounds: str = "nan"
) -> Any:
    """Interpolate every temporal variable onto new time coordinates.

    Distinct from :meth:`interpolate_na`, which fills gaps on the existing axis: this
    *relabels* the axis, resampling the series onto the times you ask for.

    Args:
        new_times: The time coordinates to interpolate onto.
        method: ``"linear"`` or ``"nearest"``. Defaults to ``"linear"``. (Not every
            ``scipy.interpolate.interp1d`` kind: the kernel accepts these two.)
        bounds: What a label outside the existing axis answers -- ``"nan"`` for NaN, or
            ``"clamp"`` to hold the first/last step's value. Defaults to ``"nan"``.

    Returns:
        UgridDataset: A new dataset on the requested time coordinates.

    Raises:
        ValueError: No variable has a time dimension; ``method`` is neither ``"linear"``
            nor ``"nearest"``; ``bounds`` is neither ``"nan"`` nor ``"clamp"``; or the time
            coordinates are not numeric.
    """
    stamps = self._time_coords("interp_time")  # type: ignore[attr-defined]
    targets = list(np.atleast_1d(np.asarray(new_times)))
    # `new_times` goes through the same converter as the existing axis, so a date target
    # is measured on the same scale as the dated coordinates it sits between. Casting it
    # with a bare `astype("float64")` was what made a `Timestamp` target unusable even
    # once the axis itself could be read.
    numeric_targets = _numeric_stamps(targets)
    new_vars: dict[str, Any] = {}
    for name, var in self._data_variables.items():  # type: ignore[attr-defined]
        if not var.has_time:
            new_vars[name] = var
            continue
        moved = _regrid.interp_time(
            _numeric_stamps(stamps),
            np.asarray(var.data),
            numeric_targets,
            method=method,
            axis=cast("int", var.time_index),
            bounds=bounds,
        )
        new_var = var.with_data(np.asarray(moved))
        new_var.attributes = {**var.attributes, "time_values": targets}
        new_vars[name] = new_var
    return self._rebuild(new_vars)  # type: ignore[attr-defined]

regrid(target, *, method='overlap', variable_name=None)

Regrid a variable onto another mesh.

Parameters:

Name	Type	Description	Default
target	Any	The destination UgridDataset (its mesh is the target).	required
method	str	"overlap" (area-weighted mean), "conservative" (integral preserving), "centroid" or "barycentric". Defaults to "overlap".	'overlap'
variable_name	str | None	The variable to regrid, or None for the only one.	None

Returns:

Name	Type	Description
UgridDataset	Any	target's mesh carrying the regridded variable.

Raises:

Type	Description
ValueError	An unknown method; the variable does not sit on faces (only "face" source data is weighted); or variable_name is None while the dataset holds no variable or more than one.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def regrid(
    self, target: Any, *, method: str = "overlap", variable_name: str | None = None
) -> Any:
    """Regrid a variable onto another mesh.

    Args:
        target: The destination ``UgridDataset`` (its mesh is the target).
        method: ``"overlap"`` (area-weighted mean), ``"conservative"`` (integral
            preserving), ``"centroid"`` or ``"barycentric"``. Defaults to ``"overlap"``.
        variable_name: The variable to regrid, or ``None`` for the only one.

    Returns:
        UgridDataset: ``target``'s mesh carrying the regridded variable.

    Raises:
        ValueError: An unknown ``method``; the variable does not sit on faces (only
            ``"face"`` source data is weighted); or ``variable_name`` is ``None`` while the
            dataset holds no variable or more than one.
    """
    var = self._facade_variable("regrid", variable_name)
    regridder = _regrid.Regridder(
        self._mesh,  # type: ignore[attr-defined]
        target.mesh,
        method=method,
        source_location=var.location,
    )
    values = regridder.regrid(np.asarray(var.data))
    # A target element the source does not cover comes back NaN, so the regridded variable
    # has to declare NaN as its no-data value -- otherwise `to_file` writes those NaNs with
    # no `_FillValue` and a reader cannot tell them from data.
    return target.with_variable(
        var.name, np.asarray(values), location="face", nodata=float("nan")
    )

to_xarray()

Hand the mesh and its variables to xarray, in the UGRID-CF encoding.

xarray is a peer dependency, imported only here (as the raster :meth:pyramids.netcdf.NetCDF.to_xarray does), so nothing here imports it at module scope. Note the parent pyramids.netcdf package does currently pull xarray and dask at import time for unrelated reasons, so that cost is not avoided in practice yet.

Returns:

Type	Description
Any	xarray.Dataset: The mesh's UGRID-CF variables plus the data variables.

Raises:

Type	Description
OptionalPackageDoesNotExist	xarray is not installed.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def to_xarray(self) -> Any:
    """Hand the mesh and its variables to xarray, in the UGRID-CF encoding.

    xarray is a **peer dependency**, imported only here (as the raster
    :meth:`pyramids.netcdf.NetCDF.to_xarray` does), so nothing here imports it at module
    scope. Note the parent ``pyramids.netcdf`` package does currently pull xarray and dask
    at import time for unrelated reasons, so that cost is not avoided in practice yet.

    Returns:
        xarray.Dataset: The mesh's UGRID-CF variables plus the data variables.

    Raises:
        OptionalPackageDoesNotExist: xarray is not installed.
    """
    return _interop.to_xarray(
        self._mesh,  # type: ignore[attr-defined]
        self._data_variables,  # type: ignore[attr-defined]
        # The dataset's own topology name, not the default: exporting a mesh called
        # anything else wrote its topology as "mesh2d" while the data variables kept
        # their own element dimension, leaving two unrelated dimensions in one file.
        mesh_name=self.mesh_name,  # type: ignore[attr-defined]
        crs_wkt=self.crs_wkt,  # type: ignore[attr-defined]
        volume_topology=self._volume_topology,  # type: ignore[attr-defined]
    )

to_zarr(store, **kwargs)

Write the mesh and its variables to a zarr store, through xarray.

Parameters:

Name	Type	Description	Default
store	Any	The zarr store (a path or a mapping).	required
**kwargs	Any	Passed to xarray.Dataset.to_zarr.	{}

Returns:

Type	Description
Any	The value xarray.Dataset.to_zarr returns.

Raises:

Type	Description
OptionalPackageDoesNotExist	xarray or zarr is not installed.

Source code in src/pyramids/netcdf/ugrid/_facades.py

def to_zarr(self, store: Any, **kwargs: Any) -> Any:
    """Write the mesh and its variables to a zarr store, through xarray.

    Args:
        store: The zarr store (a path or a mapping).
        **kwargs: Passed to ``xarray.Dataset.to_zarr``.

    Returns:
        The value ``xarray.Dataset.to_zarr`` returns.

    Raises:
        OptionalPackageDoesNotExist: xarray or zarr is not installed.
    """
    return _interop.to_zarr(
        self._mesh,  # type: ignore[attr-defined]
        self._data_variables,  # type: ignore[attr-defined]
        store,
        mesh_name=self.mesh_name,  # type: ignore[attr-defined]
        crs_wkt=self.crs_wkt,  # type: ignore[attr-defined]
        **kwargs,
    )

__init__(mesh, data_variables, global_attributes, topology_info=None, crs_wkt=None, file_name=None, volume_topology=None)

Assemble a dataset from an already-built mesh and its variables.

The low-level constructor. Prefer :meth:read_file for a file, :meth:from_arrays for raw arrays, or one of the from_* bridges -- each of those builds the mesh and the metadata record and then calls this. It is also what every derivation (:meth:isel, :meth:reduce, :meth:copy, ...) calls through type(self)(...), which is how a subclass survives them.

Parameters:

Name	Type	Description	Default
mesh	Mesh2d	The topology the variables sit on.	required
data_variables	dict[str, MeshVariable]	Mapping of variable name to :class:MeshVariable. Held by reference, not copied, so a caller must not mutate it afterwards.	required
global_attributes	dict[str, Any]	The file-level attributes, e.g. Conventions.	required
topology_info	MeshTopologyInfo | None	The record of the on-disk variable names and the declared topology dimension. None for a mesh built from arrays, where there are no file variables to name. Defaults to None.	None
crs_wkt	str | None	WKT of the mesh CRS. None leaves the mesh unreferenced, and :attr:epsg then answers None. Defaults to None.	None
file_name	str | None	The path this dataset was read from, carried through derivations so a :meth:to_file round trip and repr can name it. None for an in-memory mesh. Defaults to None.	None
volume_topology	VolumeTopology | None	The ragged (per-column) vertical layering for a genuinely 3-D mesh — the map from faces to the flat volume axis that a location="volume" variable is indexed through (issue #1330). None (the default) for a 2-D mesh or a uniform-layer 3-D one, where the dense (layer, element) path applies and :attr:n_volume answers None.	None

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __init__(
    self,
    mesh: Mesh2d,
    data_variables: dict[str, MeshVariable],
    global_attributes: dict[str, Any],
    topology_info: MeshTopologyInfo | None = None,
    crs_wkt: str | None = None,
    file_name: str | None = None,
    volume_topology: VolumeTopology | None = None,
):
    """Assemble a dataset from an already-built mesh and its variables.

    The low-level constructor. Prefer :meth:`read_file` for a file, :meth:`from_arrays`
    for raw arrays, or one of the ``from_*`` bridges -- each of those builds the mesh and
    the metadata record and then calls this. It is also what every derivation
    (:meth:`isel`, :meth:`reduce`, :meth:`copy`, ...) calls through ``type(self)(...)``,
    which is how a subclass survives them.

    Args:
        mesh: The topology the variables sit on.
        data_variables: Mapping of variable name to :class:`MeshVariable`. Held by
            reference, not copied, so a caller must not mutate it afterwards.
        global_attributes: The file-level attributes, e.g. ``Conventions``.
        topology_info: The record of the on-disk variable names and the declared
            topology dimension. ``None`` for a mesh built from arrays, where there are
            no file variables to name. Defaults to ``None``.
        crs_wkt: WKT of the mesh CRS. ``None`` leaves the mesh unreferenced, and
            :attr:`epsg` then answers ``None``. Defaults to ``None``.
        file_name: The path this dataset was read from, carried through derivations so a
            :meth:`to_file` round trip and ``repr`` can name it. ``None`` for an
            in-memory mesh. Defaults to ``None``.
        volume_topology: The ragged (per-column) vertical layering for a genuinely 3-D
            mesh — the map from faces to the flat ``volume`` axis that a
            ``location="volume"`` variable is indexed through (issue #1330). ``None`` (the
            default) for a 2-D mesh or a uniform-layer 3-D one, where the dense
            ``(layer, element)`` path applies and :attr:`n_volume` answers ``None``.
    """
    self._mesh = mesh
    self._data_variables = data_variables
    self._global_attributes = global_attributes
    self._topology_info = topology_info
    self._crs_wkt = crs_wkt
    self._file_name = file_name
    self._volume_topology = volume_topology
    self._cached_crs: Any = None

read_file(path, mesh_name=None, driver=None, lazy=True) classmethod

Open a mesh file.

UGRID NetCDF is read here. Any other format — 2DM, Selafin, DFSU, XMDF, … — is read by a third-party driver registered into the pyramids.mesh_drivers entry-point group. A non-NetCDF suffix is dispatched by detection; a NetCDF suffix is read by the built-in reader first, and only a file that opens but carries no UGRID topology falls back to a driver claiming that suffix. Either way a successful UGRID read triggers no driver discovery, so a .nc read still pays nothing.

Resolution order:

1. driver given — that driver, or an error. Never falls back.
2. A /vsi path or a NetCDF suffix — the built-in UGRID reader. If a NetCDF-suffixed file opens but declares no UGRID topology, a registered driver claiming that suffix is tried before giving up, so 3Di, TUFLOW FV and other NetCDF-based formats are reachable; discovery runs only after the built-in read has already failed.
3. Otherwise a registered driver claiming the suffix whose can_open accepts the file.
4. Nothing claimed it — the built-in reader anyway, so a NetCDF file under an unusual suffix still opens exactly as it always did.
5. That failed too — :class:UnsupportedMeshFormatError, naming what is installed.

Parameters:

Name	Type	Description	Default
path	str | Path	Path to the file. A GDAL virtual-filesystem path (/vsimem/, /vsizip/, /vsicurl/, …) is passed straight to GDAL rather than checked on disk.	required
mesh_name	str | None	Which topology to read, for a file declaring more than one (see :meth:mesh_names). None (the default) takes the first.	None
driver	str | None	Name of a registered mesh driver to use, bypassing detection — the name a plugin declares, not a GDAL driver short name. None (the default) resolves as described above.	None
lazy	bool	Defer reading variable values until first access. Forwarded to the driver, which honours it when it declares lazy_data. The built-in UGRID reader is always lazy, so it rejects lazy=False rather than ignoring it.	True

Returns:

Type	Description
UgridDataset	UgridDataset instance.

Raises:

Type	Description
FileNotFoundError	If the file does not exist.
DriverNotExistError	If driver names a driver that is not installed.
UnsupportedMeshFormatError	If no installed driver reads the file, and the built-in UGRID reader cannot either.
IsADirectoryError	The path is a directory that no driver claims.
TypeError	lazy=False was asked of the built-in UGRID reader, which is always lazy and so cannot honour it.
RuntimeError	GDAL cannot open a file that reached the built-in reader by its own suffix. A file whose suffix no driver claimed is reported as :class:UnsupportedMeshFormatError instead, so this is now the native route only.
ValueError	If no UGRID topology is found in the file, or mesh_name names a topology the file does not declare.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def read_file(
    cls,
    path: str | Path,
    mesh_name: str | None = None,
    driver: str | None = None,
    lazy: bool = True,
) -> UgridDataset:
    """Open a mesh file.

    UGRID NetCDF is read here. Any other format — 2DM, Selafin, DFSU, XMDF, … — is read by a
    third-party driver registered into the ``pyramids.mesh_drivers`` entry-point group. A
    non-NetCDF suffix is dispatched by detection; a NetCDF suffix is read by the built-in
    reader first, and only a file that opens but carries no UGRID topology falls back to a
    driver claiming that suffix. Either way a successful UGRID read triggers no driver
    discovery, so a ``.nc`` read still pays nothing.

    Resolution order:

    1. ``driver`` given — that driver, or an error. Never falls back.
    2. A ``/vsi`` path or a NetCDF suffix — the built-in UGRID reader. If a NetCDF-suffixed
       file opens but declares no UGRID topology, a registered driver claiming that suffix
       is tried before giving up, so 3Di, TUFLOW FV and other NetCDF-based formats are
       reachable; discovery runs only after the built-in read has already failed.
    3. Otherwise a registered driver claiming the suffix whose ``can_open`` accepts the file.
    4. Nothing claimed it — the built-in reader anyway, so a NetCDF file under an unusual
       suffix still opens exactly as it always did.
    5. That failed too — :class:`UnsupportedMeshFormatError`, naming what is installed.

    Args:
        path: Path to the file. A GDAL virtual-filesystem path
            (``/vsimem/``, ``/vsizip/``, ``/vsicurl/``, …) is passed straight to GDAL
            rather than checked on disk.
        mesh_name: Which topology to read, for a file declaring more than one (see
            :meth:`mesh_names`). ``None`` (the default) takes the first.
        driver: Name of a registered **mesh** driver to use, bypassing detection — the
            name a plugin declares, not a GDAL driver short name. ``None`` (the default)
            resolves as described above.
        lazy: Defer reading variable values until first access. Forwarded to the driver,
            which honours it when it declares ``lazy_data``. The built-in UGRID reader is
            always lazy, so it rejects ``lazy=False`` rather than ignoring it.

    Returns:
        UgridDataset instance.

    Raises:
        FileNotFoundError: If the file does not exist.
        DriverNotExistError: If ``driver`` names a driver that is not installed.
        UnsupportedMeshFormatError: If no installed driver reads the file, and the
            built-in UGRID reader cannot either.
        IsADirectoryError: The path is a directory that no driver claims.
        TypeError: ``lazy=False`` was asked of the built-in UGRID reader, which is always
            lazy and so cannot honour it.
        RuntimeError: GDAL cannot open a file that reached the built-in reader by its
            own suffix. A file whose suffix no driver claimed is reported as
            :class:`UnsupportedMeshFormatError` instead, so this is now the native
            route only.
        ValueError: If no UGRID topology is found in the file, or ``mesh_name`` names
            a topology the file does not declare.
    """
    selected, native = _resolve_read_driver(path, driver)
    if selected is not None:
        result = _read_through_driver(selected, path, mesh_name, cls, lazy)
    else:
        try:
            result = cls._read_ugrid_netcdf(
                path, mesh_name, wrap_unreadable=not native, lazy=lazy
            )
        except NoMeshTopologyError:
            # A NetCDF file that opened but carries no UGRID topology -- a 3Di or
            # TUFLOW FV result. Only now, after the built-in read has failed, consult a
            # driver claiming the suffix; if none does, the original error stands. `lazy`
            # is forwarded to the driver, which may honour an eager read even though the
            # built-in reader cannot. A genuine UGRID file with `lazy=False` raised
            # TypeError inside `_read_ugrid_netcdf` instead, before reading any geometry.
            fallback = _netcdf_fallback_driver(path, driver)
            if fallback is None:
                raise
            result = _read_through_driver(fallback, path, mesh_name, cls, lazy)
    return result

get_data(variable_name)

Get a data variable by name.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable.	required

Returns:

Type	Description
MeshVariable	MeshVariable instance.

Raises:

Type	Description
KeyError	If the variable name is not found.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def get_data(self, variable_name: str) -> MeshVariable:
    """Get a data variable by name.

    Args:
        variable_name: Name of the data variable.

    Returns:
        MeshVariable instance.

    Raises:
        KeyError: If the variable name is not found.
    """
    if variable_name not in self._data_variables:
        raise KeyError(
            f"Variable '{variable_name}' not found. "
            f"Available: {self.data_variable_names}"
        )
    result = self._data_variables[variable_name]
    return result

__getitem__(key)

Get a data variable by name using bracket notation.

Parameters:

Name	Type	Description	Default
key	str	The variable name.	required

Returns:

Name	Type	Description
MeshVariable	MeshVariable	The variable.

Raises:

Type	Description
KeyError	No variable is named key; the message lists the available names.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __getitem__(self, key: str) -> MeshVariable:
    """Get a data variable by name using bracket notation.

    Args:
        key: The variable name.

    Returns:
        MeshVariable: The variable.

    Raises:
        KeyError: No variable is named ``key``; the message lists the available names.
    """
    return self.get_data(key)

__contains__(key)

Whether key names a data variable of this dataset.

Parameters:

Name	Type	Description	Default
key	str	The variable name to test.	required

Returns:

Name	Type	Description
bool	bool	True when the dataset carries a variable of that name.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __contains__(self, key: str) -> bool:
    """Whether ``key`` names a data variable of this dataset.

    Args:
        key: The variable name to test.

    Returns:
        bool: True when the dataset carries a variable of that name.
    """
    return key in self._data_variables

__len__()

The number of data variables.

Returns:

Name	Type	Description
int	int	How many data variables the dataset carries. Note this counts variables,
	int	not elements -- :attr:n_face / :attr:n_node / :attr:n_edge are the mesh
	int	sizes.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __len__(self) -> int:
    """The number of data variables.

    Returns:
        int: How many data variables the dataset carries. Note this counts *variables*,
        not elements -- :attr:`n_face` / :attr:`n_node` / :attr:`n_edge` are the mesh
        sizes.
    """
    return len(self._data_variables)

__iter__()

Iterate over the data variable names.

Returns:

Type	Description
Any	Iterator[str]: The variable names, in insertion order -- so list(dataset)
Any	and dataset.keys() agree, and for name in dataset reads like a mapping.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __iter__(self) -> Any:
    """Iterate over the data variable names.

    Returns:
        Iterator[str]: The variable names, in insertion order -- so ``list(dataset)``
        and ``dataset.keys()`` agree, and ``for name in dataset`` reads like a mapping.
    """
    return iter(self._data_variables)

keys()

The data variable names (read-only mapping surface).

Returns:

Type	Description
list[str]	list[str]: The names, in insertion order. A list rather than a view, so it does
list[str]	not track later edits -- every variable edit returns a new dataset anyway.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def keys(self) -> list[str]:
    """The data variable names (read-only mapping surface).

    Returns:
        list[str]: The names, in insertion order. A list rather than a view, so it does
        not track later edits -- every variable edit returns a new dataset anyway.
    """
    return list(self._data_variables.keys())

values()

The data variables (read-only mapping surface).

Returns:

Type	Description
list[MeshVariable]	list[MeshVariable]: The variables, in the same order as :meth:keys. The
list[MeshVariable]	variables themselves are not copied, so reading .data on one still fires its
list[MeshVariable]	lazy loader.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def values(self) -> list[MeshVariable]:
    """The data variables (read-only mapping surface).

    Returns:
        list[MeshVariable]: The variables, in the same order as :meth:`keys`. The
        variables themselves are not copied, so reading ``.data`` on one still fires its
        lazy loader.
    """
    return list(self._data_variables.values())

items()

The (name, variable) pairs (read-only mapping surface).

Returns:

Type	Description
list[tuple[str, MeshVariable]]	list[tuple[str, MeshVariable]]: One pair per variable, in insertion order.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def items(self) -> list[tuple[str, MeshVariable]]:
    """The ``(name, variable)`` pairs (read-only mapping surface).

    Returns:
        list[tuple[str, MeshVariable]]: One pair per variable, in insertion order.
    """
    return list(self._data_variables.items())

get(key, default=None)

The variable key, or default when it is absent.

Parameters:

Name	Type	Description	Default
key	str	The variable name to look up.	required
default	Any	What to answer when no variable is named key. Defaults to None.	None

Returns:

Type	Description
MeshVariable | Any	MeshVariable | Any: The variable, or default.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def get(self, key: str, default: Any = None) -> MeshVariable | Any:
    """The variable ``key``, or ``default`` when it is absent.

    Args:
        key: The variable name to look up.
        default: What to answer when no variable is named ``key``. Defaults to ``None``.

    Returns:
        MeshVariable | Any: The variable, or ``default``.
    """
    return self._data_variables.get(key, default)

with_variable(name, data, *, location='face', nodata=None)

Return a new dataset with name added or replaced.

UGRID derivations are immutable — unlike NetCDF.set_variable this does not mutate in place but returns a fresh dataset. A 2-D data array is treated as temporal (leading axis = time); a 1-D array is a static per-element variable.

Parameters:

Name	Type	Description	Default
name	str	Variable name to add or replace.	required
data	ndarray	The values; (n_elements,) static or (n_time, n_elements) temporal.	required
location	str	Mesh location — "face", "node" or "edge". Defaults to "face".	'face'
nodata	float | None	Optional no-data value for the new variable.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset carrying the variable.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def with_variable(
    self,
    name: str,
    data: np.ndarray,
    *,
    location: str = "face",
    nodata: float | None = None,
) -> UgridDataset:
    """Return a new dataset with ``name`` added or replaced.

    UGRID derivations are immutable — unlike ``NetCDF.set_variable`` this does not mutate
    in place but returns a fresh dataset. A 2-D ``data`` array is treated as temporal
    (leading axis = time); a 1-D array is a static per-element variable.

    Args:
        name: Variable name to add or replace.
        data: The values; ``(n_elements,)`` static or ``(n_time, n_elements)`` temporal.
        location: Mesh location — ``"face"``, ``"node"`` or ``"edge"``. Defaults to
            ``"face"``.
        nodata: Optional no-data value for the new variable.

    Returns:
        UgridDataset: A new dataset carrying the variable.
    """
    arr = np.asarray(data)
    variable = MeshVariable(
        name=name,
        location=location,
        mesh_name=self.mesh_name,
        shape=arr.shape,
        nodata=nodata,
        _data=arr,
    )
    new_vars = dict(self._data_variables)
    new_vars[name] = variable
    return self._rebuild(new_vars)

drop_variables(names)

Return a new dataset without the named variable(s).

Parameters:

Name	Type	Description	Default
names	str | list[str]	A variable name, or a list of them, to drop.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset without those variables.

Raises:

Type	Description
KeyError	A named variable is not present.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def drop_variables(self, names: str | list[str]) -> UgridDataset:
    """Return a new dataset without the named variable(s).

    Args:
        names: A variable name, or a list of them, to drop.

    Returns:
        UgridDataset: A new dataset without those variables.

    Raises:
        KeyError: A named variable is not present.
    """
    drop = [names] if isinstance(names, str) else list(names)
    new_vars = dict(self._data_variables)
    for name in drop:
        if name not in new_vars:
            raise KeyError(
                f"Variable {name!r} not found. Available: {self.data_variable_names}"
            )
        del new_vars[name]
    return self._rebuild(new_vars)

rename_variable(old, new)

Return a new dataset with variable old renamed to new.

Parameters:

Name	Type	Description	Default
old	str	The current variable name.	required
new	str	The new name.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with the variable renamed.

Raises:

Type	Description
KeyError	old is not present.
ValueError	new already names a different variable.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def rename_variable(self, old: str, new: str) -> UgridDataset:
    """Return a new dataset with variable ``old`` renamed to ``new``.

    Args:
        old: The current variable name.
        new: The new name.

    Returns:
        UgridDataset: A new dataset with the variable renamed.

    Raises:
        KeyError: ``old`` is not present.
        ValueError: ``new`` already names a different variable.
    """
    if old not in self._data_variables:
        raise KeyError(
            f"Variable {old!r} not found. Available: {self.data_variable_names}"
        )
    if new in self._data_variables and new != old:
        raise ValueError(f"Variable {new!r} already exists.")
    new_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if name == old:
            # `replace` keeps the lazy loader and every other field, only re-labelling
            # the variable, so a rename never forces a read.
            new_vars[new] = replace(var, name=new)
        else:
            new_vars[name] = var
    return self._rebuild(new_vars)

to_dataset(variable_name, cell_size, method='nearest', bounds=None, epsg=None, nodata=-9999.0)

Convert a mesh variable to a regular-grid Dataset.

Interpolates mesh data onto a regular grid and returns a standard pyramids Dataset. This is the bridge between unstructured (UGRID) and structured (raster) worlds.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable to rasterize.	required
cell_size	float	Target grid cell size in coordinate units.	required
method	str	Interpolation method ("nearest" or "linear").	'nearest'
bounds	tuple[float, float, float, float] | None	Target (xmin, ymin, xmax, ymax). Defaults to mesh bounds.	None
epsg	int | None	Target EPSG code. Defaults to mesh CRS.	None
nodata	float	No-data value for the output raster.	-9999.0

Returns:

Type	Description
Dataset	pyramids Dataset with the interpolated data.

Raises:

Type	Description
KeyError	No variable is named variable_name.
ValueError	The variable exists but holds no data (nothing was loaded and it carries no lazy loader), so there is nothing to interpolate.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_dataset(
    self,
    variable_name: str,
    cell_size: float,
    method: str = "nearest",
    bounds: tuple[float, float, float, float] | None = None,
    epsg: int | None = None,
    nodata: float = -9999.0,
) -> Dataset:
    """Convert a mesh variable to a regular-grid Dataset.

    Interpolates mesh data onto a regular grid and returns a
    standard pyramids Dataset. This is the bridge between
    unstructured (UGRID) and structured (raster) worlds.

    Args:
        variable_name: Name of the data variable to rasterize.
        cell_size: Target grid cell size in coordinate units.
        method: Interpolation method ("nearest" or "linear").
        bounds: Target (xmin, ymin, xmax, ymax). Defaults to mesh bounds.
        epsg: Target EPSG code. Defaults to mesh CRS.
        nodata: No-data value for the output raster.

    Returns:
        pyramids Dataset with the interpolated data.

    Raises:
        KeyError: No variable is named ``variable_name``.
        ValueError: The variable exists but holds no data (nothing was loaded and it
            carries no lazy loader), so there is nothing to interpolate.
    """
    var = self.get_data(variable_name)
    data = var.data
    if data is None:
        raise ValueError(f"Variable '{variable_name}' has no data loaded.")
    if var.has_time:
        # First step along the variable's own time axis (not blindly axis 0, which for
        # a trailing-time variable would take the first face) — see `_element_values`.
        data = np.take(np.asarray(data), 0, axis=cast("int", var.time_index))

    grid_array, geotransform = mesh_to_grid(
        mesh=self._mesh,
        data=data,
        location=var.location,
        cell_size=cell_size,
        method=method,
        bounds=bounds,
        nodata=nodata,
    )

    # `crs_spec` rather than `self.epsg or 4326`: a mesh carrying a projected
    # WKT with no EPSG code has `epsg is None`, and the old default stamped
    # EPSG:4326 onto metre coordinates. That is the case this resolves.
    #
    # The `or 4326` behind it keeps the *other* case as it was. A mesh with
    # no CRS at all -- no code and no WKT -- has always rasterised to WGS 84
    # here, and `to_dataset(...).to_file("out.tif")` writing a GeoTIFF with
    # no CRS instead would be a silent behaviour change for every caller
    # holding such a mesh. Narrowing that default is a decision for its own
    # change, not a side effect of teaching this line about WKT.
    target_epsg = (
        epsg if epsg is not None else crs_spec(self.epsg, self.crs_wkt) or 4326
    )
    result = Dataset.from_array(
        grid_array,
        no_data_value=nodata,
        geo_ref=GeoReference(
            geo=cast(
                "tuple[float, float, float, float, float, float]", geotransform
            ),
            epsg=target_epsg,
        ),
    )
    return result

crop(mask=None, touch=True, *, bbox=None, epsg=None)

Crop the mesh to a polygon mask or a bbox — the unstructured-mesh analogue of crop.

The mesh equivalent of :meth:pyramids.dataset.Dataset.crop / :meth:NetCDF.crop. Rather than warping a raster, it selects the faces that fall inside the region (renumbering the node connectivity for the resulting sub-mesh) and keeps the data on the surviving elements. Delegates to :meth:clip for a polygon and :meth:subset_by_bounds for a bbox; this method exists so the spatial-subset call is named crop across the raster and mesh classes alike.

Parameters:

Name	Type	Description	Default
mask	Any	Polygon mask — a shapely geometry, GeoDataFrame, or FeatureCollection. Mutually exclusive with bbox.	None
touch	bool	Applies only to a polygon mask: if True (default), keep faces that touch the mask boundary; if False, keep only faces fully inside it. Ignored for bbox, which always selects faces by its axis-aligned envelope (subset_by_bounds). Defaults to True.	True
bbox	tuple or list of 4 floats, keyword-only	(west, south, east, north) in the mesh CRS, or in epsg when supplied. Accepts a tuple or a list. Selects faces by the axis-aligned envelope; not affected by touch. Mutually exclusive with mask.	None
epsg	(int, keyword - only)	CRS of bbox. When it differs from the mesh CRS the box is reprojected to the mesh CRS and subset by its envelope; when it equals the mesh CRS it is a no-op. Defaults to the mesh CRS.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new sub-mesh — faces inside the region, connectivity renumbered, and data variables subset to the surviving elements.

Raises:

Type	Description
ValueError	If both mask and bbox are supplied, if bbox is not a 4-tuple, or if epsg is given for a bbox but the mesh has no CRS to reproject into.
TypeError	If neither mask nor bbox is supplied.

Examples:

• Crop a mesh to a polygon (faces intersecting it survive):

  >>> from shapely.geometry import Polygon
  >>> from pyramids.netcdf import UgridDataset
  >>> ug = UgridDataset.read_file("mesh.nc")                        # doctest: +SKIP
  >>> sub = ug.crop(Polygon([(-1, -1), (0, -1), (0, 1), (-1, 1)]))  # doctest: +SKIP
  >>> sub.n_face <= ug.n_face                                       # doctest: +SKIP
  True
  

• Crop to a bounding box in the mesh's own CRS:

  >>> sub = ug.crop(bbox=(-1.0, -1.0, 0.0, 1.0))                    # doctest: +SKIP
  

See Also

clip: Polygon-mask subsetting that crop delegates to when mask is given. subset_by_bounds: Bounding-box subsetting that crop delegates to when bbox is given. pyramids.dataset.Dataset.crop: The raster equivalent on a gridded dataset.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def crop(
    self,
    mask: Any = None,
    touch: bool = True,
    *,
    bbox: tuple[float, float, float, float] | list[float] | None = None,
    epsg: int | None = None,
) -> UgridDataset:
    """Crop the mesh to a polygon mask or a bbox — the unstructured-mesh analogue of crop.

    The mesh equivalent of :meth:`pyramids.dataset.Dataset.crop` / :meth:`NetCDF.crop`. Rather
    than warping a raster, it selects the **faces** that fall inside the region (renumbering the
    node connectivity for the resulting sub-mesh) and keeps the data on the surviving elements.
    Delegates to :meth:`clip` for a polygon and :meth:`subset_by_bounds` for a bbox; this method
    exists so the spatial-subset call is named ``crop`` across the raster and mesh classes alike.

    Args:
        mask (Any):
            Polygon mask — a shapely geometry, ``GeoDataFrame``, or ``FeatureCollection``.
            Mutually exclusive with ``bbox``.
        touch (bool):
            Applies only to a polygon ``mask``: if ``True`` (default), keep faces that touch the
            mask boundary; if ``False``, keep only faces fully inside it. Ignored for ``bbox``,
            which always selects faces by its axis-aligned envelope (``subset_by_bounds``).
            Defaults to True.
        bbox (tuple or list of 4 floats, keyword-only):
            ``(west, south, east, north)`` in the mesh CRS, or in ``epsg`` when supplied. Accepts
            a tuple or a list. Selects faces by the axis-aligned envelope; not affected by
            ``touch``. Mutually exclusive with ``mask``.
        epsg (int, keyword-only):
            CRS of ``bbox``. When it differs from the mesh CRS the box is reprojected to the mesh
            CRS and subset by its envelope; when it equals the mesh CRS it is a no-op. Defaults to
            the mesh CRS.

    Returns:
        UgridDataset: A new sub-mesh — faces inside the region, connectivity renumbered, and data
            variables subset to the surviving elements.

    Raises:
        ValueError: If both ``mask`` and ``bbox`` are supplied, if ``bbox`` is not a 4-tuple, or
            if ``epsg`` is given for a ``bbox`` but the mesh has no CRS to reproject into.
        TypeError: If neither ``mask`` nor ``bbox`` is supplied.

    Examples:
        - Crop a mesh to a polygon (faces intersecting it survive):
            ```python
            >>> from shapely.geometry import Polygon
            >>> from pyramids.netcdf import UgridDataset
            >>> ug = UgridDataset.read_file("mesh.nc")                        # doctest: +SKIP
            >>> sub = ug.crop(Polygon([(-1, -1), (0, -1), (0, 1), (-1, 1)]))  # doctest: +SKIP
            >>> sub.n_face <= ug.n_face                                       # doctest: +SKIP
            True

            ```
        - Crop to a bounding box in the mesh's own CRS:
            ```python
            >>> sub = ug.crop(bbox=(-1.0, -1.0, 0.0, 1.0))                    # doctest: +SKIP

            ```

    See Also:
        clip: Polygon-mask subsetting that ``crop`` delegates to when ``mask`` is given.
        subset_by_bounds: Bounding-box subsetting that ``crop`` delegates to when ``bbox`` is
            given.
        pyramids.dataset.Dataset.crop: The raster equivalent on a gridded dataset.
    """
    if bbox is not None:
        if mask is not None:
            raise ValueError("crop accepts either `mask` or `bbox`, not both")
        if len(bbox) != 4:
            raise ValueError(
                "bbox must be a 4-tuple of (west, south, east, north), "
                f"got {len(bbox)} value(s)"
            )
        west, south, east, north = bbox
        if epsg is not None and (self.epsg is None or int(epsg) != int(self.epsg)):
            if self.epsg is None:
                raise ValueError(
                    f"cannot reproject a bbox given in EPSG:{int(epsg)} into a mesh that has no "
                    "CRS; drop epsg to treat the bbox as native coordinates"
                )
            # Reproject the bbox to the mesh CRS and subset by its envelope, so the bbox path
            # selects faces with the same rule (subset_by_bounds) regardless of the source CRS.
            west, south, east, north = (
                gpd.GeoSeries([box(west, south, east, north)], crs=epsg)
                .to_crs(self.epsg)
                .total_bounds
            )
        result = self.subset_by_bounds(west, south, east, north)
    elif mask is not None:
        result = self.clip(mask, touch=touch)
    else:
        raise TypeError(
            "crop requires a `mask` (polygon) or a `bbox` (west, south, east, north) tuple"
        )
    return result

clip(mask, touch=True)

Clip the mesh to a polygon mask.

Selects faces that intersect (touch=True) or are fully contained within (touch=False) the mask polygon.

Parameters:

Name	Type	Description	Default
mask	Any	Polygon mask (GeoDataFrame, FeatureCollection, or Shapely geometry).	required
touch	bool	If True, include faces touching the boundary.	True

Returns:

Type	Description
UgridDataset	New UgridDataset with clipped mesh and data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def clip(self, mask: Any, touch: bool = True) -> UgridDataset:
    """Clip the mesh to a polygon mask.

    Selects faces that intersect (touch=True) or are fully
    contained within (touch=False) the mask polygon.

    Args:
        mask: Polygon mask (GeoDataFrame, FeatureCollection,
            or Shapely geometry).
        touch: If True, include faces touching the boundary.

    Returns:
        New UgridDataset with clipped mesh and data.
    """
    mesh, data_variables = clip_mesh(self, mask, touch=touch)
    return self._wrap_subset(mesh, data_variables)

subset_by_bounds(xmin, ymin, xmax, ymax)

Subset mesh to faces within a bounding box.

Parameters:

Name	Type	Description	Default
xmin	float	Minimum x-coordinate.	required
ymin	float	Minimum y-coordinate.	required
xmax	float	Maximum x-coordinate.	required
ymax	float	Maximum y-coordinate.	required

Returns:

Type	Description
UgridDataset	New UgridDataset with subset mesh and data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def subset_by_bounds(
    self,
    xmin: float,
    ymin: float,
    xmax: float,
    ymax: float,
) -> UgridDataset:
    """Subset mesh to faces within a bounding box.

    Args:
        xmin: Minimum x-coordinate.
        ymin: Minimum y-coordinate.
        xmax: Maximum x-coordinate.
        ymax: Maximum y-coordinate.

    Returns:
        New UgridDataset with subset mesh and data.
    """
    mesh, data_variables = subset_by_bounds(self, xmin, ymin, xmax, ymax)
    return self._wrap_subset(mesh, data_variables)

to_crs(to_epsg)

Reproject all node coordinates to a new CRS.

Uses pyproj.Transformer to reproject node coordinates. Face/edge center coordinates are recomputed after reprojection. Data values are preserved — only coordinates change.

Parameters:

Name	Type	Description	Default
to_epsg	int	Target EPSG code.	required

Returns:

Type	Description
UgridDataset	New UgridDataset with reprojected coordinates.

Raises:

Type	Description
ValueError	The mesh declares neither an EPSG code nor a WKT, so there is no source CRS to reproject from.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_crs(self, to_epsg: int) -> UgridDataset:
    """Reproject all node coordinates to a new CRS.

    Uses pyproj.Transformer to reproject node coordinates.
    Face/edge center coordinates are recomputed after reprojection.
    Data values are preserved — only coordinates change.

    Args:
        to_epsg: Target EPSG code.

    Returns:
        New UgridDataset with reprojected coordinates.

    Raises:
        ValueError: The mesh declares neither an EPSG code nor a WKT, so there is no
            source CRS to reproject from.
    """
    # The mesh may describe its CRS by WKT alone, with no EPSG code to
    # resolve; `crs_spec` returns whichever of the two is usable, so a
    # projected mesh stops being refused for a CRS it plainly has.
    source_crs = crs_spec(self.epsg, self.crs_wkt)
    if source_crs is None:
        raise ValueError(
            "Cannot reproject: source CRS is unknown. "
            "Set CRS before calling to_crs()."
        )

    # Through `crs_from_user_input` so a mesh in a CRS whose code only GDAL's
    # PROJ database carries still reprojects (issue #943). The source goes in
    # as-is rather than as `EPSG:{...}`: `crs_spec` yields the mesh's WKT when
    # there is no usable code, and prefixing that builds the nonsense
    # `EPSG:PROJCS[...]`. The target is always a bare integer code.
    transformer = Transformer.from_crs(
        crs_from_user_input(source_crs),
        crs_from_user_input(f"EPSG:{to_epsg}"),
        always_xy=True,
    )
    new_node_x, new_node_y = transformer.transform(
        self._mesh.node_x,
        self._mesh.node_y,
    )

    new_face_x = None
    new_face_y = None
    if self._mesh.has_face_coords:
        new_face_x, new_face_y = transformer.transform(
            self._mesh.face_x,
            self._mesh.face_y,
        )

    new_edge_x = None
    new_edge_y = None
    if self._mesh.has_edge_coords:
        new_edge_x, new_edge_y = transformer.transform(
            self._mesh.edge_x,
            self._mesh.edge_y,
        )

    new_mesh = Mesh2d(
        node_x=new_node_x,
        node_y=new_node_y,
        face_node_connectivity=self._mesh.face_node_connectivity,
        edge_node_connectivity=self._mesh.edge_node_connectivity,
        face_edge_connectivity=self._mesh.face_edge_connectivity,
        face_face_connectivity=self._mesh.face_face_connectivity,
        edge_face_connectivity=self._mesh.edge_face_connectivity,
        face_x=new_face_x,
        face_y=new_face_y,
        edge_x=new_edge_x,
        edge_y=new_edge_y,
    )

    srs = sr_from_epsg(to_epsg)
    new_crs_wkt = srs.ExportToWkt()

    new_topo_info = None
    if self._topology_info is not None:
        new_topo_info = replace(self._topology_info, crs_wkt=new_crs_wkt)

    # `type(self)` and `file_name`, as every other derivation does: hard-coding
    # `UgridDataset(` silently downgraded a subclass, and dropping `file_name` made the
    # attribute unreliable rather than absent.
    result = type(self)(
        mesh=new_mesh,
        data_variables=self._data_variables,
        global_attributes=self._global_attributes,
        topology_info=new_topo_info,
        crs_wkt=new_crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )
    return result

sel_time(index)

Select a single time step from all temporal variables.

Non-temporal variables are kept unchanged.

Parameters:

Name	Type	Description	Default
index	int	Time step index.	required

Returns:

Type	Description
UgridDataset	New UgridDataset with single time step data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sel_time(self, index: int) -> UgridDataset:
    """Select a single time step from all temporal variables.

    Non-temporal variables are kept unchanged.

    Args:
        index: Time step index.

    Returns:
        New UgridDataset with single time step data.
    """
    new_data_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if var.has_time:
            new_data_vars[name] = var.with_data(
                var.sel_time(index), active=var._time_sliced_active(index, None)
            )
        else:
            new_data_vars[name] = var

    # See `to_crs`: `type(self)` keeps a subclass, and `file_name` is carried.
    result = type(self)(
        mesh=self._mesh,
        data_variables=new_data_vars,
        global_attributes=self._global_attributes,
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )
    return result

sel_time_range(start, stop)

Select a time range from all temporal variables.

Parameters:

Name	Type	Description	Default
start	int	Start index (inclusive).	required
stop	int	Stop index (exclusive).	required

Returns:

Type	Description
UgridDataset	New UgridDataset with the selected time range.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sel_time_range(self, start: int, stop: int) -> UgridDataset:
    """Select a time range from all temporal variables.

    Args:
        start: Start index (inclusive).
        stop: Stop index (exclusive).

    Returns:
        New UgridDataset with the selected time range.
    """
    new_data_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if var.has_time:
            new_data_vars[name] = var.sel_time_range(start, stop)
        else:
            new_data_vars[name] = var

    # See `to_crs`: `type(self)` keeps a subclass, and `file_name` is carried.
    result = type(self)(
        mesh=self._mesh,
        data_variables=new_data_vars,
        global_attributes=self._global_attributes,
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )
    return result

attach_data(path, driver=None, lazy=True)

Attach a results-only file's variables to this mesh.

Some formats carry values but no topology — an SMS .dat, an XMDF, an XDMF — and must be paired with a mesh read from elsewhere. Those drivers declare needs_mesh, and :meth:read_file refuses them, pointing here.

>>> mesh = UgridDataset.read_file("domain.2dm")       # doctest: +SKIP
>>> result = mesh.attach_data("velocity.dat")         # doctest: +SKIP

The driver returns only the variables it read; this method merges them onto the dataset. That division is deliberate — it makes a name the dataset already defines an unambiguous collision, and means a driver cannot drop the dataset's own variables by rebuilding the mapping.

The driver checks the file's declared element count against this mesh. That check is the only safeguard there is: a bare results file carries nothing identifying the mesh it belongs to, so attaching it to the wrong one would otherwise produce a plausible and entirely wrong dataset.

Parameters:

Name	Type	Description	Default
path	str | Path	The results file to read.	required
driver	str | None	Name of a registered driver to use, bypassing detection. None (the default) picks the needs_mesh driver claiming the file's suffix.	None
lazy	bool	Defer reading variable values until first access. Defaults to True.	True

Returns:

Type	Description
UgridDataset	A new dataset carrying this mesh plus the attached variables. This dataset is
UgridDataset	unchanged.

Raises:

Type	Description
FileNotFoundError	If the results file does not exist.
DriverNotExistError	If driver names a driver that is not installed.
UnsupportedMeshFormatError	If no installed driver can attach this file, the named driver does not declare needs_mesh, or the driver returned something other than a mapping of name to :class:MeshVariable.
ValueError	If the driver returns a variable whose name this dataset already uses, or reports an element-count mismatch against this mesh.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def attach_data(
    self, path: str | Path, driver: str | None = None, lazy: bool = True
) -> UgridDataset:
    """Attach a results-only file's variables to this mesh.

    Some formats carry values but no topology — an SMS ``.dat``, an XMDF, an XDMF — and must
    be paired with a mesh read from elsewhere. Those drivers declare ``needs_mesh``, and
    :meth:`read_file` refuses them, pointing here.

        >>> mesh = UgridDataset.read_file("domain.2dm")       # doctest: +SKIP
        >>> result = mesh.attach_data("velocity.dat")         # doctest: +SKIP

    The driver returns **only the variables it read**; this method merges them onto the
    dataset. That division is deliberate — it makes a name the dataset already defines an
    unambiguous collision, and means a driver cannot drop the dataset's own variables by
    rebuilding the mapping.

    The driver checks the file's declared element count against this mesh. That check is the
    only safeguard there is: a bare results file carries nothing identifying the mesh it
    belongs to, so attaching it to the wrong one would otherwise produce a plausible and
    entirely wrong dataset.

    Args:
        path: The results file to read.
        driver: Name of a registered driver to use, bypassing detection. ``None`` (the
            default) picks the ``needs_mesh`` driver claiming the file's suffix.
        lazy: Defer reading variable values until first access. Defaults to True.

    Returns:
        A new dataset carrying this mesh plus the attached variables. This dataset is
        unchanged.

    Raises:
        FileNotFoundError: If the results file does not exist.
        DriverNotExistError: If ``driver`` names a driver that is not installed.
        UnsupportedMeshFormatError: If no installed driver can attach this file, the
            named driver does not declare ``needs_mesh``, or the driver returned
            something other than a mapping of name to :class:`MeshVariable`.
        ValueError: If the driver returns a variable whose name this dataset already
            uses, or reports an element-count mismatch against this mesh.
    """
    _require_existing_file(path)
    selected = _resolve_attach_driver(path, driver)
    produced = _read_data_through_driver(selected, path, self, lazy)
    collisions = sorted(set(produced) & set(self._data_variables))
    if collisions:
        raise ValueError(
            f"The {selected.name!r} driver read variable(s) {collisions} from "
            f"{str(path)!r}, which this dataset already defines. Rename one side before "
            f"attaching, or drop the existing variable first."
        )
    merged = dict(self._data_variables)
    merged.update(produced)
    return type(self)(
        mesh=self._mesh,
        data_variables=merged,
        global_attributes=self._global_attributes,
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        volume_topology=self._volume_topology,
        file_name=self._file_name,
    )

to_file(path, driver=None, **options)

Write the dataset to a file.

Writes UGRID-compliant NetCDF by default: a topology variable, node coordinates, connectivity arrays, face/edge centres, data variables, and global attributes following the UGRID convention.

With driver, writes through a registered third-party driver instead — 2DM, Selafin, PLY and others are writable. The output format is never inferred from the suffix, so to_file("mesh.slf") still writes NetCDF; name the driver to change that.

Parameters:

Name	Type	Description	Default
path	str | Path	Output file path.	required
driver	str | None	Name of a registered mesh driver to write with, as declared by the plugin providing it (see :func:~pyramids.netcdf.ugrid.mesh_drivers) — not a GDAL driver short name, which is what driver means elsewhere in pyramids. None (the default) writes UGRID NetCDF through the built-in writer, and the output format is never inferred from path's suffix.	None
**options	Any	Format-specific settings for the driver, passed through uninterpreted (for example Selafin endianness, or PLY ascii versus binary). The built-in writer takes none, so passing any without a driver raises TypeError rather than dropping them silently.	{}

Raises:

Type	Description
DriverNotExistError	If driver names a driver that is not installed.
UnsupportedMeshFormatError	If the named driver cannot write a mesh.
TypeError	If options are given without a driver.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_file(
    self, path: str | Path, driver: str | None = None, **options: Any
) -> None:
    """Write the dataset to a file.

    Writes UGRID-compliant NetCDF by default: a topology variable, node coordinates,
    connectivity arrays, face/edge centres, data variables, and global attributes following
    the UGRID convention.

    With ``driver``, writes through a registered third-party driver instead — 2DM, Selafin,
    PLY and others are writable. The output format is never inferred from the suffix, so
    ``to_file("mesh.slf")`` still writes NetCDF; name the driver to change that.

    Args:
        path: Output file path.
        driver: Name of a registered **mesh** driver to write with, as declared by the
            plugin providing it (see :func:`~pyramids.netcdf.ugrid.mesh_drivers`) — not a
            GDAL driver short name, which is what ``driver`` means elsewhere in pyramids.
            ``None`` (the default) writes UGRID NetCDF through the built-in writer, and
            the output format is never inferred from ``path``'s suffix.
        **options: Format-specific settings for the driver, passed through uninterpreted
            (for example Selafin endianness, or PLY ascii versus binary). The built-in
            writer takes none, so passing any without a ``driver`` raises ``TypeError``
            rather than dropping them silently.

    Raises:
        DriverNotExistError: If ``driver`` names a driver that is not installed.
        UnsupportedMeshFormatError: If the named driver cannot write a mesh.
        TypeError: If ``options`` are given without a ``driver``.
    """
    if driver is not None:
        self._write_through_driver(path, driver, options)
    elif options:
        raise TypeError(
            f"to_file() got unexpected keyword argument(s) "
            f"{sorted(options)}; format options apply to a `driver=` write only."
        )
    else:
        self._write_ugrid_netcdf(Path(path))

to_geodataframe(variable_name=None, location='face')

Convert mesh to a GeoDataFrame.

For faces: each row is a Polygon with data columns. For nodes: each row is a Point. For edges: each row is a LineString.

Parameters:

Name	Type	Description	Default
variable_name	str | None	Optional data variable to include as a column.	None
location	str	Mesh location ("face", "node", or "edge").	'face'

Returns:

Type	Description
GeoDataFrame	geopandas GeoDataFrame.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_geodataframe(
    self,
    variable_name: str | None = None,
    location: str = "face",
) -> gpd.GeoDataFrame:
    """Convert mesh to a GeoDataFrame.

    For faces: each row is a Polygon with data columns.
    For nodes: each row is a Point.
    For edges: each row is a LineString.

    Args:
        variable_name: Optional data variable to include as a column.
        location: Mesh location ("face", "node", or "edge").

    Returns:
        geopandas GeoDataFrame.
    """
    geometries = self._build_geometries(location)

    data_dict: dict[str, Any] = {}
    if variable_name is not None:
        var = self.get_data(variable_name)
        if var.location == location:
            # For a temporal variable only the first step is tabulated; `sel_time(0)` reads just
            # that slab instead of loading every step to slice `[0]` (#982). `has_data_source`
            # avoids a temporal-specific `sel_time` error for a variable with no readable data
            # (checked without forcing a load — review L3).
            if var.has_time:
                var_data = var.sel_time(0) if var.has_data_source else None
            else:
                var_data = var.data
            # A variable with no readable data becomes a length-correct null column rather than
            # raising — pandas rejects a scalar `None` column ("must pass an index") (review L3).
            if var_data is None:
                var_data = np.full(len(geometries), np.nan)
            data_dict[variable_name] = var_data

    gdf = gpd.GeoDataFrame(data_dict, geometry=geometries)
    if self.crs is not None:
        gdf = gdf.set_crs(self.crs)

    result = gdf
    return result

to_feature_collection(variable_name=None, location='face')

Convert mesh to a pyramids FeatureCollection.

Parameters:

Name	Type	Description	Default
variable_name	str | None	Optional data variable to include.	None
location	str	Mesh location ("face", "node", or "edge").	'face'

Returns:

Type	Description
FeatureCollection	pyramids FeatureCollection.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_feature_collection(
    self,
    variable_name: str | None = None,
    location: str = "face",
) -> FeatureCollection:
    """Convert mesh to a pyramids FeatureCollection.

    Args:
        variable_name: Optional data variable to include.
        location: Mesh location ("face", "node", or "edge").

    Returns:
        pyramids FeatureCollection.
    """
    gdf = self.to_geodataframe(variable_name, location)
    result = FeatureCollection(gdf)
    return result

from_arrays(node_x, node_y, face_node_connectivity, data=None, data_locations=None, epsg=4326, mesh_name=DEFAULT_MESH_NAME) classmethod

Create a UgridDataset programmatically from arrays.

Plural because an unstructured mesh is not one array: the topology needs node coordinates and a face-node connectivity table before any data can be attached. It therefore takes a flat epsg rather than the :class:~pyramids.base.georeference.GeoReference the gridded constructors take — a mesh carries its own coordinates, so there is no affine transform to describe.

Parameters:

Name	Type	Description	Default
node_x	ndarray	Node x-coordinates.	required
node_y	ndarray	Node y-coordinates.	required
face_node_connectivity	ndarray	(n_faces, max_nodes) array of node indices. Use -1 as fill value for mixed meshes.	required
data	dict[str, ndarray] | None	Optional dict mapping variable name to data array.	None
data_locations	dict[str, str] | None	Optional dict mapping variable name to location ("face", "node", "edge"). Defaults to "face".	None
epsg	int	EPSG code for the CRS.	4326
mesh_name	str	Name for the topology variable.	DEFAULT_MESH_NAME

Returns:

Type	Description
UgridDataset	UgridDataset instance.

Examples:

• Build the smallest possible mesh — two triangles — and inspect its topology:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ... )
  >>> (mesh.n_node, mesh.n_face)
  (4, 2)
  >>> mesh.bounds
  (0.0, 0.0, 1.0, 1.0)
  

• Attach a per-face variable and read it back:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"depth": np.array([1.5, 2.5])},
  ... )
  >>> mesh.data_variable_names
  ['depth']
  >>> mesh["depth"].location
  'face'
  >>> mesh["depth"].data.tolist()
  [1.5, 2.5]
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_arrays(
    cls,
    node_x: np.ndarray,
    node_y: np.ndarray,
    face_node_connectivity: np.ndarray,
    data: dict[str, np.ndarray] | None = None,
    data_locations: dict[str, str] | None = None,
    epsg: int = 4326,
    mesh_name: str = DEFAULT_MESH_NAME,
) -> UgridDataset:
    """Create a UgridDataset programmatically from arrays.

    Plural because an unstructured mesh is not one array: the topology
    needs node coordinates *and* a face-node connectivity table before any
    data can be attached. It therefore takes a flat `epsg` rather than the
    :class:`~pyramids.base.georeference.GeoReference` the gridded
    constructors take — a mesh carries its own coordinates, so there is no
    affine transform to describe.

    Args:
        node_x: Node x-coordinates.
        node_y: Node y-coordinates.
        face_node_connectivity: (n_faces, max_nodes) array of node
            indices. Use -1 as fill value for mixed meshes.
        data: Optional dict mapping variable name to data array.
        data_locations: Optional dict mapping variable name to
            location ("face", "node", "edge"). Defaults to "face".
        epsg: EPSG code for the CRS.
        mesh_name: Name for the topology variable.

    Returns:
        UgridDataset instance.

    Examples:
        - Build the smallest possible mesh — two triangles — and inspect
          its topology:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ... )
            >>> (mesh.n_node, mesh.n_face)
            (4, 2)
            >>> mesh.bounds
            (0.0, 0.0, 1.0, 1.0)

            ```
        - Attach a per-face variable and read it back:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"depth": np.array([1.5, 2.5])},
            ... )
            >>> mesh.data_variable_names
            ['depth']
            >>> mesh["depth"].location
            'face'
            >>> mesh["depth"].data.tolist()
            [1.5, 2.5]

            ```
    """
    fnc = Connectivity(
        data=np.asarray(face_node_connectivity, dtype=np.intp),
        fill_value=-1,
        cf_role="face_node_connectivity",
        original_start_index=0,
    )
    mesh = Mesh2d(
        node_x=np.asarray(node_x, dtype=np.float64),
        node_y=np.asarray(node_y, dtype=np.float64),
        face_node_connectivity=fnc,
    )

    data_variables: dict[str, MeshVariable] = {}
    topo_data_vars: dict[str, str] = {}
    if data is not None:
        if data_locations is None:
            data_locations = {}
        for name, arr in data.items():
            loc = data_locations.get(name, "face")
            topo_data_vars[name] = loc
            data_variables[name] = MeshVariable(
                name=name,
                location=loc,
                mesh_name=mesh_name,
                shape=arr.shape,
                _data=arr,
            )

    srs = sr_from_epsg(epsg)
    crs_wkt = srs.ExportToWkt()

    topo_info = MeshTopologyInfo(
        mesh_name=mesh_name,
        topology_dimension=2,
        node_x_var=f"{mesh_name}_node_x",
        node_y_var=f"{mesh_name}_node_y",
        face_node_var=f"{mesh_name}_face_nodes",
        data_variables=topo_data_vars,
        crs_wkt=crs_wkt,
    )

    result = cls(
        mesh=mesh,
        data_variables=data_variables,
        global_attributes={"Conventions": UGRID_CONVENTIONS},
        topology_info=topo_info,
        crs_wkt=crs_wkt,
    )
    return result

from_structured(dataset, band_names=None, mesh_name=DEFAULT_MESH_NAME) classmethod

Build a mesh from a raster, one quad face per cell.

The bridge from the structured world: each cell becomes a quad over the corner lattice (shared corners deduplicated) and each band becomes a per-face variable, so a raster can be fed to the mesh operations.

Parameters:

Name	Type	Description	Default
dataset	Any	The source raster (a pyramids Dataset / NetCDF variable).	required
band_names	list[str] | None	Names for the per-face variables, one per band. Defaults to band_1, band_2, ….	None
mesh_name	str	The topology name to stamp. Defaults to the package default.	DEFAULT_MESH_NAME

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The quad mesh carrying the band values.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_structured(
    cls,
    dataset: Any,
    band_names: list[str] | None = None,
    mesh_name: str = DEFAULT_MESH_NAME,
) -> UgridDataset:
    """Build a mesh from a raster, one quad face per cell.

    The bridge from the structured world: each cell becomes a quad over the corner
    lattice (shared corners deduplicated) and each band becomes a per-face variable, so a
    raster can be fed to the mesh operations.

    Args:
        dataset: The source raster (a pyramids ``Dataset`` / ``NetCDF`` variable).
        band_names: Names for the per-face variables, one per band. Defaults to
            ``band_1``, ``band_2``, ….
        mesh_name: The topology name to stamp. Defaults to the package default.

    Returns:
        UgridDataset: The quad mesh carrying the band values.
    """
    node_x, node_y, faces, data = _construct.from_structured(dataset, band_names)
    return cls.from_arrays(
        node_x,
        node_y,
        faces,
        data=data,
        epsg=dataset.epsg,
        mesh_name=mesh_name,
    )

from_geodataframe(gdf, mesh_name=DEFAULT_MESH_NAME) classmethod

Build a mesh from polygon geometries, one face per polygon.

Vertices are deduplicated into the node set and each ring is oriented counter-clockwise; the frame's attribute columns become per-face variables.

Parameters:

Name	Type	Description	Default
gdf	Any	A GeoDataFrame of polygons. Its crs is carried onto the mesh verbatim, so a projected CRS stays projected even when it has no EPSG code.	required
mesh_name	str	The topology name to stamp. Defaults to the package default.	DEFAULT_MESH_NAME

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The mesh built from the polygons.

Raises:

Type	Description
ValueError	The frame declares no CRS, so the mesh would carry no georeferencing.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_geodataframe(
    cls, gdf: Any, mesh_name: str = DEFAULT_MESH_NAME
) -> UgridDataset:
    """Build a mesh from polygon geometries, one face per polygon.

    Vertices are deduplicated into the node set and each ring is oriented
    counter-clockwise; the frame's attribute columns become per-face variables.

    Args:
        gdf: A GeoDataFrame of polygons. Its ``crs`` is carried onto the mesh verbatim,
            so a projected CRS stays projected even when it has no EPSG code.
        mesh_name: The topology name to stamp. Defaults to the package default.

    Returns:
        UgridDataset: The mesh built from the polygons.

    Raises:
        ValueError: The frame declares no CRS, so the mesh would carry no
            georeferencing.
    """
    node_x, node_y, faces, data = _construct.from_geodataframe(gdf)
    # Carry the frame's own CRS. Reading an EPSG code and defaulting to 4326 stamped degrees
    # on a projected frame whose CRS has no EPSG code, and on a frame with no CRS at all --
    # silent georeferencing corruption, since everything downstream trusts `epsg`.
    crs = getattr(gdf, "crs", None)
    if crs is None:
        raise ValueError(
            "from_geodataframe: the GeoDataFrame declares no CRS, so the mesh would have no "
            "georeferencing. Set `gdf.crs` (e.g. gdf.set_crs(4326)) before converting."
        )
    epsg = crs.to_epsg()
    built = cls.from_arrays(
        node_x,
        node_y,
        faces,
        data=data,
        epsg=epsg if epsg is not None else 4326,
        mesh_name=mesh_name,
    )
    if epsg is not None:
        return built
    # A CRS with no EPSG code (a projected one given as WKT, say) would otherwise be
    # reduced to the 4326 placeholder `from_arrays` needs, stamping degrees on metres.
    # Keep the frame's WKT verbatim instead.
    wkt = crs.to_wkt()
    return cls(
        mesh=built.mesh,
        data_variables=built.data_vars,
        global_attributes={"Conventions": UGRID_CONVENTIONS},
        topology_info=_topology_info_named(mesh_name, crs_wkt=wkt),
        crs_wkt=wkt,
        file_name=None,
    )

from_xarray(ds) classmethod

Read a mesh back from an xarray.Dataset in the UGRID-CF encoding.

The inverse of :meth:to_xarray. xarray is a peer dependency, imported only inside the bridge, so nothing in this module imports it at module scope.

Parameters:

Name	Type	Description	Default
ds	Any	The xarray.Dataset to read.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The reconstructed mesh and variables.

Raises:

Type	Description
OptionalPackageDoesNotExist	xarray is not installed.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_xarray(cls, ds: Any) -> UgridDataset:
    """Read a mesh back from an ``xarray.Dataset`` in the UGRID-CF encoding.

    The inverse of :meth:`to_xarray`. xarray is a peer dependency, imported only inside
    the bridge, so nothing in this module imports it at module scope.

    Args:
        ds: The ``xarray.Dataset`` to read.

    Returns:
        UgridDataset: The reconstructed mesh and variables.

    Raises:
        OptionalPackageDoesNotExist: xarray is not installed.
    """
    mesh, data_variables = _interop.from_xarray(ds)
    mesh_name = _interop.read_mesh_name(ds)
    return cls(
        mesh=mesh,
        data_variables=data_variables,
        global_attributes={},
        # Keep the topology's own name: without it a mesh called anything other than
        # "mesh2d" came back renamed, and a re-export then wrote the wrong dimensions.
        topology_info=_topology_info_named(mesh_name),
        crs_wkt=_interop.read_crs_wkt(ds),
        file_name=None,
        # Rebuild the ragged volume topology from its companion arrays when present
        # (issue #1330); `None` for a 2-D / uniform mesh, leaving the dense path in force.
        volume_topology=_interop.volume_topology_from_xarray(ds),
    )

from_zarr(store, **kwargs) classmethod

Read a mesh from a zarr store written by :meth:to_zarr.

Parameters:

Name	Type	Description	Default
store	Any	The zarr store (a path or a mapping).	required
**kwargs	Any	Passed to xarray.open_zarr.	{}

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The reconstructed mesh and variables.

Raises:

Type	Description
OptionalPackageDoesNotExist	xarray or zarr is not installed.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_zarr(cls, store: Any, **kwargs: Any) -> UgridDataset:
    """Read a mesh from a zarr store written by :meth:`to_zarr`.

    Args:
        store: The zarr store (a path or a mapping).
        **kwargs: Passed to ``xarray.open_zarr``.

    Returns:
        UgridDataset: The reconstructed mesh and variables.

    Raises:
        OptionalPackageDoesNotExist: xarray or zarr is not installed.
    """
    # One open, read twice: the mesh and the CRS both come off the same dataset, so the
    # store is not opened again just to recover the grid mapping.
    opened = _interop.open_zarr_dataset(store, **kwargs)
    mesh, data_variables = _interop.from_xarray(opened)
    return cls(
        mesh=mesh,
        data_variables=data_variables,
        global_attributes={},
        topology_info=_topology_info_named(_interop.read_mesh_name(opened)),
        crs_wkt=_interop.read_crs_wkt(opened),
        file_name=None,
    )

mesh_names(path, driver=None) staticmethod

The names of every topology a file declares.

A mesh file may carry several topologies; :meth:read_file takes the first unless told otherwise, so this is how a caller discovers what else is in there.

Dispatch mirrors :meth:read_file exactly, including the short-circuit that keeps a NetCDF suffix from triggering driver discovery. Every driver is asked, not only those declaring multi_mesh: a single-mesh format answers with a one-item list, which is the right answer rather than an error.

Parameters:

Name	Type	Description	Default
path	str | Path	Path to the file.	required
driver	str | None	Name of a registered driver to ask, bypassing detection. None (the default) resolves as described above.	None

Returns:

Type	Description
list[str]	list[str]: One name per declared topology, in file order.

Raises:

Type	Description
FileNotFoundError	The file does not exist.
DriverNotExistError	driver names a driver that is not installed.
UnsupportedMeshFormatError	No installed driver claims the suffix and the built-in reader cannot open it either, or a driver failed or returned something other than a list of topology names.
IsADirectoryError	The path is a directory that no driver claims.
RuntimeError	GDAL cannot open a file that reached the built-in reader by its own suffix. GDAL exceptions are enabled package-wide, so the open failure surfaces as GDAL's own error.
ValueError	The file declares no UGRID topology.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@staticmethod
def mesh_names(path: str | Path, driver: str | None = None) -> list[str]:
    """The names of every topology a file declares.

    A mesh file may carry several topologies; :meth:`read_file` takes the first unless told
    otherwise, so this is how a caller discovers what else is in there.

    Dispatch mirrors :meth:`read_file` exactly, including the short-circuit that keeps a
    NetCDF suffix from triggering driver discovery. Every driver is asked, not only those
    declaring ``multi_mesh``: a single-mesh format answers with a one-item list, which is
    the right answer rather than an error.

    Args:
        path: Path to the file.
        driver: Name of a registered driver to ask, bypassing detection. ``None`` (the
            default) resolves as described above.

    Returns:
        list[str]: One name per declared topology, in file order.

    Raises:
        FileNotFoundError: The file does not exist.
        DriverNotExistError: ``driver`` names a driver that is not installed.
        UnsupportedMeshFormatError: No installed driver claims the suffix and the
            built-in reader cannot open it either, or a driver failed or returned
            something other than a list of topology names.
        IsADirectoryError: The path is a directory that no driver claims.
        RuntimeError: GDAL cannot open a file that reached the built-in reader by its own
            suffix. GDAL exceptions are enabled package-wide, so the open failure
            surfaces as GDAL's own error.
        ValueError: The file declares no UGRID topology.
    """
    selected, native = _resolve_read_driver(path, driver)
    if selected is not None:
        result = _mesh_names_through_driver(selected, path)
    else:
        try:
            result = UgridDataset._ugrid_netcdf_mesh_names(
                path, wrap_unreadable=not native
            )
        except NoMeshTopologyError:
            fallback = _netcdf_fallback_driver(path, driver)
            if fallback is None:
                raise
            result = _mesh_names_through_driver(fallback, path)
    return result

read_all(path, driver=None, lazy=True) classmethod

Read every topology in a file, one dataset each.

:meth:read_file answers a single mesh (the first, or the one named). This answers all of them, so nothing in a multi-mesh file is silently dropped. Dispatch follows :meth:mesh_names and :meth:read_file, so a third-party format works here too.

Parameters:

Name	Type	Description	Default
path	str | Path	Path to the file.	required
driver	str | None	Name of a registered mesh driver to use, bypassing detection. None (the default) detects.	None
lazy	bool	Defer reading variable values until first access. Forwarded to a driver, which may honour an eager read; the built-in UGRID reader has no eager mode, so lazy=False on a native read raises :class:TypeError, exactly as :meth:read_file does. Without it a multi-topology third-party format could only ever be read lazily.	True

Returns:

Type	Description
dict[str, UgridDataset]	dict[str, UgridDataset]: Each topology name mapped to its dataset.

Raises:

Type	Description
FileNotFoundError	The file does not exist.
DriverNotExistError	driver names a driver that is not installed.
UnsupportedMeshFormatError	No installed driver reads the file and the built-in reader cannot either — raised from :meth:mesh_names, which this delegates to, so every error that method raises surfaces here too.
IsADirectoryError	The path is a directory that no driver claims.
RuntimeError	GDAL cannot open a file that reached the built-in reader by its own suffix. GDAL exceptions are enabled package-wide, so the open failure surfaces as GDAL's own error.
TypeError	lazy=False on a native (built-in UGRID) read, which has no eager mode -- the same rejection :meth:read_file makes.
ValueError	The file declares no UGRID topology.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def read_all(
    cls, path: str | Path, driver: str | None = None, lazy: bool = True
) -> dict[str, UgridDataset]:
    """Read **every** topology in a file, one dataset each.

    :meth:`read_file` answers a single mesh (the first, or the one named). This answers
    all of them, so nothing in a multi-mesh file is silently dropped. Dispatch follows
    :meth:`mesh_names` and :meth:`read_file`, so a third-party format works here too.

    Args:
        path: Path to the file.
        driver: Name of a registered **mesh** driver to use, bypassing detection.
            ``None`` (the default) detects.
        lazy: Defer reading variable values until first access. Forwarded to a driver,
            which may honour an eager read; the built-in UGRID reader has no eager mode, so
            ``lazy=False`` on a native read raises :class:`TypeError`, exactly as
            :meth:`read_file` does. Without it a multi-topology third-party format could
            only ever be read lazily.

    Returns:
        dict[str, UgridDataset]: Each topology name mapped to its dataset.

    Raises:
        FileNotFoundError: The file does not exist.
        DriverNotExistError: ``driver`` names a driver that is not installed.
        UnsupportedMeshFormatError: No installed driver reads the file and the built-in
            reader cannot either — raised from :meth:`mesh_names`, which this delegates
            to, so every error that method raises surfaces here too.
        IsADirectoryError: The path is a directory that no driver claims.
        RuntimeError: GDAL cannot open a file that reached the built-in reader by its own
            suffix. GDAL exceptions are enabled package-wide, so the open failure
            surfaces as GDAL's own error.
        TypeError: ``lazy=False`` on a native (built-in UGRID) read, which has no eager
            mode -- the same rejection :meth:`read_file` makes.
        ValueError: The file declares no UGRID topology.
    """
    # Resolve once, for real. Passing a driver *name* through only short-circuited the
    # route where detection had found something: when nothing claimed the suffix the name
    # was None, so `mesh_names` and every per-topology read re-ran full detection anyway
    # -- measured at two `can_open` calls for one `read_all`, growing per topology.
    # Threading the resolved object through covers the fallback route too.
    selected, native = _resolve_read_driver(path, driver)
    # Set in the native branch below; stays empty whenever a driver handles the file --
    # resolved up front, or picked by the fallback -- in which case it is never read.
    native_names: list[str] = []
    if selected is None:
        try:
            native_names = cls._ugrid_netcdf_mesh_names(
                path, wrap_unreadable=not native
            )
        except NoMeshTopologyError:
            # The file opened as NetCDF but declares no UGRID topology; fall back to a
            # driver claiming the suffix, as read_file does. If none claims it, the
            # original error stands.
            selected = _netcdf_fallback_driver(path, driver)
            if selected is None:
                raise
    if selected is None:
        result = {
            name: cls._read_ugrid_netcdf(
                path, name, wrap_unreadable=not native, lazy=lazy
            )
            for name in native_names
        }
    else:
        names = _mesh_names_through_driver(selected, path)
        result = {
            name: _read_through_driver(selected, path, name, cls, lazy)
            for name in names
        }
    return result

isel_layer(index)

Select along the layer axis by position.

A layered variable — (layer, element), or (time, layer, element) — is indexed on its layer axis. An integer collapses that axis (the variable loses its layer dimension); a slice or sequence keeps it, trimmed. Variables with no layer axis are carried through untouched, as the time selectors carry static variables.

For a ragged 3-D mesh (:attr:volume_topology set) a location="volume" variable has no rectangular layer axis to index; the selection is per column instead. index must be an int k: each column yields its k-th layer where it has one, and a column shallower than k + 1 layers yields NaN — the absent layers are made explicit, never padded silently. The volume variable becomes a plain location="face" field; other variables carry through as on the uniform path.

Parameters:

Name	Type	Description	Default
index	int | slice | list[int] | ndarray	An int layer, a slice, or a sequence of layer positions. A ragged mesh accepts only an int.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with the layer selection applied.

Raises:

Type	Description
ValueError	No variable has a layer dimension, or a non-int index was given for a ragged mesh.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def isel_layer(self, index: int | slice | list[int] | np.ndarray) -> UgridDataset:
    """Select along the layer axis by position.

    A layered variable — ``(layer, element)``, or ``(time, layer, element)`` — is indexed
    on its layer axis. An integer collapses that axis (the variable loses its layer
    dimension); a slice or sequence keeps it, trimmed. Variables with no layer axis are
    carried through untouched, as the time selectors carry static variables.

    For a **ragged** 3-D mesh (:attr:`volume_topology` set) a ``location="volume"``
    variable has no rectangular layer axis to index; the selection is per column instead.
    ``index`` must be an ``int`` ``k``: each column yields its ``k``-th layer where it has
    one, and a column shallower than ``k + 1`` layers yields NaN — the absent layers are
    made explicit, never padded silently. The volume variable becomes a plain
    ``location="face"`` field; other variables carry through as on the uniform path.

    Args:
        index: An ``int`` layer, a ``slice``, or a sequence of layer positions. A ragged
            mesh accepts only an ``int``.

    Returns:
        UgridDataset: A new dataset with the layer selection applied.

    Raises:
        ValueError: No variable has a layer dimension, or a non-``int`` ``index`` was given
            for a ragged mesh.
    """
    if self._volume_topology is not None:
        return self._isel_layer_ragged(index)
    new_vars: dict[str, MeshVariable] = {}
    layered = False
    for name, var in self._data_variables.items():
        axis = self._layer_axis(var)
        if axis is None:
            new_vars[name] = var
            continue
        layered = True
        data = np.asarray(var.data)
        if isinstance(index, (int, np.integer)):
            picked = np.take(data, int(index), axis=axis)
            moved = var.with_data(picked)
            moved.dimensions = tuple(
                dim
                for position, dim in enumerate(var.dimensions)
                if position != axis
            )
        else:
            positions = (
                np.arange(data.shape[axis])[index]
                if isinstance(index, slice)
                else np.asarray(index, dtype=np.intp)
            )
            moved = var.with_data(np.take(data, positions, axis=axis))
        new_vars[name] = moved
    if not layered:
        raise ValueError(
            "isel_layer needs at least one variable with a layer dimension; this "
            "dataset has none."
        )
    return self._rebuild(new_vars)

sel_layer(value)

Select along the layer axis by coordinate value.

Matches value against :attr:layer_values, falling back to layer positions when no layer coordinate is present -- which is the usual case, since the reader does not populate layer_values (see :attr:layer_values). So on a mesh read from a file, sel_layer(1) selects layer position 1.

Parameters:

Name	Type	Description	Default
value	Any	The layer coordinate value to select.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with that layer selected.

Raises:

Type	Description
ValueError	No layer dimension, or the value is not among the layer coordinates.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sel_layer(self, value: Any) -> UgridDataset:
    """Select along the layer axis by coordinate value.

    Matches ``value`` against :attr:`layer_values`, falling back to layer positions when
    no layer coordinate is present -- which is the usual case, since the reader does not
    populate `layer_values` (see :attr:`layer_values`). So on a mesh read from a file,
    ``sel_layer(1)`` selects layer position 1.

    Args:
        value: The layer coordinate value to select.

    Returns:
        UgridDataset: A new dataset with that layer selected.

    Raises:
        ValueError: No layer dimension, or the value is not among the layer coordinates.
    """
    count = self.n_layers
    if count is None:
        raise ValueError(
            "sel_layer needs at least one variable with a layer dimension; this "
            "dataset has none."
        )
    # Read the property once: it walks every variable, so evaluating it twice walked
    # them twice.
    stored = self.layer_values
    coords = stored if stored is not None else list(range(count))
    if value not in coords:
        raise ValueError(
            f"sel_layer: layer value {value!r} not found in the layer coordinate "
            f"{coords!r}."
        )
    return self.isel_layer(coords.index(value))

collapse_layer(layer, *, location='face')

Collapse a ragged 3-D mesh's volume variables to a 2-D field (issue #1330).

The explicit 2-D collapse of a ragged mesh: each column's layer-th value becomes a plain (n_face,) (or (time, n_face)) face field, absent layers NaN. It is :meth:isel_layer under a name that says "give me a 2-D slice", so a caller can ask for the surface/bed layer as a flat field without reaching for the per-column internals. (Named collapse_layer rather than to_location because the latter already moves a variable between mesh locations by graph interpolation — a different operation.)

Parameters:

Name	Type	Description	Default
layer	int	The 0-based layer position to take from each column.	required
location	str	The target mesh location. Only "face" is supported — a ragged volume collapses onto its columns' faces. Defaults to "face".	'face'

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset whose volume variables are 2-D face fields.

Raises:

Type	Description
ValueError	location is not "face", or the dataset has no volume topology to collapse.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def collapse_layer(self, layer: int, *, location: str = "face") -> UgridDataset:
    """Collapse a ragged 3-D mesh's ``volume`` variables to a 2-D field (issue #1330).

    The explicit 2-D collapse of a ragged mesh: each column's ``layer``-th value becomes a
    plain ``(n_face,)`` (or ``(time, n_face)``) face field, absent layers NaN. It is
    :meth:`isel_layer` under a name that says "give me a 2-D slice", so a caller can ask for
    the surface/bed layer as a flat field without reaching for the per-column internals.
    (Named ``collapse_layer`` rather than ``to_location`` because the latter already moves a
    variable *between* mesh locations by graph interpolation — a different operation.)

    Args:
        layer: The 0-based layer position to take from each column.
        location: The target mesh location. Only ``"face"`` is supported — a ragged volume
            collapses onto its columns' faces. Defaults to ``"face"``.

    Returns:
        UgridDataset: A new dataset whose volume variables are 2-D face fields.

    Raises:
        ValueError: ``location`` is not ``"face"``, or the dataset has no volume topology
            to collapse.
    """
    if self._volume_topology is None:
        raise ValueError(
            "collapse_layer needs a ragged volume topology; this dataset has none (it is "
            "2-D or uniformly layered). Use isel_layer for a uniform mesh."
        )
    if location != "face":
        raise ValueError(
            f"collapse_layer supports only 'face' for a ragged volume mesh, not "
            f"{location!r}."
        )
    return self._isel_layer_ragged(int(layer))

to_dataframe(variables=None, *, dropna=False)

A tidy frame of the mesh values, one row per element (and step).

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.to_dataframe, in the tidy shape rather than the raster (y, x) MultiIndex one: an unstructured element has no row/column address, so each row carries the element index, its centroid coordinates, the time step where the variable has one, and the value.

Parameters:

Name	Type	Description	Default
variables	Sequence[str] | None	Which data variables to include. None (the default) takes them all.	None
dropna	bool	Drop rows whose value is no-data. Defaults to False.	False

Returns:

Type	Description
DataFrame	pandas.DataFrame: Columns variable, element, x, y, layer,
DataFrame	time and value. element indexes the facet the variable sits on and
DataFrame	x/y are that facet's coordinates, so a node variable is addressed by node.
DataFrame	layer is the layer coordinate, or None for an unlayered variable, and time
DataFrame	the step coordinate, or None for a static one.

Raises:

Type	Description
KeyError	A requested variable is not a data variable of this dataset.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def to_dataframe(
    self, variables: Sequence[str] | None = None, *, dropna: bool = False
) -> pd.DataFrame:
    """A tidy frame of the mesh values, one row per element (and step).

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.to_dataframe`, in the **tidy**
    shape rather than the raster ``(y, x)`` MultiIndex one: an unstructured element has no
    row/column address, so each row carries the element index, its centroid coordinates,
    the time step where the variable has one, and the value.

    Args:
        variables: Which data variables to include. ``None`` (the default) takes them all.
        dropna: Drop rows whose value is no-data. Defaults to False.

    Returns:
        pandas.DataFrame: Columns ``variable``, ``element``, ``x``, ``y``, ``layer``,
        ``time`` and ``value``. ``element`` indexes the facet the variable sits on and
        ``x``/``y`` are that facet's coordinates, so a node variable is addressed by node.
        ``layer`` is the layer coordinate, or None for an unlayered variable, and ``time``
        the step coordinate, or None for a static one.

    Raises:
        KeyError: A requested variable is not a data variable of this dataset.
    """
    names = list(self._data_variables) if variables is None else list(variables)
    frames: list[pd.DataFrame] = []
    for name in names:
        if name not in self._data_variables:
            raise KeyError(name)
        var = self._data_variables[name]
        if var.data is None:
            continue
        frames.extend(self._variable_frames(name, var))
    if not frames:
        # Match the populated frame's dtypes, so a caller that concatenates or
        # dtype-checks sees one schema either way.
        return pd.DataFrame(
            {
                "variable": pd.Series(dtype="str"),
                "element": pd.Series(dtype="int64"),
                "x": pd.Series(dtype="float64"),
                "y": pd.Series(dtype="float64"),
                "layer": pd.Series(dtype="object"),
                "time": pd.Series(dtype="object"),
                "value": pd.Series(dtype="float64"),
            }
        )
    frame = pd.concat(frames, ignore_index=True)
    if dropna:
        frame = frame[~frame["value"].isna()].reset_index(drop=True)
    return frame

copy()

An independent deep copy of this dataset.

Every variable's values are copied, so writing into the copy never shows in the original — the counterpart of :meth:pyramids.netcdf.NetCDF.copy.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The copy.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def copy(self) -> UgridDataset:
    """An independent deep copy of this dataset.

    Every variable's values are copied, so writing into the copy never shows in the
    original — the counterpart of :meth:`pyramids.netcdf.NetCDF.copy`.

    Returns:
        UgridDataset: The copy.
    """
    copied: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        copied[name] = (
            var
            if var.data is None
            else var.with_data(np.array(var.data, copy=True))
        )
    return type(self)(
        mesh=self._mesh,
        data_variables=copied,
        global_attributes=dict(self._global_attributes),
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )

from_bytes(data, *, suffix='.nc') classmethod

Read a UGRID mesh from an in-memory buffer.

Writes data into GDAL's in-memory filesystem and reads it as a file, mirroring :meth:pyramids.netcdf.NetCDF.from_bytes — for a mesh fetched over the network or held in a database blob. The values are materialised before the buffer is unlinked, so the result does not depend on it.

Parameters:

Name	Type	Description	Default
data	bytes	The bytes of a UGRID NetCDF file.	required
suffix	str	The extension to present to GDAL. Defaults to ".nc".	'.nc'

Returns:

Name	Type	Description
UgridDataset	UgridDataset	The mesh read from the buffer.

Raises:

Type	Description
RuntimeError	GDAL cannot read the buffer at all. It is written to /vsimem and read through :meth:read_file, so GDAL's own error comes back -- naming the in-memory path rather than the caller's bytes.
ValueError	The buffer reads as a file but declares no UGRID topology.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@classmethod
def from_bytes(cls, data: bytes, *, suffix: str = ".nc") -> UgridDataset:
    """Read a UGRID mesh from an in-memory buffer.

    Writes ``data`` into GDAL's in-memory filesystem and reads it as a file, mirroring
    :meth:`pyramids.netcdf.NetCDF.from_bytes` — for a mesh fetched over the network or
    held in a database blob. The values are materialised before the buffer is unlinked, so
    the result does not depend on it.

    Args:
        data: The bytes of a UGRID NetCDF file.
        suffix: The extension to present to GDAL. Defaults to ``".nc"``.

    Returns:
        UgridDataset: The mesh read from the buffer.

    Raises:
        RuntimeError: GDAL cannot read the buffer at all. It is written to ``/vsimem`` and
            read through :meth:`read_file`, so GDAL's own error comes back -- naming the
            in-memory path rather than the caller's bytes.
        ValueError: The buffer reads as a file but declares no UGRID topology.
    """
    name = f"/vsimem/pyramids_ugrid_{uuid4().hex}{suffix}"
    gdal.FileFromMemBuffer(name, data)
    try:
        result = cls.read_file(name).copy()
    finally:
        gdal.Unlink(name)
    return result

info(buf=None, drivers=False)

Print a summary of the mesh, its variables and its attributes.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.info: one readable block naming the topology, the element counts, the CRS, and every data variable with its location, dtype and shape.

Parameters:

Name	Type	Description	Default
buf	Any	Where to write. Defaults to sys.stdout.	None
drivers	bool	Also list the mesh file-format drivers available in this interpreter — the built-in UGRID reader plus any installed plugin. Off by default because resolving them walks every installed distribution, which a summary of one dataset should not pay for.	False

Source code in src/pyramids/netcdf/ugrid/dataset.py

def info(self, buf: Any = None, drivers: bool = False) -> None:
    """Print a summary of the mesh, its variables and its attributes.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.info`: one readable block naming
    the topology, the element counts, the CRS, and every data variable with its location,
    dtype and shape.

    Args:
        buf: Where to write. Defaults to ``sys.stdout``.
        drivers: Also list the mesh file-format drivers available in this interpreter —
            the built-in UGRID reader plus any installed plugin. Off by default because
            resolving them walks every installed distribution, which a summary of one
            dataset should not pay for.
    """
    stream = sys.stdout if buf is None else buf
    lines = [
        f"UgridDataset: {self.mesh_name}",
        f"  nodes: {self.n_node}  faces: {self.n_face}  edges: {self.n_edge}",
        f"  epsg: {self.epsg}",
        f"  variables ({len(self._data_variables)}):",
    ]
    for name, var in self._data_variables.items():
        # Metadata only: `var.data` would fire the lazy loader and pull every variable's
        # full array off disk, which is the opposite of what a summary is for. The declared
        # shape and `has_data_source` answer the same questions without a read.
        shape = str(tuple(var.shape)) if var.has_data_source else "unloaded"
        temporal = "temporal" if var.has_time else "static"
        lines.append(
            f"    {name}: location={var.location} dtype={var.dtype} "
            f"shape={shape} ({temporal})"
        )
    if self._global_attributes:
        lines.append(f"  global attributes ({len(self._global_attributes)}):")
        lines.extend(
            f"    {key}: {value}" for key, value in self._global_attributes.items()
        )
    if drivers:
        driver_lines = describe_drivers()
        # Count the plugins, not the lines: reporting "mesh drivers (1)" on a bare install
        # counted the built-in reader as a driver, while DriverNotExistError described the
        # same state as having none installed.
        plugins = len(driver_lines) - 1
        suffix = "" if plugins == 1 else "s"
        lines.append(f"  mesh drivers (built-in + {plugins} plugin{suffix}):")
        lines.extend(f"    {line}" for line in driver_lines)
    stream.write("\n".join(lines) + "\n")

set_global_attribute(name, value)

Return a new dataset carrying one more global attribute.

A UGRID derivation is immutable here (as :meth:with_variable is), so this answers a new dataset rather than writing in place; :meth:to_file then persists the attributes through the shared CF writer.

Parameters:

Name	Type	Description	Default
name	str	The attribute name.	required
value	Any	The attribute value.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset whose global attributes include name.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def set_global_attribute(self, name: str, value: Any) -> UgridDataset:
    """Return a new dataset carrying one more global attribute.

    A UGRID derivation is immutable here (as :meth:`with_variable` is), so this answers a
    new dataset rather than writing in place; :meth:`to_file` then persists the attributes
    through the shared CF writer.

    Args:
        name: The attribute name.
        value: The attribute value.

    Returns:
        UgridDataset: A new dataset whose global attributes include ``name``.
    """
    return type(self)(
        mesh=self._mesh,
        data_variables=dict(self._data_variables),
        global_attributes={**self._global_attributes, name: value},
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )

delete_global_attribute(name)

Return a new dataset without the global attribute name.

Parameters:

Name	Type	Description	Default
name	str	The attribute to drop. Dropping an absent attribute is not an error, mirroring the raster delete_global_attribute.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset without that attribute.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def delete_global_attribute(self, name: str) -> UgridDataset:
    """Return a new dataset without the global attribute ``name``.

    Args:
        name: The attribute to drop. Dropping an absent attribute is not an error,
            mirroring the raster ``delete_global_attribute``.

    Returns:
        UgridDataset: A new dataset without that attribute.
    """
    remaining = {
        key: value for key, value in self._global_attributes.items() if key != name
    }
    return type(self)(
        mesh=self._mesh,
        data_variables=dict(self._data_variables),
        global_attributes=remaining,
        topology_info=self._topology_info,
        crs_wkt=self._crs_wkt,
        file_name=self._file_name,
        volume_topology=self._volume_topology,
    )

active_mask(variable_name)

The variable's per-element active (wet/dry) mask, or None when it has none.

The first-class companion of the data (issue #1331): True marks an active/wet element, False a dry one, kept separate from nodata/NaN so a consumer can tell a dry element from a missing value. The mask is element-indexed — (n_element,) when it is static or (n_time, n_element) when it varies per step — and is returned as read, never applied to the values. Use :meth:masked_values to get the values with it applied.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable.	required

Returns:

Type	Description
NDArray | None	np.ndarray | None: The boolean mask, or None when the variable carries none.

Raises:

Type	Description
KeyError	variable_name is not a data variable of this dataset.

Examples:

• A variable with a mask reports it; one without answers None:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> ds = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"depth": np.array([2.0, 4.0])},
  ... )
  >>> ds["depth"].active = np.array([True, False])
  >>> ds.active_mask("depth").tolist()
  [True, False]
  >>> ds.active_mask("depth") is not None
  True
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def active_mask(self, variable_name: str) -> np.typing.NDArray | None:
    """The variable's per-element active (wet/dry) mask, or ``None`` when it has none.

    The first-class companion of the data (issue #1331): ``True`` marks an active/wet
    element, ``False`` a dry one, kept separate from ``nodata``/NaN so a consumer can tell
    a dry element from a missing value. The mask is element-indexed — ``(n_element,)``
    when it is static or ``(n_time, n_element)`` when it varies per step — and is returned
    as read, never applied to the values. Use :meth:`masked_values` to get the values with
    it applied.

    Args:
        variable_name: Name of the data variable.

    Returns:
        np.ndarray | None: The boolean mask, or ``None`` when the variable carries none.

    Raises:
        KeyError: ``variable_name`` is not a data variable of this dataset.

    Examples:
        - A variable with a mask reports it; one without answers ``None``:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> ds = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"depth": np.array([2.0, 4.0])},
            ... )
            >>> ds["depth"].active = np.array([True, False])
            >>> ds.active_mask("depth").tolist()
            [True, False]
            >>> ds.active_mask("depth") is not None
            True

            ```
    """
    var = self.get_data(variable_name)
    return None if var.active is None else np.asarray(var.active, dtype=bool)

masked_values(variable_name)

The variable's values with both nodata and the active mask blanked to NaN.

The opt-in counterpart of a bare .data read (issue #1331): dry elements (where :meth:active_mask is False) become NaN on top of the usual nodata blanking, so a caller that wants "treat dry as missing" gets it explicitly — the values are never masked by default. Keeps the variable's full shape (every time step and layer), unlike the single-step :meth:sample / :meth:weighted path. When the variable has no mask the result is just the nodata-blanked values.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable.	required

Returns:

Type	Description
NDArray	np.ndarray: The float64 values, dry and missing elements alike as NaN.

Raises:

Type	Description
KeyError	variable_name is not a data variable of this dataset.
ValueError	The variable has no loaded data.

Examples:

• Dry elements become NaN, so a mean over the wet cells is straightforward:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> ds = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"depth": np.array([2.0, 4.0])},
  ... )
  >>> ds["depth"].active = np.array([True, False])
  >>> ds.masked_values("depth").tolist()
  [2.0, nan]
  >>> float(np.nanmean(ds.masked_values("depth")))
  2.0
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def masked_values(self, variable_name: str) -> np.typing.NDArray:
    """The variable's values with both ``nodata`` **and** the active mask blanked to NaN.

    The **opt-in** counterpart of a bare ``.data`` read (issue #1331): dry elements (where
    :meth:`active_mask` is ``False``) become NaN on top of the usual ``nodata`` blanking,
    so a caller that wants "treat dry as missing" gets it explicitly — the values are
    never masked by default. Keeps the variable's full shape (every time step and layer),
    unlike the single-step :meth:`sample` / :meth:`weighted` path. When the variable has no
    mask the result is just the ``nodata``-blanked values.

    Args:
        variable_name: Name of the data variable.

    Returns:
        np.ndarray: The float64 values, dry and missing elements alike as NaN.

    Raises:
        KeyError: ``variable_name`` is not a data variable of this dataset.
        ValueError: The variable has no loaded data.

    Examples:
        - Dry elements become NaN, so a mean over the wet cells is straightforward:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> ds = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"depth": np.array([2.0, 4.0])},
            ... )
            >>> ds["depth"].active = np.array([True, False])
            >>> ds.masked_values("depth").tolist()
            [2.0, nan]
            >>> float(np.nanmean(ds.masked_values("depth")))
            2.0

            ```
    """
    var = self.get_data(variable_name)
    data = var.data
    if data is None:
        raise ValueError(f"Variable {variable_name!r} has no loaded data.")
    values = gaps_as_nan(np.asarray(data), var.nodata)
    if var.active is not None:
        values = _apply_active_mask(var, values)
    return cast("np.typing.NDArray", values)

summary(variables=None, *, metrics=DEFAULT_METRICS, skipna=True, ddof=0)

A per-variable summary table over the whole mesh.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.summary, built on the same shared assembly (:func:pyramids.base._summary.variable_summary) so the two classes answer in an identical shape. Each variable is reduced over all of its samples — every element and, for a temporal variable, every step — with the declared no-data value left out.

Parameters:

Name	Type	Description	Default
variables	Sequence[str] | None	Which data variables to summarise, in output-row order. None (the default) takes every data variable. An unknown name raises KeyError.	None
metrics	Sequence[str]	Which statistics to report, in column order. Defaults to ("count", "min", "max", "mean", "std"); median / var / sum / prod are available too.	DEFAULT_METRICS
skipna	bool	Leave no-data out of the statistics (the default) or let it propagate.	True
ddof	int	Delta degrees of freedom for std / var. Defaults to 0.	0

Returns:

Type	Description
DataFrame	pandas.DataFrame: One row per variable (index name "variable"), one column per
DataFrame	metric. count is int64; the rest are float64. A variable with no valid
DataFrame	element yields NaN statistics and a count of 0.

Raises:

Type	Description
KeyError	A requested variable is not a data variable of this dataset.
TypeError	A requested variable is non-numeric.
ValueError	A requested variable has no loaded data, or an unknown metric.

Examples:

• Per-variable summary of a two-triangle mesh:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"depth": np.array([2.0, 4.0])},
  ... )
  >>> mesh.summary().loc["depth", ["count", "min", "max", "mean"]].tolist()
  [2.0, 2.0, 4.0, 3.0]
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def summary(
    self,
    variables: Sequence[str] | None = None,
    *,
    metrics: Sequence[str] = DEFAULT_METRICS,
    skipna: bool = True,
    ddof: int = 0,
) -> pd.DataFrame:
    """A per-variable summary table over the whole mesh.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.summary`, built on the same shared
    assembly (:func:`pyramids.base._summary.variable_summary`) so the two classes answer in an
    identical shape. Each variable is reduced over **all** of its samples — every element and,
    for a temporal variable, every step — with the declared no-data value left out.

    Args:
        variables: Which data variables to summarise, in output-row order. ``None`` (the
            default) takes every data variable. An unknown name raises ``KeyError``.
        metrics: Which statistics to report, in column order. Defaults to
            ``("count", "min", "max", "mean", "std")``; ``median`` / ``var`` / ``sum`` /
            ``prod`` are available too.
        skipna: Leave no-data out of the statistics (the default) or let it propagate.
        ddof: Delta degrees of freedom for ``std`` / ``var``. Defaults to 0.

    Returns:
        pandas.DataFrame: One row per variable (index name ``"variable"``), one column per
        metric. ``count`` is ``int64``; the rest are ``float64``. A variable with no valid
        element yields NaN statistics and a ``count`` of 0.

    Raises:
        KeyError: A requested variable is not a data variable of this dataset.
        TypeError: A requested variable is non-numeric.
        ValueError: A requested variable has no loaded data, or an unknown metric.

    Examples:
        - Per-variable summary of a two-triangle mesh:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"depth": np.array([2.0, 4.0])},
            ... )
            >>> mesh.summary().loc["depth", ["count", "min", "max", "mean"]].tolist()
            [2.0, 2.0, 4.0, 3.0]

            ```
    """
    names = list(self._data_variables) if variables is None else list(variables)
    arrays: dict[str, np.typing.NDArray] = {}
    for name in names:
        if name not in self._data_variables:
            raise KeyError(name)
        var = self._data_variables[name]
        if var.data is None:
            raise ValueError(f"Variable {name!r} has no loaded data.")
        data = np.asarray(var.data)
        if not np.issubdtype(data.dtype, np.number):
            if variables is None:
                warnings.warn(
                    f"skipping non-numeric variable {name!r} in summary",
                    stacklevel=2,
                )
                continue
            raise TypeError(f"variable {name!r} is non-numeric; cannot summarise")
        arrays[name] = gaps_as_nan(data, var.nodata)
    return variable_summary(arrays, metrics=metrics, skipna=skipna, ddof=ddof)

sample(variable_name, x, y, *, method='contains', time_index=0)

Sample a mesh variable at point locations.

The mesh counterpart of :meth:pyramids.dataset.Dataset.sample. Where the raster version inverts an affine geotransform to find the pixel under each point, a mesh resolves the point to an element through :class:MeshSpatialIndex: a face-located variable by which face contains the point (method="contains") or the nearest face centroid (method="nearest"); a node-located variable by nearest node.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable.	required
x	float | ndarray	Query x-coordinate(s), in the mesh CRS.	required
y	float | ndarray	Query y-coordinate(s), in the mesh CRS.	required
method	str	"contains" (point-in-face, face variables only) or "nearest". Defaults to "contains".	'contains'
time_index	int	Step for a temporal variable. Defaults to 0.	0

Returns:

Type	Description
NDArray	numpy.ndarray: One value per query point; NaN where a "contains" query
NDArray	falls outside every face.

Raises:

Type	Description
ValueError	An unknown method, "contains" on a non-face variable, or a variable located on edges (unsupported).

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sample(
    self,
    variable_name: str,
    x: float | np.ndarray,
    y: float | np.ndarray,
    *,
    method: str = "contains",
    time_index: int = 0,
) -> np.typing.NDArray:
    """Sample a mesh variable at point locations.

    The mesh counterpart of :meth:`pyramids.dataset.Dataset.sample`. Where the raster
    version inverts an affine geotransform to find the pixel under each point, a mesh
    resolves the point to an element through :class:`MeshSpatialIndex`: a face-located
    variable by which face contains the point (``method="contains"``) or the nearest
    face centroid (``method="nearest"``); a node-located variable by nearest node.

    Args:
        variable_name: Name of the data variable.
        x: Query x-coordinate(s), in the mesh CRS.
        y: Query y-coordinate(s), in the mesh CRS.
        method: ``"contains"`` (point-in-face, face variables only) or ``"nearest"``.
            Defaults to ``"contains"``.
        time_index: Step for a temporal variable. Defaults to 0.

    Returns:
        numpy.ndarray: One value per query point; NaN where a ``"contains"`` query
        falls outside every face.

    Raises:
        ValueError: An unknown ``method``, ``"contains"`` on a non-face variable, or a
            variable located on edges (unsupported).
    """
    var, arr = self._element_values(variable_name, time_index=time_index)
    xs = np.atleast_1d(np.asarray(x, dtype="float64"))
    ys = np.atleast_1d(np.asarray(y, dtype="float64"))
    index = MeshSpatialIndex(self._mesh)
    if var.location == "face":
        if method == "contains":
            idx = index.locate_faces(xs, ys)
        elif method == "nearest":
            idx = np.atleast_1d(index.locate_nearest_face(xs, ys)).ravel()
        else:
            raise ValueError(
                f"sample method must be 'contains' or 'nearest', got {method!r}"
            )
    elif var.location == "node":
        # A node is a point, so "contains" has no meaning here — only "nearest"
        # applies. Reject "contains" rather than silently doing a nearest lookup, as
        # the docstring promises.
        if method == "contains":
            raise ValueError(
                "sample method='contains' is only valid for a face-located variable; "
                f"{variable_name!r} is on 'node' — use method='nearest'."
            )
        if method != "nearest":
            raise ValueError(
                f"sample method must be 'contains' or 'nearest', got {method!r}"
            )
        idx = np.atleast_1d(index.locate_nearest_node(xs, ys)).ravel()
    else:
        raise ValueError(
            f"sample supports 'face' and 'node' variables; {variable_name!r} is on "
            f"{var.location!r}."
        )
    out = np.full(len(xs), np.nan, dtype="float64")
    inside = idx >= 0
    out[inside] = arr[idx[inside]]
    return out

weighted(variable_name, *, how='mean', time_index=0, q=None)

Area-weighted statistic of a face variable over the mesh.

An irregular mesh has faces of different sizes, so the meaningful aggregate weights each face by its area. Delegates to the shared :func:pyramids.base._reductions.weighted_statistic with :attr:Mesh2d.face_areas as the weights — the same kernel the raster weighted reduction uses.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of a face-located data variable.	required
how	str	One of "mean", "sum", "sum_of_weights", "std", "var", "quantile" (which needs q). Defaults to "mean".	'mean'
time_index	int	Step for a temporal variable. Defaults to 0.	0
q	float | None	The quantile in [0, 1] when how="quantile" (the area-weighted quantile uses the Hazen convention and does not match reduce(how="quantile")); must be None otherwise.	None

Returns:

Name	Type	Description
float	float	The area-weighted statistic over all faces.

Raises:

Type	Description
ValueError	how is not one of WEIGHTED_HOWS; the variable is not face-located; or its length does not match the number of faces.

Examples:

• Area-weighted mean: the larger face (area 4, value 10) outweighs the smaller (area 2, value 20), pulling the mean below the unweighted 15.0:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"v": np.array([10.0, 20.0])},
  ... )
  >>> round(mesh.weighted("v"), 3)
  13.333
  

• The area-weighted quantile clamps to the data range at the extremes:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"v": np.array([10.0, 20.0])},
  ... )
  >>> mesh.weighted("v", how="quantile", q=0.0)
  10.0
  >>> mesh.weighted("v", how="quantile", q=1.0)
  20.0
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def weighted(
    self,
    variable_name: str,
    *,
    how: str = "mean",
    time_index: int = 0,
    q: float | None = None,
) -> float:
    """Area-weighted statistic of a face variable over the mesh.

    An irregular mesh has faces of different sizes, so the meaningful aggregate weights
    each face by its area. Delegates to the shared
    :func:`pyramids.base._reductions.weighted_statistic` with :attr:`Mesh2d.face_areas`
    as the weights — the same kernel the raster ``weighted`` reduction uses.

    Args:
        variable_name: Name of a **face-located** data variable.
        how: One of ``"mean"``, ``"sum"``, ``"sum_of_weights"``, ``"std"``, ``"var"``,
            ``"quantile"`` (which needs ``q``). Defaults to ``"mean"``.
        time_index: Step for a temporal variable. Defaults to 0.
        q: The quantile in ``[0, 1]`` when ``how="quantile"`` (the area-weighted quantile
            uses the Hazen convention and does not match ``reduce(how="quantile")``); must be
            ``None`` otherwise.

    Returns:
        float: The area-weighted statistic over all faces.

    Raises:
        ValueError: ``how`` is not one of ``WEIGHTED_HOWS``; the variable is not
            face-located; or its length does not match the number of faces.

    Examples:
        - Area-weighted mean: the larger face (area 4, value 10) outweighs the
          smaller (area 2, value 20), pulling the mean below the unweighted 15.0:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"v": np.array([10.0, 20.0])},
            ... )
            >>> round(mesh.weighted("v"), 3)
            13.333

            ```
        - The area-weighted quantile clamps to the data range at the extremes:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 4.0, 4.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 2.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"v": np.array([10.0, 20.0])},
            ... )
            >>> mesh.weighted("v", how="quantile", q=0.0)
            10.0
            >>> mesh.weighted("v", how="quantile", q=1.0)
            20.0

            ```
    """
    if how not in WEIGHTED_HOWS:
        # Validate up front, mirroring the raster weighted path: the kernel's catch-all
        # else-branch would otherwise return the standard deviation for any unsupported
        # `how`, a silent wrong answer.
        raise ValueError(
            f"weighted: how must be one of {sorted(WEIGHTED_HOWS)}, got {how!r}."
        )
    if how == "quantile":
        if q is None or isinstance(q, bool) or not 0.0 <= float(q) <= 1.0:
            raise ValueError(
                f"weighted(how='quantile') needs q in [0, 1], got {q!r}."
            )
    elif q is not None:
        raise ValueError(
            f"weighted(q=...) is only valid with how='quantile', not {how!r}."
        )
    var, arr = self._element_values(variable_name, time_index=time_index)
    if var.location != "face":
        raise ValueError(
            "area-weighted statistics need a face-located variable; "
            f"{variable_name!r} is on {var.location!r}."
        )
    areas = self._mesh.face_areas
    if arr.shape[-1] != areas.shape[0]:
        raise ValueError(
            f"variable {variable_name!r} has {arr.shape[-1]} face value(s) but the "
            f"mesh has {areas.shape[0]} face(s)."
        )
    # `arr` already carries NaN for gaps, so `ndv=None` and `skipna=True` reduce over
    # the single face axis; the kernel keeps the axis as length 1, squeezed off here.
    result = weighted_statistic(arr, areas, (0,), how, None, True, q)
    return float(np.asarray(result).ravel()[0])

zonal_stats(zones, *, variable_name, stats=('mean',), weighted=True, time_index=0)

Aggregate a face variable within polygons, weighted by face area.

The mesh counterpart of :func:pyramids.dataset.ops._zonal.zonal_stats. Each face is assigned to the zone whose polygon contains its centroid (the mesh analogue of a raster pixel's cell centre), then the shared :func:pyramids.base._reductions.reduce_by_label reduces the face values per zone. With weighted (the default) the averaging statistics (mean / std / var) are area-weighted, so a zone's mean is area-exact — a capability the raster path does not yet have. sum is always the plain Σ value (never an area integral), and count / min / max are weight-invariant.

Parameters:

Name	Type	Description	Default
zones	Any	Polygons, as a :class:~pyramids.feature.FeatureCollection (or any object exposing geometry, index and len). CRS must match the mesh; reproject first if not.	required
variable_name	str	Name of a face-located data variable.	required
stats	tuple[str, ...]	Statistics per zone, any of "mean", "sum", "min", "max", "std", "var", "count". Defaults to ("mean",).	('mean',)
weighted	bool	Area-weight the averaging statistics (mean / std / var). Defaults to True. sum / count / min / max are unaffected.	True
time_index	int	Step for a temporal variable. Defaults to 0.	0

Returns:

Type	Description
DataFrame	pandas.DataFrame: Indexed like zones; one column per statistic. A zone
DataFrame	containing no face centroid is NaN (count 0.0).

Raises:

Type	Description
ValueError	The variable is not face-located, or the zones' CRS disagrees with the mesh CRS.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def zonal_stats(
    self,
    zones: Any,
    *,
    variable_name: str,
    stats: tuple[str, ...] = ("mean",),
    weighted: bool = True,
    time_index: int = 0,
) -> pd.DataFrame:
    """Aggregate a face variable within polygons, weighted by face area.

    The mesh counterpart of :func:`pyramids.dataset.ops._zonal.zonal_stats`. Each face
    is assigned to the zone whose polygon contains its centroid (the mesh analogue of a
    raster pixel's cell centre), then the shared
    :func:`pyramids.base._reductions.reduce_by_label` reduces the face values per zone.
    With ``weighted`` (the default) the **averaging** statistics (``mean`` / ``std`` /
    ``var``) are area-weighted, so a zone's mean is area-exact — a capability the raster
    path does not yet have. ``sum`` is always the plain Σ value (never an area integral),
    and ``count`` / ``min`` / ``max`` are weight-invariant.

    Args:
        zones: Polygons, as a :class:`~pyramids.feature.FeatureCollection` (or any
            object exposing ``geometry``, ``index`` and ``len``). CRS must match the
            mesh; reproject first if not.
        variable_name: Name of a **face-located** data variable.
        stats: Statistics per zone, any of ``"mean"``, ``"sum"``, ``"min"``, ``"max"``,
            ``"std"``, ``"var"``, ``"count"``. Defaults to ``("mean",)``.
        weighted: Area-weight the averaging statistics (``mean`` / ``std`` / ``var``).
            Defaults to True. ``sum`` / ``count`` / ``min`` / ``max`` are unaffected.
        time_index: Step for a temporal variable. Defaults to 0.

    Returns:
        pandas.DataFrame: Indexed like ``zones``; one column per statistic. A zone
        containing no face centroid is NaN (``count`` ``0.0``).

    Raises:
        ValueError: The variable is not face-located, or the zones' CRS disagrees with
            the mesh CRS.
    """
    var, arr = self._element_values(variable_name, time_index=time_index)
    if var.location != "face":
        raise ValueError(
            "mesh zonal_stats needs a face-located variable; "
            f"{variable_name!r} is on {var.location!r}."
        )
    zone_crs = getattr(zones, "crs", None)
    if zone_crs is not None and self.epsg is not None:
        zone_epsg = zone_crs.to_epsg()
        if zone_epsg is not None and zone_epsg != self.epsg:
            raise ValueError(
                f"zonal_stats: zones CRS (EPSG:{zone_epsg}) does not match mesh CRS "
                f"(EPSG:{self.epsg}). Reproject the zones to the mesh CRS first."
            )
    geometries = list(zones.geometry)
    cx, cy = self._mesh.face_centroids
    centroids = shapely.points(cx, cy)
    tree = shapely.STRtree(geometries)
    # Face centroid within a zone polygon -> that zone. A centroid in more than one
    # (overlapping zones) keeps the first, mirroring a raster pixel landing in one cell.
    centroid_idx, zone_idx = tree.query(centroids, predicate="within")
    labels = np.full(self._mesh.n_face, -1, dtype=np.intp)
    for face_i, zone_i in zip(centroid_idx, zone_idx):
        if labels[face_i] == -1:
            labels[face_i] = zone_i
    n_zones = len(geometries)
    columns: dict[str, np.typing.NDArray] = {}
    # Area weighting applies only to the averaging statistics. `sum` stays the plain
    # Σ value (a weighted sum would be an area integral — a different quantity, and a
    # silent surprise); `count` / `min` / `max` are weight-invariant anyway.
    averaging = [s for s in stats if s in ("mean", "std", "var")]
    plain = [s for s in stats if s not in ("mean", "std", "var")]
    if weighted and averaging:
        columns.update(
            reduce_by_label(
                arr, labels, n_zones, averaging, weights=self._mesh.face_areas
            )
        )
        if plain:
            columns.update(reduce_by_label(arr, labels, n_zones, plain))
    else:
        columns.update(reduce_by_label(arr, labels, n_zones, list(stats)))
    return pd.DataFrame({stat: columns[stat] for stat in stats}, index=zones.index)

reduce(how, *, skipna=True, q=None, groupby=None)

Reduce the time dimension of every temporal variable, whole or in groups.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.reduce over time. Each temporal variable is reduced along its time axis through the shared :func:pyramids.base._reductions.reduce_axis — the very kernel the raster reductions use. Non-temporal variables are kept unchanged.

Without groupby the whole axis collapses to one statistic and the variable becomes static. With groupby each group is reduced on its own slab and the results are stacked back along the time axis, so the variable stays temporal with one step per group — the same shape contract the raster grouped reduce has. For a (4, 2) variable over four monthly steps, reduce("mean") answers shape (2,) with has_time False, while reduce("mean", groupby="MS") answers shape (2, 2) with has_time still True.

Parameters:

Name	Type	Description	Default
how	str	One of "mean", "sum", "min", "max", "std", "var", "median", "prod", "quantile", "count", "all" or "any".	required
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True
q	float | None	The quantile for how="quantile"; must stay None otherwise.	None
groupby	Any	Reduce within groups of steps instead of over the whole axis. Two spellings, matching the raster grouped reduce: a sequence of per-step labels the same length as the time axis (grouped in first-appearance order, so the caller controls the order), or a pandas offset alias ("MS", "YS", …) binned against the dataset's date-like time coordinates. Each group's label becomes the result's time_values entry, and empty bins are dropped. None reduces the whole axis. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset. Without groupby each temporal variable is
	UgridDataset	collapsed over time into a static per-element variable; with groupby each one
	UgridDataset	keeps its time dimension, at one step per group.

Raises:

Type	Description
ValueError	how is not a known reduction; q is given for a non-quantile how or missing for how="quantile"; no variable has a time dimension; a temporal variable has no loaded data; a groupby label sequence's length is not the time length; or a groupby offset alias is asked of time coordinates that are step positions, or that pandas cannot read as dates.

Examples:

• Mean over two time steps of a per-face variable:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0]])},
  ...     data_locations={"d": "face"},
  ... )
  >>> mesh["d"].has_time
  True
  >>> reduced = mesh.reduce("mean")
  >>> reduced["d"].data.tolist()
  [2.0, 3.0]
  >>> reduced["d"].has_time
  False
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def reduce(
    self,
    how: str,
    *,
    skipna: bool = True,
    q: float | None = None,
    groupby: Any = None,
) -> UgridDataset:
    """Reduce the time dimension of every temporal variable, whole or in groups.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.reduce` over time. Each
    temporal variable is reduced along its time axis through the shared
    :func:`pyramids.base._reductions.reduce_axis` — the very kernel the raster reductions
    use. Non-temporal variables are kept unchanged.

    Without ``groupby`` the whole axis collapses to one statistic and the variable becomes
    static. With ``groupby`` each group is reduced on its own slab and the results are
    stacked back along the time axis, so the variable stays temporal with one step per
    group — the same shape contract the raster grouped reduce has. For a ``(4, 2)``
    variable over four monthly steps, ``reduce("mean")`` answers shape ``(2,)`` with
    ``has_time`` False, while ``reduce("mean", groupby="MS")`` answers shape ``(2, 2)``
    with ``has_time`` still True.

    Args:
        how: One of ``"mean"``, ``"sum"``, ``"min"``, ``"max"``, ``"std"``, ``"var"``,
            ``"median"``, ``"prod"``, ``"quantile"``, ``"count"``, ``"all"`` or ``"any"``.
        skipna: Skip the declared no-data value and NaN. Defaults to True.
        q: The quantile for ``how="quantile"``; must stay ``None`` otherwise.
        groupby: Reduce within groups of steps instead of over the whole axis. Two
            spellings, matching the raster grouped reduce: a **sequence of per-step
            labels** the same length as the time axis (grouped in first-appearance order,
            so the caller controls the order), or a **pandas offset alias** (``"MS"``,
            ``"YS"``, …) binned against the dataset's date-like time coordinates. Each
            group's label becomes the result's ``time_values`` entry, and empty bins are
            dropped. ``None`` reduces the whole axis. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset. Without ``groupby`` each temporal variable is
        collapsed over time into a static per-element variable; with ``groupby`` each one
        keeps its time dimension, at one step per group.

    Raises:
        ValueError: ``how`` is not a known reduction; ``q`` is given for a non-quantile
            ``how`` or missing for ``how="quantile"``; no variable has a time dimension;
            a temporal variable has no loaded data; a ``groupby`` label sequence's length
            is not the time length; or a ``groupby`` offset alias is asked of time
            coordinates that are step positions, or that pandas cannot read as dates.

    Examples:
        - Mean over two time steps of a per-face variable:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0]])},
            ...     data_locations={"d": "face"},
            ... )
            >>> mesh["d"].has_time
            True
            >>> reduced = mesh.reduce("mean")
            >>> reduced["d"].data.tolist()
            [2.0, 3.0]
            >>> reduced["d"].has_time
            False

            ```
    """
    _check_reduce_how(how, q)
    self._require_temporal("reduce")
    # all/any collapse to a uint8 flag band whose all-gap value is FLAG_NO_DATA (255);
    # the collapsed variable must declare that so a consumer does not read 255 as data.
    result_nodata = FLAG_NO_DATA if how in ("all", "any") else None
    grouped = (
        _group_positions(self._time_coords("reduce"), groupby)
        if groupby is not None
        else None
    )
    new_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if not var.has_time:
            new_vars[name] = var
            continue
        axis = cast("int", var.time_index)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {name!r} has no loaded data to reduce.")
        arr = np.asarray(data)
        nodata = result_nodata if result_nodata is not None else var.nodata
        if grouped is None:
            reduced = reduce_axis(arr, axis, how, skipna, var.nodata, q)
            new_vars[name] = _static_from(
                var, np.asarray(reduced), axis, nodata=nodata
            )
            continue
        labels, positions = grouped
        # Each group is reduced on its own slab and the results stacked back along the
        # time axis, so a grouped reduce keeps the dimension (shorter) instead of
        # collapsing it — the same shape contract the raster grouped reduce has.
        parts = [
            np.asarray(
                reduce_axis(
                    np.take(arr, group, axis=axis), axis, how, skipna, var.nodata, q
                )
            )
            for group in positions
        ]
        # A grouped reduce re-bins the time axis, so a per-step mask no longer aligns —
        # even when n_groups == n_time keeps the shape, which would slip past the
        # length-only implicit guard (review L3). Drop it; a static mask is step-
        # independent and kept.
        new_var = var.with_data(
            np.stack(parts, axis=axis),
            active=None if var.active_has_time else _KEEP_ACTIVE,
        )
        new_var.nodata = nodata
        new_var.attributes = {**var.attributes, "time_values": list(labels)}
        new_vars[name] = new_var
    return self._rebuild(new_vars)

resample(freq, how='mean', *, skipna=True, q=None)

Bin the time axis by a calendar frequency and reduce each bin.

Sugar over reduce(how, groupby=freq): the mesh counterpart of the raster resample. The time coordinates must be date-like, since the bins come from a pandas offset alias ("1MS", "YS", "7D", …). The time dimension is kept, one step per non-empty bin, stamped with pandas' own label for that bin — which is the bin's start for a start-anchored alias and its end for an end-anchored one. Over steps in January, February and March 2020, "MS" labels the bins 01-01 / 02-01 / 03-01 while "ME" labels them 01-31 / 02-29 / 03-31, and "W" labels each week by its Sunday.

Parameters:

Name	Type	Description	Default
freq	str	A pandas offset alias naming the bin width.	required
how	str	The statistic each bin is reduced with. Defaults to "mean".	'mean'
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True
q	float | None	The quantile, required when how="quantile".	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with one time step per bin.

Raises:

Type	Description
ValueError	No variable has a time dimension, the coordinates are not date-like, or how/q is invalid.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def resample(
    self,
    freq: str,
    how: str = "mean",
    *,
    skipna: bool = True,
    q: float | None = None,
) -> UgridDataset:
    """Bin the time axis by a calendar frequency and reduce each bin.

    Sugar over ``reduce(how, groupby=freq)``: the mesh counterpart of the raster
    ``resample``. The time coordinates must be date-like, since the bins come from a pandas
    offset alias (``"1MS"``, ``"YS"``, ``"7D"``, …). The time dimension is kept, one step
    per non-empty bin, stamped with **pandas' own label for that bin** — which is the bin's
    start for a start-anchored alias and its end for an end-anchored one. Over steps in
    January, February and March 2020, ``"MS"`` labels the bins 01-01 / 02-01 / 03-01 while
    ``"ME"`` labels them 01-31 / 02-29 / 03-31, and ``"W"`` labels each week by its Sunday.

    Args:
        freq: A pandas offset alias naming the bin width.
        how: The statistic each bin is reduced with. Defaults to ``"mean"``.
        skipna: Skip the declared no-data value and NaN. Defaults to True.
        q: The quantile, required when ``how="quantile"``.

    Returns:
        UgridDataset: A new dataset with one time step per bin.

    Raises:
        ValueError: No variable has a time dimension, the coordinates are not date-like,
            or ``how``/``q`` is invalid.
    """
    return self.reduce(how, skipna=skipna, q=q, groupby=freq)

mean(*, skipna=True)

Mean over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time mean.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def mean(self, *, skipna: bool = True) -> UgridDataset:
    """Mean over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time mean.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("mean", skipna=skipna)

sum(*, skipna=True)

Sum over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time sum.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sum(self, *, skipna: bool = True) -> UgridDataset:
    """Sum over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time sum.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("sum", skipna=skipna)

min(*, skipna=True)

Minimum over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time minimum.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def min(self, *, skipna: bool = True) -> UgridDataset:
    """Minimum over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time minimum.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("min", skipna=skipna)

max(*, skipna=True)

Maximum over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time maximum.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def max(self, *, skipna: bool = True) -> UgridDataset:
    """Maximum over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time maximum.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("max", skipna=skipna)

std(*, skipna=True)

Std-dev over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time std.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def std(self, *, skipna: bool = True) -> UgridDataset:
    """Std-dev over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time std.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("std", skipna=skipna)

var(*, skipna=True)

Variance over time of every temporal variable (thin wrapper over :meth:reduce).

Parameters:

Name	Type	Description	Default
skipna	bool	Skip the declared no-data value and NaN. Defaults to True.	True

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset; each temporal variable collapsed to its time variance.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def var(self, *, skipna: bool = True) -> UgridDataset:
    """Variance over time of every temporal variable (thin wrapper over :meth:`reduce`).

    Args:
        skipna: Skip the declared no-data value and NaN. Defaults to True.

    Returns:
        UgridDataset: A new dataset; each temporal variable collapsed to its time variance.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self.reduce("var", skipna=skipna)

cumsum()

Cumulative sum along time for every temporal variable.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset in which each temporal variable holds the running
	UgridDataset	sum of its steps along the time axis; static variables are unchanged.

Raises:

Type	Description
ValueError	No variable has a time dimension, or a temporal variable has no loaded data.

Examples:

• Running sum over three time steps of a per-face variable:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0], [5.0, 6.0]])},
  ...     data_locations={"d": "face"},
  ... )
  >>> mesh.cumsum()["d"].data.tolist()
  [[1.0, 2.0], [4.0, 6.0], [9.0, 12.0]]
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def cumsum(self) -> UgridDataset:
    """Cumulative sum along time for every temporal variable.

    Returns:
        UgridDataset: A new dataset in which each temporal variable holds the running
        sum of its steps along the time axis; static variables are unchanged.

    Raises:
        ValueError: No variable has a time dimension, or a temporal variable has no
            loaded data.

    Examples:
        - Running sum over three time steps of a per-face variable:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"d": np.array([[1.0, 2.0], [3.0, 4.0], [5.0, 6.0]])},
            ...     data_locations={"d": "face"},
            ... )
            >>> mesh.cumsum()["d"].data.tolist()
            [[1.0, 2.0], [4.0, 6.0], [9.0, 12.0]]

            ```
    """
    return self._transform_time("cumsum", lambda d, axis: np.cumsum(d, axis=axis))

cumprod()

Cumulative product along time for every temporal variable.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset in which each temporal variable holds the running
	UgridDataset	product of its steps along the time axis; static variables are unchanged.

Raises:

Type	Description
ValueError	No variable has a time dimension, or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def cumprod(self) -> UgridDataset:
    """Cumulative product along time for every temporal variable.

    Returns:
        UgridDataset: A new dataset in which each temporal variable holds the running
        product of its steps along the time axis; static variables are unchanged.

    Raises:
        ValueError: No variable has a time dimension, or a temporal variable has no
            loaded data.
    """
    return self._transform_time("cumprod", lambda d, axis: np.cumprod(d, axis=axis))

shift(periods=1)

Shift each temporal variable periods steps along time, vacated steps NaN.

Parameters:

Name	Type	Description	Default
periods	int	Steps to shift; negative shifts towards the start. Defaults to 1.	1

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable shifted along time, the
	UgridDataset	vacated steps filled with NaN; static variables are unchanged.

Raises:

Type	Description
ValueError	No variable has a time dimension, or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def shift(self, periods: int = 1) -> UgridDataset:
    """Shift each temporal variable ``periods`` steps along time, vacated steps NaN.

    Args:
        periods: Steps to shift; negative shifts towards the start. Defaults to 1.

    Returns:
        UgridDataset: A new dataset with each temporal variable shifted along time, the
        vacated steps filled with NaN; static variables are unchanged.

    Raises:
        ValueError: No variable has a time dimension, or a temporal variable has no
            loaded data.
    """
    return self._transform_time(
        "shift",
        lambda d, axis: shifted(d, axis, periods, np.nan),
        # Move a per-step mask with the snapshots (time on axis 0), vacated steps inactive —
        # a shifted-in step holds NaN data, so False is right (review H1).
        active_fn=lambda m: shifted(m, 0, periods, False),
    )

ffill(*, limit=None, dim=None)

Forward-fill gaps along time from the last valid step.

Parameters:

Name	Type	Description	Default
limit	int | None	Maximum consecutive gaps one valid step may fill, or None for no limit.	None
dim	str | None	The dimension to fill along. A mesh fills along "time" only, so any other name is refused; None means time. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable's gaps carried forward
	UgridDataset	from the previous valid step; static variables are unchanged.

Raises:

Type	Description
ValueError	dim names anything but "time", no variable has a time dimension, or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def ffill(
    self, *, limit: int | None = None, dim: str | None = None
) -> UgridDataset:
    """Forward-fill gaps along time from the last valid step.

    Args:
        limit: Maximum consecutive gaps one valid step may fill, or ``None`` for no limit.
        dim: The dimension to fill along. A mesh fills along ``"time"`` only, so any other
            name is refused; ``None`` means time. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with each temporal variable's gaps carried forward
        from the previous valid step; static variables are unchanged.

    Raises:
        ValueError: ``dim`` names anything but ``"time"``, no variable has a time
            dimension, or a temporal variable has no loaded data.
    """
    self._check_time_dim("ffill", dim)
    return self._transform_time(
        "ffill", lambda d, axis: pushed(d, axis, limit, False)
    )

bfill(*, limit=None, dim=None)

Back-fill gaps along time from the next valid step.

Parameters:

Name	Type	Description	Default
limit	int | None	Maximum consecutive gaps one valid step may fill, or None for no limit.	None
dim	str | None	The dimension to fill along. A mesh fills along "time" only, so any other name is refused; None means time. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable's gaps carried backward
	UgridDataset	from the next valid step; static variables are unchanged.

Raises:

Type	Description
ValueError	dim names anything but "time", no variable has a time dimension, or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def bfill(
    self, *, limit: int | None = None, dim: str | None = None
) -> UgridDataset:
    """Back-fill gaps along time from the next valid step.

    Args:
        limit: Maximum consecutive gaps one valid step may fill, or ``None`` for no limit.
        dim: The dimension to fill along. A mesh fills along ``"time"`` only, so any other
            name is refused; ``None`` means time. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with each temporal variable's gaps carried backward
        from the next valid step; static variables are unchanged.

    Raises:
        ValueError: ``dim`` names anything but ``"time"``, no variable has a time
            dimension, or a temporal variable has no loaded data.
    """
    self._check_time_dim("bfill", dim)
    return self._transform_time(
        "bfill", lambda d, axis: pushed(d, axis, limit, True)
    )

interpolate_na(*, method='linear', limit=None, use_coordinate=True, dim=None)

Fill each interior time gap by interpolating between the steps on either side.

Distance is measured along the time coordinate by default, so unevenly spaced steps weight their neighbours accordingly; pass use_coordinate=False to measure by step position instead. A leading or trailing gap has only one neighbour and is left alone.

Parameters:

Name	Type	Description	Default
method	str	One of "linear" (distance-weighted), "nearest", or the scipy spline kinds "slinear" / "quadratic" / "cubic" (each falling back to linear on a variable with too few valid steps). Defaults to "linear".	'linear'
limit	int | None	Maximum consecutive gaps a run may fill, or None for no limit.	None
use_coordinate	bool	Measure distance along the time coordinate rather than by step position. With time values [0, 1, 10] and a gap at the middle step, the coordinate reading fills 1.0 and the positional reading 5.0. Date-like coordinates count as distances too (converted to nanoseconds), so unevenly spaced dates weight their neighbours; a coordinate with no measurable spacing (plain labels) falls back to step positions. Defaults to True.	True
dim	str | None	The dimension to interpolate along. A mesh interpolates along "time" only, so any other name is refused; None means time. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable's interior gaps
	UgridDataset	interpolated along time; static variables are unchanged.

Raises:

Type	Description
ValueError	method is not one of the accepted interpolations; no variable has a time dimension; or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def interpolate_na(
    self,
    *,
    method: str = "linear",
    limit: int | None = None,
    use_coordinate: bool = True,
    dim: str | None = None,
) -> UgridDataset:
    """Fill each interior time gap by interpolating between the steps on either side.

    Distance is measured along the time coordinate by default, so unevenly spaced steps
    weight their neighbours accordingly; pass ``use_coordinate=False`` to measure by step
    position instead. A leading or trailing gap has only one neighbour and is left alone.

    Args:
        method: One of ``"linear"`` (distance-weighted), ``"nearest"``, or the scipy spline
            kinds ``"slinear"`` / ``"quadratic"`` / ``"cubic"`` (each falling back to linear on
            a variable with too few valid steps). Defaults to ``"linear"``.
        limit: Maximum consecutive gaps a run may fill, or ``None`` for no limit.
        use_coordinate: Measure distance along the time coordinate rather than by step
            position. With time values ``[0, 1, 10]`` and a gap at the middle step, the
            coordinate reading fills ``1.0`` and the positional reading ``5.0``. Date-like
            coordinates count as distances too (converted to nanoseconds), so unevenly
            spaced dates weight their neighbours; a coordinate with no measurable spacing
            (plain labels) falls back to step positions. Defaults to ``True``.
        dim: The dimension to interpolate along. A mesh interpolates along ``"time"``
            only, so any other name is refused; ``None`` means time. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with each temporal variable's interior gaps
        interpolated along time; static variables are unchanged.

    Raises:
        ValueError: ``method`` is not one of the accepted interpolations; no variable has a
            time dimension; or a temporal variable has no loaded data.
    """
    if method not in INTERP_METHODS:
        # Validate up front, before the time check, so a bogus method is reported as such
        # even on a mesh with no temporal variable — matching the raster interpolate_na.
        raise ValueError(
            f"interpolate_na() takes method="
            f"{' or '.join(repr(one) for one in INTERP_METHODS)}, got {method!r}."
        )

    self._check_time_dim("interpolate_na", dim)
    stamps = self.time_values if use_coordinate else None
    axis_values = None
    if stamps is not None:
        axis_values = _axis_distances(stamps)

    def _fill(data: np.ndarray, axis: int) -> np.ndarray:
        size = data.shape[axis]
        if axis_values is not None and axis_values.shape[0] == size:
            positions = axis_values
        else:
            positions = np.arange(size, dtype="float64")
        return np.asarray(interpolated(data, axis, positions, method, limit))

    return self._transform_time("interpolate_na", _fill)

rolling(window, how='mean', *, center=False, min_periods=None, q=None, dim=None)

Rolling-window reduction along time for every temporal variable.

Each time step becomes a statistic of the window of steps it owns (ending at it, or centred on it), cut where the window runs off the axis. Gaps are skipped; a step whose window holds fewer than min_periods valid cells is NaN. The time dimension keeps its length.

Parameters:

Name	Type	Description	Default
window	int	Steps per window.	required
how	str	The reduction over each window ("mean", "sum", "min", "max", "std", "var", "median", "quantile"). Defaults to "mean".	'mean'
center	bool	Centre each window on its step rather than ending at it. Defaults to False.	False
min_periods	int | None	Valid cells a window needs before its step holds a value. None means the full window, matching the raster rolling, so a step whose window runs off the axis is NaN rather than a partial statistic. Defaults to None.	None
q	float | None	The quantile in [0, 1], required when how="quantile" and rejected otherwise. Defaults to None.	None
dim	str | None	The dimension to roll along. A mesh rolls along "time" only, so any other name is refused; None means time. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable replaced by its rolling
	UgridDataset	statistic along time; static variables are unchanged.

Raises:

Type	Description
ValueError	how is not a known reduction; q is given for a non-quantile how or missing for how="quantile"; window is below 1; min_periods falls outside [1, window]; dim names anything but "time"; no variable has a time dimension; or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def rolling(
    self,
    window: int,
    how: str = "mean",
    *,
    center: bool = False,
    min_periods: int | None = None,
    q: float | None = None,
    dim: str | None = None,
) -> UgridDataset:
    """Rolling-window reduction along time for every temporal variable.

    Each time step becomes a statistic of the window of steps it owns (ending at it, or
    centred on it), cut where the window runs off the axis. Gaps are skipped; a step whose
    window holds fewer than ``min_periods`` valid cells is NaN. The time dimension keeps
    its length.

    Args:
        window: Steps per window.
        how: The reduction over each window (``"mean"``, ``"sum"``, ``"min"``, ``"max"``,
            ``"std"``, ``"var"``, ``"median"``, ``"quantile"``). Defaults to ``"mean"``.
        center: Centre each window on its step rather than ending at it. Defaults to False.
        min_periods: Valid cells a window needs before its step holds a value. ``None``
            means the full ``window``, matching the raster `rolling`, so a step whose
            window runs off the axis is NaN rather than a partial statistic. Defaults to
            ``None``.
        q: The quantile in ``[0, 1]``, required when ``how="quantile"`` and rejected
            otherwise. Defaults to ``None``.
        dim: The dimension to roll along. A mesh rolls along ``"time"`` only, so any
            other name is refused; ``None`` means time. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with each temporal variable replaced by its rolling
        statistic along time; static variables are unchanged.

    Raises:
        ValueError: ``how`` is not a known reduction; ``q`` is given for a non-quantile
            ``how`` or missing for ``how="quantile"``; ``window`` is below 1;
            ``min_periods`` falls outside ``[1, window]``; ``dim`` names anything but
            ``"time"``; no variable has a time dimension; or a temporal variable has no
            loaded data.
    """
    if how not in _ROLLING_HOWS:
        # Restrict to the documented statistics. The counting reductions
        # (count/all/any) are excluded on purpose: a window's all/any flag carries the
        # 255 FLAG_NO_DATA sentinel, which would surface as a literal data value here.
        raise ValueError(
            f"rolling how must be one of {sorted(_ROLLING_HOWS)}, got {how!r}."
        )
    # Same guard as every other along-time member, through the shared check rather than
    # a second copy of it with its own wording.
    self._check_time_dim("rolling", dim)
    if how == "quantile" and q is None:
        raise ValueError("rolling(how='quantile') needs q in [0, 1].")
    if how != "quantile" and q is not None:
        raise ValueError(
            f"rolling(q=...) is only valid with how='quantile', not {how!r}."
        )
    if window < 1:
        raise ValueError(f"rolling: window must be >= 1, got {window!r}.")
    # `None` means the full window, as the raster rolling defaults — a step whose window
    # runs off the axis then has no statistic rather than a partial one.
    needed = window if min_periods is None else int(min_periods)
    if not 1 <= needed <= window:
        raise ValueError(
            f"rolling: min_periods must be between 1 and window ({window}), got "
            f"{min_periods!r}."
        )

    def _roll(data: np.ndarray, axis: int) -> np.ndarray:
        moved = np.moveaxis(data, axis, 0)
        size = moved.shape[0]
        out = np.full_like(moved, np.nan, dtype="float64")
        for step in range(size):
            members = window_members(step, size, window, center)
            block = moved[members]
            valid = np.sum(~np.isnan(block), axis=0)
            reduced = reduce_axis(block, 0, how, True, None, q)
            out[step] = np.where(valid >= needed, reduced, np.nan)
        return np.moveaxis(out, 0, axis)

    return self._transform_time("rolling", _roll)

isel(time=None, *, element=None)

Select by position along the time axis, the element axis, or both.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.isel. Over time an integer selects a single step and collapses the time dimension (the variables become static); a slice or sequence keeps the time dimension, trimmed to the chosen steps. Over element the selection is a topology change: the chosen faces are kept and the mesh is renumbered around them (as :meth:clip does), so the result is a smaller mesh. Giving both applies the element selection first, then the time selection.

Parameters:

Name	Type	Description	Default
time	int | slice | list[int] | ndarray | None	An int step, a slice, or a sequence of integer step positions. None (the default) leaves the time axis alone.	None
element	int | slice | list[int] | ndarray | None	An int face index, a slice, or a sequence of face indices. None (the default) leaves the mesh alone.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with the selection applied.

Raises:

Type	Description
ValueError	Neither time nor element was given, or time was asked of a dataset with no time dimension.
IndexError	A face index is out of range for this mesh.

Examples:

• Keep one face, renumbering the mesh around it:

  >>> import numpy as np
  >>> from pyramids.netcdf.ugrid import UgridDataset
  >>> mesh = UgridDataset.from_arrays(
  ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
  ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
  ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
  ...     data={"depth": np.array([2.0, 4.0])},
  ... )
  >>> one = mesh.isel(element=0)
  >>> one.n_face, one.n_node
  (1, 3)
  >>> one["depth"].data.tolist()
  [2.0]
  

Source code in src/pyramids/netcdf/ugrid/dataset.py

def isel(
    self,
    time: int | slice | list[int] | np.ndarray | None = None,
    *,
    element: int | slice | list[int] | np.ndarray | None = None,
) -> UgridDataset:
    """Select by position along the time axis, the element axis, or both.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.isel`. Over **time** an integer
    selects a single step and collapses the time dimension (the variables become static);
    a slice or sequence keeps the time dimension, trimmed to the chosen steps. Over
    **element** the selection is a topology change: the chosen faces are kept and the mesh
    is renumbered around them (as :meth:`clip` does), so the result is a smaller mesh.
    Giving both applies the element selection first, then the time selection.

    Args:
        time: An ``int`` step, a ``slice``, or a sequence of integer step positions.
            ``None`` (the default) leaves the time axis alone.
        element: An ``int`` face index, a ``slice``, or a sequence of face indices.
            ``None`` (the default) leaves the mesh alone.

    Returns:
        UgridDataset: A new dataset with the selection applied.

    Raises:
        ValueError: Neither ``time`` nor ``element`` was given, or ``time`` was asked of a
            dataset with no time dimension.
        IndexError: A face index is out of range for this mesh.

    Examples:
        - Keep one face, renumbering the mesh around it:
            ```python
            >>> import numpy as np
            >>> from pyramids.netcdf.ugrid import UgridDataset
            >>> mesh = UgridDataset.from_arrays(
            ...     node_x=np.array([0.0, 1.0, 1.0, 0.0]),
            ...     node_y=np.array([0.0, 0.0, 1.0, 1.0]),
            ...     face_node_connectivity=np.array([[0, 1, 2], [0, 2, 3]]),
            ...     data={"depth": np.array([2.0, 4.0])},
            ... )
            >>> one = mesh.isel(element=0)
            >>> one.n_face, one.n_node
            (1, 3)
            >>> one["depth"].data.tolist()
            [2.0]

            ```
    """
    if time is None and element is None:
        raise ValueError("isel needs `time`, `element`, or both.")
    if element is not None:
        selected = self._select_elements(element)
        return selected if time is None else selected.isel(time)
    n = self._time_length("isel")
    if isinstance(time, (int, np.integer)):
        return self._collapse_to_step(int(time))
    if isinstance(time, slice):
        indices = np.arange(n)[time]
    else:
        indices = np.asarray(time, dtype=np.intp)
    return self._select_steps(indices)

head(n=5)

Keep the first n time steps (fewer if the axis is shorter).

Parameters:

Name	Type	Description	Default
n	int	Number of leading steps to keep. Defaults to 5.	5

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset trimmed to the first n steps.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def head(self, n: int = 5) -> UgridDataset:
    """Keep the first ``n`` time steps (fewer if the axis is shorter).

    Args:
        n: Number of leading steps to keep. Defaults to 5.

    Returns:
        UgridDataset: A new dataset trimmed to the first ``n`` steps.

    Raises:
        ValueError: No variable has a time dimension.
    """
    length = self._time_length("head")
    return self._select_steps(np.arange(min(n, length)))

tail(n=5)

Keep the last n time steps (fewer if the axis is shorter).

Parameters:

Name	Type	Description	Default
n	int	Number of trailing steps to keep. Defaults to 5.	5

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset trimmed to the last n steps.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def tail(self, n: int = 5) -> UgridDataset:
    """Keep the last ``n`` time steps (fewer if the axis is shorter).

    Args:
        n: Number of trailing steps to keep. Defaults to 5.

    Returns:
        UgridDataset: A new dataset trimmed to the last ``n`` steps.

    Raises:
        ValueError: No variable has a time dimension.
    """
    length = self._time_length("tail")
    return self._select_steps(np.arange(max(length - n, 0), length))

thin(step)

Keep every step-th time step.

Parameters:

Name	Type	Description	Default
step	int	Stride; must be >= 1.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset keeping every step-th step.

Raises:

Type	Description
ValueError	step is less than 1, or no variable is temporal.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def thin(self, step: int) -> UgridDataset:
    """Keep every ``step``-th time step.

    Args:
        step: Stride; must be >= 1.

    Returns:
        UgridDataset: A new dataset keeping every ``step``-th step.

    Raises:
        ValueError: ``step`` is less than 1, or no variable is temporal.
    """
    if step < 1:
        raise ValueError(f"thin step must be >= 1, got {step}.")
    length = self._time_length("thin")
    return self._select_steps(np.arange(0, length, step))

drop_isel(indices)

Drop the time steps at indices by position, keeping the rest.

Parameters:

Name	Type	Description	Default
indices	int | list[int] | ndarray	A step position, or a sequence of them, to drop (negative indices count from the end).	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset without the dropped steps.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def drop_isel(self, indices: int | list[int] | np.ndarray) -> UgridDataset:
    """Drop the time steps at ``indices`` by position, keeping the rest.

    Args:
        indices: A step position, or a sequence of them, to drop (negative indices
            count from the end).

    Returns:
        UgridDataset: A new dataset without the dropped steps.

    Raises:
        ValueError: No variable has a time dimension.
    """
    length = self._time_length("drop_isel")
    drop = {int(i) % length for i in np.atleast_1d(np.asarray(indices))}
    keep = np.array([i for i in range(length) if i not in drop], dtype=np.intp)
    return self._select_steps(keep)

squeeze()

Drop the time dimension when it has length 1; otherwise return unchanged.

Mirrors :meth:pyramids.netcdf.NetCDF.squeeze for the time axis: a single-step temporal variable becomes static, a multi-step one is left alone.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with single-step temporal variables collapsed to
	UgridDataset	static, or self when there is nothing to squeeze.

Raises:

Type	Description
ValueError	A single-step temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def squeeze(self) -> UgridDataset:
    """Drop the time dimension when it has length 1; otherwise return unchanged.

    Mirrors :meth:`pyramids.netcdf.NetCDF.squeeze` for the time axis: a single-step
    temporal variable becomes static, a multi-step one is left alone.

    Returns:
        UgridDataset: A new dataset with single-step temporal variables collapsed to
        static, or ``self`` when there is nothing to squeeze.

    Raises:
        ValueError: A single-step temporal variable has no loaded data.
    """
    # Decide per variable, not from the first temporal one: with mixed time lengths a
    # later single-step variable must still be squeezed even if the first has many steps.
    if not any(
        var.has_time and var.n_time_steps == 1
        for var in self._data_variables.values()
    ):
        return self
    new_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if not var.has_time or var.n_time_steps != 1:
            new_vars[name] = var
            continue
        axis = cast("int", var.time_index)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {name!r} has no loaded data to squeeze.")
        # Squeezing the size-1 time axis is taking its single step, so collapse a per-step
        # mask to that step (`mask[0]`) rather than letting the implicit carry drop it as a
        # now-rank-mismatched 2-D mask (review L3). A static mask passes straight through.
        new_vars[name] = _static_from(
            var,
            np.squeeze(np.asarray(data), axis=axis),
            axis,
            active=var._time_sliced_active(0, None),
        )
    return self._rebuild(new_vars)

diff(n=1)

Discrete difference along time (out[i] = x[i] - x[i-1]), n times.

The time dimension shortens by n; a time_values coordinate, if present, drops its first n entries to stay aligned with the result.

Parameters:

Name	Type	Description	Default
n	int	The number of successive differences. Defaults to 1.	1

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with each temporal variable differenced along time
	UgridDataset	(its time axis shorter by n); static variables are unchanged.

Raises:

Type	Description
ValueError	No variable has a time dimension, or a temporal variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def diff(self, n: int = 1) -> UgridDataset:
    """Discrete difference along time (``out[i] = x[i] - x[i-1]``), ``n`` times.

    The time dimension shortens by ``n``; a ``time_values`` coordinate, if present, drops
    its first ``n`` entries to stay aligned with the result.

    Args:
        n: The number of successive differences. Defaults to 1.

    Returns:
        UgridDataset: A new dataset with each temporal variable differenced along time
        (its time axis shorter by ``n``); static variables are unchanged.

    Raises:
        ValueError: No variable has a time dimension, or a temporal variable has no
            loaded data.
    """
    self._require_temporal("diff")
    new_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if not var.has_time:
            new_vars[name] = var
            continue
        axis = cast("int", var.time_index)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {name!r} has no loaded data to diff.")
        diffed = np.diff(gaps_as_nan(np.asarray(data), var.nodata), n=n, axis=axis)
        new_var = var.with_data(diffed)
        times = var.attributes.get("time_values")
        if times is not None:
            new_var.attributes = {**var.attributes, "time_values": list(times[n:])}
        new_vars[name] = new_var
    return self._rebuild(new_vars)

argmin()

The time-step position of each element's minimum.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	Static int64 variables holding the step index of the minimum,
	UgridDataset	declaring -1 for an element with no valid step.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def argmin(self) -> UgridDataset:
    """The time-step position of each element's minimum.

    Returns:
        UgridDataset: Static ``int64`` variables holding the step index of the minimum,
        declaring ``-1`` for an element with no valid step.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self._extremum_along_time("argmin", largest=False, as_coordinate=False)

argmax()

The time-step position of each element's maximum.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	Static int64 variables holding the step index of the maximum,
	UgridDataset	declaring -1 for an element with no valid step.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def argmax(self) -> UgridDataset:
    """The time-step position of each element's maximum.

    Returns:
        UgridDataset: Static ``int64`` variables holding the step index of the maximum,
        declaring ``-1`` for an element with no valid step.

    Raises:
        ValueError: No variable has a time dimension.
    """
    return self._extremum_along_time("argmax", largest=True, as_coordinate=False)

idxmin()

The time coordinate of each element's minimum.

Where :meth:argmin answers a step position, this answers the value of the time coordinate at that step, so it needs numeric time coordinates.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	Static float64 variables holding the time coordinate of the
	UgridDataset	minimum, NaN where an element has no valid step.

Raises:

Type	Description
ValueError	No variable has a time dimension, or the time coordinates are date-like rather than numeric (use argmin / argmax for the step position instead).

Source code in src/pyramids/netcdf/ugrid/dataset.py

def idxmin(self) -> UgridDataset:
    """The time **coordinate** of each element's minimum.

    Where :meth:`argmin` answers a step position, this answers the value of the time
    coordinate at that step, so it needs numeric time coordinates.

    Returns:
        UgridDataset: Static ``float64`` variables holding the time coordinate of the
        minimum, NaN where an element has no valid step.

    Raises:
        ValueError: No variable has a time dimension, or the time coordinates are
            date-like rather than numeric (use `argmin` / `argmax` for the step
            position instead).
    """
    return self._extremum_along_time("idxmin", largest=False, as_coordinate=True)

idxmax()

The time coordinate of each element's maximum.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	Static float64 variables holding the time coordinate of the
	UgridDataset	maximum, NaN where an element has no valid step.

Raises:

Type	Description
ValueError	No variable has a time dimension, or the time coordinates are date-like rather than numeric (use argmin / argmax for the step position instead).

Source code in src/pyramids/netcdf/ugrid/dataset.py

def idxmax(self) -> UgridDataset:
    """The time **coordinate** of each element's maximum.

    Returns:
        UgridDataset: Static ``float64`` variables holding the time coordinate of the
        maximum, NaN where an element has no valid step.

    Raises:
        ValueError: No variable has a time dimension, or the time coordinates are
            date-like rather than numeric (use `argmin` / `argmax` for the step
            position instead).
    """
    return self._extremum_along_time("idxmax", largest=True, as_coordinate=True)

sel(time=None, *, method=None, tolerance=None)

Select time steps by coordinate value, exactly or by nearest.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.sel over time. A scalar selects one step and collapses the time dimension; a sequence keeps it. Values are matched against the dataset's :attr:time_values (which fall back to step positions when the file carries no explicit time coordinate). method="nearest" snaps each value to the closest coordinate instead of requiring an exact hit, and tolerance caps how far it may snap — the same contract the raster sel uses, including raising KeyError (not ValueError) when a snap would exceed tolerance.

Parameters:

Name	Type	Description	Default
time	Any	A coordinate value, or a sequence of them.	None
method	str | None	None (the default) matches exactly; "nearest" snaps to the closest coordinate, which needs numeric coordinates.	None
tolerance	float | None	The furthest a method="nearest" snap may travel. Only valid with "nearest", and must not be negative.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with the selected steps.

Raises:

Type	Description
ValueError	No time dimension; an unknown method; tolerance without method="nearest" or negative; a value is not among the coordinates (exact match); or "nearest" was asked of non-numeric coordinates.
KeyError	A "nearest" snap would exceed tolerance.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sel(
    self,
    time: Any = None,
    *,
    method: str | None = None,
    tolerance: float | None = None,
) -> UgridDataset:
    """Select time steps by coordinate value, exactly or by nearest.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.sel` over time. A scalar selects
    one step and collapses the time dimension; a sequence keeps it. Values are matched
    against the dataset's :attr:`time_values` (which fall back to step positions when the
    file carries no explicit time coordinate). ``method="nearest"`` snaps each value to the
    closest coordinate instead of requiring an exact hit, and ``tolerance`` caps how far it
    may snap — the same contract the raster ``sel`` uses, including raising ``KeyError`` (not
    ``ValueError``) when a snap would exceed ``tolerance``.

    Args:
        time: A coordinate value, or a sequence of them.
        method: ``None`` (the default) matches exactly; ``"nearest"`` snaps to the closest
            coordinate, which needs numeric coordinates.
        tolerance: The furthest a ``method="nearest"`` snap may travel. Only valid with
            ``"nearest"``, and must not be negative.

    Returns:
        UgridDataset: A new dataset with the selected steps.

    Raises:
        ValueError: No time dimension; an unknown ``method``; ``tolerance`` without
            ``method="nearest"`` or negative; a value is not among the coordinates (exact
            match); or ``"nearest"`` was asked of non-numeric coordinates.
        KeyError: A ``"nearest"`` snap would exceed ``tolerance``.
    """
    if time is None:
        # `time` is optional in the signature for parity with the raster `sel`, but a
        # missing value has no meaning here. Say so, as `isel` does, instead of
        # reporting that the coordinate [None] was not found.
        raise ValueError(
            "sel needs a time value to select; pass one, or use isel() to select by "
            "position."
        )
    coords = self._time_coords("sel")
    if method not in (None, "nearest"):
        raise ValueError(f"sel: method must be None or 'nearest', got {method!r}.")
    if tolerance is not None:
        if method != "nearest":
            raise ValueError("sel: tolerance is only valid with method='nearest'.")
        if float(tolerance) < 0:
            raise ValueError(
                f"sel: tolerance must not be negative, got {tolerance!r}."
            )
    wanted = list(time) if isinstance(time, (list, tuple, np.ndarray)) else [time]
    positions = _resolve_time_positions(coords, wanted, method, tolerance)
    if isinstance(time, (list, tuple, np.ndarray)):
        result = self._select_steps(np.asarray(positions, dtype=np.intp))
    else:
        result = self.isel(positions[0])
    return result

drop_sel(values)

Drop the time steps whose coordinate is in values, keeping the rest.

Parameters:

Name	Type	Description	Default
values	Any	A coordinate value, or a sequence of them, to drop.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset without the steps whose coordinate is in values.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def drop_sel(self, values: Any) -> UgridDataset:
    """Drop the time steps whose coordinate is in ``values``, keeping the rest.

    Args:
        values: A coordinate value, or a sequence of them, to drop.

    Returns:
        UgridDataset: A new dataset without the steps whose coordinate is in ``values``.

    Raises:
        ValueError: No variable has a time dimension.
    """
    coords = self._time_coords("drop_sel")
    drop = (
        set(values)
        if isinstance(values, (list, tuple, set))
        else {*np.atleast_1d(np.asarray(values)).tolist()}
    )
    keep = [i for i, coord in enumerate(coords) if coord not in drop]
    return self._select_steps(np.asarray(keep, dtype=np.intp))

sortby()

Sort the time steps by their coordinate value (stable).

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with steps ordered by ascending time coordinate.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def sortby(self) -> UgridDataset:
    """Sort the time steps by their coordinate value (stable).

    Returns:
        UgridDataset: A new dataset with steps ordered by ascending time coordinate.

    Raises:
        ValueError: No variable has a time dimension.
    """
    coords = self._time_coords("sortby")
    order = np.argsort(np.asarray(coords), kind="stable")
    return self._select_steps(order)

drop_duplicates()

Keep the first step of each distinct time coordinate value.

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with duplicate-coordinate steps removed, first kept.

Raises:

Type	Description
ValueError	No variable has a time dimension.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def drop_duplicates(self) -> UgridDataset:
    """Keep the first step of each distinct time coordinate value.

    Returns:
        UgridDataset: A new dataset with duplicate-coordinate steps removed, first kept.

    Raises:
        ValueError: No variable has a time dimension.
    """
    coords = self._time_coords("drop_duplicates")
    seen: set = set()
    keep: list[int] = []
    for i, coord in enumerate(coords):
        if coord not in seen:
            seen.add(coord)
            keep.append(i)
    return self._select_steps(np.asarray(keep, dtype=np.intp))

dropna(how='any', thresh=None, *, dim=None)

Drop time steps that do not hold enough valid cells.

A step is kept only when every temporal variable meets the threshold at that step, so the variables stay time-aligned. how="any" requires every cell of a variable to be valid (a single gap drops the step); how="all" requires at least one; thresh overrides how with an explicit valid-cell count per variable.

Parameters:

Name	Type	Description	Default
how	str	"any" or "all". Defaults to "any".	'any'
thresh	int | None	Explicit minimum valid-cell count per variable, overriding how.	None
dim	str | None	The dimension to drop along. A mesh drops along "time" only, so any other name is refused; None means time. Defaults to None.	None

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset with the surviving steps.

Raises:

Type	Description
ValueError	how is neither "any" nor "all"; dim names anything but "time"; no variable has a time dimension; a temporal variable has no loaded data; or the threshold would drop every step, which would leave a temporal dataset with no steps for anything downstream to read.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def dropna(
    self, how: str = "any", thresh: int | None = None, *, dim: str | None = None
) -> UgridDataset:
    """Drop time steps that do not hold enough valid cells.

    A step is kept only when **every** temporal variable meets the threshold at that
    step, so the variables stay time-aligned. ``how="any"`` requires every cell of a
    variable to be valid (a single gap drops the step); ``how="all"`` requires at least
    one; ``thresh`` overrides ``how`` with an explicit valid-cell count per variable.

    Args:
        how: ``"any"`` or ``"all"``. Defaults to ``"any"``.
        thresh: Explicit minimum valid-cell count per variable, overriding ``how``.
        dim: The dimension to drop along. A mesh drops along ``"time"`` only, so any other
            name is refused; ``None`` means time. Defaults to ``None``.

    Returns:
        UgridDataset: A new dataset with the surviving steps.

    Raises:
        ValueError: ``how`` is neither ``"any"`` nor ``"all"``; ``dim`` names anything but
            ``"time"``; no variable has a time dimension; a temporal variable has no
            loaded data; or the threshold would drop **every** step, which would leave a
            temporal dataset with no steps for anything downstream to read.
    """
    if how not in ("any", "all"):
        raise ValueError(f"dropna how must be 'any' or 'all', got {how!r}.")
    self._check_time_dim("dropna", dim)
    temporal = self._require_temporal("dropna")
    length = self._time_length("dropna")
    keep = np.ones(length, dtype=bool)
    for name in temporal:
        var = self._data_variables[name]
        axis = cast("int", var.time_index)
        data = var.data
        if data is None:
            raise ValueError(f"Variable {name!r} has no loaded data to dropna.")
        moved = np.moveaxis(gaps_as_nan(np.asarray(data), var.nodata), axis, 0)
        flat = moved.reshape(moved.shape[0], -1)
        per_step_valid = np.sum(~np.isnan(flat), axis=1)
        needed = flat.shape[1] if how == "any" else 1
        if thresh is not None:
            needed = thresh
        keep &= per_step_valid >= needed
    if not keep.any():
        # Returning a zero-length time axis would leave a "temporal" dataset with no
        # steps, which nothing downstream can read; the raster dropna refuses too.
        raise ValueError(
            f"dropna(how={how!r}, thresh={thresh!r}) would drop every one of the "
            f"{length} time step(s); nothing would be left."
        )
    return self._select_steps(np.flatnonzero(keep))

concat(others)

Concatenate same-topology datasets along time.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.concat. Every dataset must sit on the same mesh. Each temporal variable's data is concatenated along its time axis (and its time_values coordinate, when present); a static variable is taken from self unchanged. The variable set must match across datasets.

A per-step active mask (:attr:MeshVariable.active of shape (n_time, n_element)) is dropped on concatenation — the parts' masks are not joined along time, and the concatenated time length no longer matches self's mask, so the implicit carry discards it (issue #1331). A static (n_element,) mask survives, since it applies to every step. Re-attach a per-step mask after concatenating if you need one.

Parameters:

Name	Type	Description	Default
others	UgridDataset | list[UgridDataset]	One dataset, or a list of them, to append after self in order.	required

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset spanning the concatenated time axis.

Raises:

Type	Description
AlignmentError	A dataset sits on a different mesh. Note this is not a ValueError subclass, so a caller catching ValueError around concat has to catch AlignmentError instead.
ValueError	A dataset carries a different variable set, or a variable has no loaded data.

Source code in src/pyramids/netcdf/ugrid/dataset.py

@_joins_meshes
def concat(self, others: UgridDataset | list[UgridDataset]) -> UgridDataset:
    """Concatenate same-topology datasets along time.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.concat`. Every dataset must sit
    on the same mesh. Each temporal variable's data is concatenated along its time axis
    (and its ``time_values`` coordinate, when present); a static variable is taken from
    ``self`` unchanged. The variable set must match across datasets.

    A **per-step** active mask (:attr:`MeshVariable.active` of shape ``(n_time, n_element)``)
    is **dropped** on concatenation — the parts' masks are not joined along time, and the
    concatenated time length no longer matches ``self``'s mask, so the implicit carry
    discards it (issue #1331). A **static** ``(n_element,)`` mask survives, since it applies
    to every step. Re-attach a per-step mask after concatenating if you need one.

    Args:
        others: One dataset, or a list of them, to append after ``self`` in order.

    Returns:
        UgridDataset: A new dataset spanning the concatenated time axis.

    Raises:
        AlignmentError: A dataset sits on a different mesh. Note this is **not** a
            `ValueError` subclass, so a caller catching `ValueError` around `concat` has
            to catch `AlignmentError` instead.
        ValueError: A dataset carries a different variable set, or a variable has no
            loaded data.
    """
    parts = [
        self,
        *([others] if isinstance(others, UgridDataset) else list(others)),
    ]
    for other in parts[1:]:
        self._assert_same_topology(other, "concat")
        if set(other._data_variables) != set(self._data_variables):
            raise ValueError(
                "concat requires the same variables in every dataset; got "
                f"{sorted(self._data_variables)} vs {sorted(other._data_variables)}."
            )
    new_vars: dict[str, MeshVariable] = {}
    for name, var in self._data_variables.items():
        if not var.has_time:
            # Static in self: refuse if any other part has it temporal, rather than
            # silently drop that part's time series (the mirror of the temporal-in-self
            # / static-in-other case _concat_temporal_variable rejects).
            if any(part._data_variables[name].has_time for part in parts[1:]):
                raise ValueError(
                    f"concat: variable {name!r} is static in one dataset but temporal "
                    "in another; they cannot be joined along time."
                )
            new_vars[name] = var  # a static variable is taken from self, as-is
        else:
            new_vars[name] = _concat_temporal_variable(name, var, parts)
    return self._rebuild(new_vars)

merge(others, *, compat='no_conflicts')

Merge the variables of same-topology datasets into one.

The mesh counterpart of :meth:pyramids.netcdf.NetCDF.merge. Every dataset must sit on the same mesh; the union of their variables is returned. A variable name present in more than one dataset is a conflict and is refused.

Parameters:

Name	Type	Description	Default
others	UgridDataset | list[UgridDataset]	One dataset, or a list of them, whose variables join self's. Under the class spelling UgridDataset.merge([a, b, c]) this one argument is the whole join: its first element becomes self and the rest are merged onto it.	required
compat	str	How to handle a variable name present in more than one dataset -- "no_conflicts" refuses the collision, "override" keeps the value from the first dataset in join order that carries the name: self, then each of others in turn. That is self's only when self has the variable; when it does not, the first of others carrying it wins. (Under the class spelling UgridDataset.merge([a, b, c]), self is a.) Defaults to "no_conflicts".	'no_conflicts'

Returns:

Name	Type	Description
UgridDataset	UgridDataset	A new dataset carrying every variable.

Raises:

Type	Description
AlignmentError	A dataset sits on a different mesh.
ValueError	An unknown compat, or a variable name collides under compat="no_conflicts".

Source code in src/pyramids/netcdf/ugrid/dataset.py

@_joins_meshes
def merge(
    self,
    others: UgridDataset | list[UgridDataset],
    *,
    compat: str = "no_conflicts",
) -> UgridDataset:
    """Merge the variables of same-topology datasets into one.

    The mesh counterpart of :meth:`pyramids.netcdf.NetCDF.merge`. Every dataset must sit
    on the same mesh; the union of their variables is returned. A variable name present
    in more than one dataset is a conflict and is refused.

    Args:
        others: One dataset, or a list of them, whose variables join ``self``'s. Under
            the class spelling ``UgridDataset.merge([a, b, c])`` this one argument is the
            whole join: its first element becomes ``self`` and the rest are merged onto it.
        compat: How to handle a variable name present in more than one dataset --
            ``"no_conflicts"`` refuses the collision, ``"override"`` keeps the value from
            the **first dataset in join order that carries the name**: ``self``, then each
            of ``others`` in turn. That is ``self``'s only when ``self`` has the variable;
            when it does not, the first of ``others`` carrying it wins. (Under the class
            spelling ``UgridDataset.merge([a, b, c])``, ``self`` is ``a``.) Defaults to
            ``"no_conflicts"``.

    Returns:
        UgridDataset: A new dataset carrying every variable.

    Raises:
        AlignmentError: A dataset sits on a different mesh.
        ValueError: An unknown ``compat``, or a variable name collides under
            ``compat="no_conflicts"``.
    """
    if compat not in ("no_conflicts", "override"):
        raise ValueError(
            f"merge: compat must be 'no_conflicts' or 'override', got {compat!r}."
        )
    parts = [others] if isinstance(others, UgridDataset) else list(others)
    new_vars: dict[str, MeshVariable] = dict(self._data_variables)
    for other in parts:
        self._assert_same_topology(other, "merge")
        for name, var in other._data_variables.items():
            if name in new_vars:
                if compat == "no_conflicts":
                    raise ValueError(
                        f"merge: variable {name!r} is present in more than one dataset; "
                        "rename it first, or pass compat='override'."
                    )
                # compat="override": the first dataset that carries the name wins, which
                # is what already sits in new_vars, so the duplicate is simply skipped.
                continue
            new_vars[name] = var
    return self._rebuild(new_vars)

plot(variable_name, ax=None, cmap='viridis', title=None, basemap=None, colorbar=None, points=None, kind='auto', color=None, contour=None, data_style=None, **kwargs)

Plot a mesh data variable.

N-6 — this facade now goes through the same module-level helper as the raster path. The mesh-specific dispatch lives in :func:pyramids.dataset._plot_helpers.mesh_render; both Dataset.plot/NetCDF.plot and UgridDataset.plot share the "resolve data, hand to a single helper" contract so the two formats no longer maintain independent plotting code paths.

Parameters:

Name	Type	Description	Default
variable_name	str	Name of the data variable to plot.	required
ax	Any	matplotlib Axes. Created if None.	None
cmap	str	Colormap name.	'viridis'
title	str | None	Plot title. Defaults to variable name.	None
basemap	bool | str | None	If True, add an OpenStreetMap basemap. If a string, use it as the tile provider name (e.g. "CartoDB.Positron"). Default is None (no basemap). Requires the [viz] extra.	None
colorbar	bool or ColorBar	Colour-bar spec, part of the shared plot signature. A pyramids.plot.ColorBar(label=…, …) configures the bar; False hides it and None (default) uses cleopatra's default (a bar is drawn). Only forwarded when set.	None
points	ndarray or PointOverlay	Accepted for signature symmetry with the raster plot family, but a no-op here — a mesh has no point-overlay layer (the mesh geometry is the data). Ignored.	None
kind	str	Accepted for signature symmetry with the raster plot family, but a no-op here — kind selects a raster renderer (imshow/pcolormesh); the mesh always renders via tripcolor/tricontour. Ignored.	'auto'
color	ColorScaling	Colour-scale spec pyramids.plot.ColorScaling (linear / power / sym-log / boundary / midpoint norm), e.g. ColorScaling.power(gamma=0.7). Default None.	None
contour	Contour	Contour-line spec pyramids.plot.Contour(levels=…, label_kw=…). Default None.	None
data_style	DataStyle	Data-style / relief spec pyramids.plot.DataStyle(style=…, hillshade=…). (A mesh has no cell-value overlay, so there is no cells param here.) Default None.	None
**kwargs	Any	Additional arguments passed to mesh_render (forwarded to plot_mesh_data). Notably colorbar (bool, default True): pass colorbar=False to suppress the per-mesh colorbar when you want to attach a custom or shared one to glyph.ax. Also style (name of a cleopatra DATA_STYLES preset, e.g. "flow_accumulation") and hillshade (True or a params dict) to colour / relief- shade the mesh; both require cleopatra >= 0.24 (hillshade needs node-centered data). Distinct from :meth:pyramids.dataset.Dataset.hillshade, which returns a shaded-relief array.	{}

Returns:

Type

Description

Any

cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the plot rendered. Use the returned object to access the matplotlib handles and the mappable: glyph.fig / glyph.ax — Figure and Axes. glyph.im — the mesh mappable (the tripcolor/tricontour(f) artist) set by plot(); use it for a custom colorbar or glyph.im.set_clim(...). It is None after :meth:plot_outline (an outline carries no scalar mapping). glyph.apply_style(style) (cleopatra >= 0.25) — re-apply a DATA_STYLES preset by name in place, without re-plotting.

Raises:

Type	Description
ValueError	If the selected variable has no loaded data, or if basemap is requested while the dataset has no CRS (epsg).

Source code in src/pyramids/netcdf/ugrid/dataset.py

def plot(
    self,
    variable_name: str,
    ax: Any = None,
    cmap: str = "viridis",
    title: str | None = None,
    basemap: bool | str | None = None,
    colorbar: bool | ColorBar | None = None,
    points: np.ndarray | PointOverlay | None = None,
    kind: str = "auto",
    color: ColorScaling | None = None,
    contour: Contour | None = None,
    data_style: DataStyle | None = None,
    **kwargs: Any,
) -> Any:
    """Plot a mesh data variable.

    N-6 — this facade now goes through the same module-level
    helper as the raster path. The mesh-specific dispatch lives in
    :func:`pyramids.dataset._plot_helpers.mesh_render`; both
    ``Dataset.plot``/``NetCDF.plot`` and ``UgridDataset.plot`` share
    the "resolve data, hand to a single helper" contract so the
    two formats no longer maintain independent plotting code paths.

    Args:
        variable_name: Name of the data variable to plot.
        ax: matplotlib Axes. Created if None.
        cmap: Colormap name.
        title: Plot title. Defaults to variable name.
        basemap: If True, add an OpenStreetMap basemap. If a string,
            use it as the tile provider name (e.g. "CartoDB.Positron").
            Default is None (no basemap). Requires the [viz] extra.
        colorbar (bool or ColorBar, optional): Colour-bar spec, part of the
            shared plot signature. A ``pyramids.plot.ColorBar(label=…, …)``
            configures the bar; ``False`` hides it and ``None`` (default) uses
            cleopatra's default (a bar is drawn). Only forwarded when set.
        points (np.ndarray or PointOverlay, optional): Accepted for signature
            symmetry with the raster plot family, but a **no-op here** — a mesh
            has no point-overlay layer (the mesh geometry is the data). Ignored.
        kind (str, optional): Accepted for signature symmetry with the raster
            plot family, but a **no-op here** — ``kind`` selects a raster
            renderer (``imshow``/``pcolormesh``); the mesh always renders via
            ``tripcolor``/``tricontour``. Ignored.
        color (ColorScaling, optional): Colour-scale spec
            ``pyramids.plot.ColorScaling`` (linear / power / sym-log / boundary /
            midpoint norm), e.g. ``ColorScaling.power(gamma=0.7)``. Default ``None``.
        contour (Contour, optional): Contour-line spec
            ``pyramids.plot.Contour(levels=…, label_kw=…)``. Default ``None``.
        data_style (DataStyle, optional): Data-style / relief spec
            ``pyramids.plot.DataStyle(style=…, hillshade=…)``. (A mesh has no
            cell-value overlay, so there is no ``cells`` param here.) Default ``None``.
        **kwargs: Additional arguments passed to mesh_render
            (forwarded to plot_mesh_data). Notably ``colorbar``
            (``bool``, default ``True``): pass ``colorbar=False`` to
            suppress the per-mesh colorbar when you want to attach a
            custom or shared one to ``glyph.ax``. Also ``style`` (name of
            a cleopatra ``DATA_STYLES`` preset, e.g. ``"flow_accumulation"``)
            and ``hillshade`` (``True`` or a params dict) to colour / relief-
            shade the mesh; both require cleopatra >= 0.24 (``hillshade``
            needs node-centered data). Distinct from
            :meth:`pyramids.dataset.Dataset.hillshade`, which *returns* a
            shaded-relief array.

    Returns:
        cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the plot
            rendered. Use the returned object to access the matplotlib
            handles and the mappable:

            - ``glyph.fig`` / ``glyph.ax`` — Figure and Axes.
            - ``glyph.im`` — the mesh mappable (the
              ``tripcolor``/``tricontour(f)`` artist) set by ``plot()``;
              use it for a custom colorbar or ``glyph.im.set_clim(...)``.
              It is ``None`` after :meth:`plot_outline` (an outline
              carries no scalar mapping).
            - ``glyph.apply_style(style)`` (cleopatra >= 0.25) — re-apply a
              ``DATA_STYLES`` preset by name in place, without re-plotting.

    Raises:
        ValueError: If the selected variable has no loaded data, or if
            `basemap` is requested while the dataset has no CRS (`epsg`).
    """
    var = self.get_data(variable_name)
    data = var.data
    if data is None:
        raise ValueError(f"Variable {variable_name!r} has no loaded data to plot.")
    if var.has_time:
        # First step along the variable's own time axis (see `_element_values`).
        data = np.take(np.asarray(data), 0, axis=cast("int", var.time_index))
    if title is None:
        title = variable_name
    if basemap and self.epsg is None:
        raise ValueError("UgridDataset must have a CRS (epsg) to use basemap.")
    # ``points`` / ``kind`` are part of the shared raster-family plot signature
    # but have no meaning for a mesh (no point overlay; the renderer is fixed to
    # tripcolor/tricontour), so they are accepted and ignored. ``colorbar`` and the
    # typed render groups (``color`` / ``contour`` / ``data_style``) map onto the mesh
    # backend and are forwarded only when set (so cleopatra's backend defaults are
    # preserved otherwise).
    if colorbar is not None:
        kwargs["colorbar"] = colorbar
    kwargs.update(
        _nonnull_group_kwargs(color=color, contour=contour, data_style=data_style)
    )
    result = _mesh_render(
        mesh=self._mesh,
        data=data,
        location=var.location,
        ax=ax,
        cmap=cmap,
        title=title,
        basemap=basemap,
        basemap_epsg=self.epsg,
        **kwargs,
    )
    return result

plot_outline(ax=None, **kwargs)

Plot mesh wireframe.

Parameters:

Name	Type	Description	Default
ax	Any	matplotlib Axes. Created if None.	None
**kwargs	Any	Additional arguments passed to plot_mesh_outline.	{}

Returns:

Type	Description
Any	cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the wireframe rendered. glyph.fig / glyph.ax are the matplotlib handles; glyph.im is None (an outline carries no scalar mapping, so no mappable is produced).

Source code in src/pyramids/netcdf/ugrid/dataset.py

def plot_outline(self, ax: Any = None, **kwargs: Any) -> Any:
    """Plot mesh wireframe.

    Args:
        ax: matplotlib Axes. Created if None.
        **kwargs: Additional arguments passed to plot_mesh_outline.

    Returns:
        cleopatra.glyphs.gridded.mesh_glyph.MeshGlyph instance with the wireframe
            rendered. ``glyph.fig`` / ``glyph.ax`` are the matplotlib
            handles; ``glyph.im`` is ``None`` (an outline carries no
            scalar mapping, so no mappable is produced).
    """
    from pyramids.netcdf.ugrid.plot import plot_mesh_outline

    result = plot_mesh_outline(self._mesh, ax=ax, **kwargs)
    return result

__str__()

Human-readable summary of the dataset.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __str__(self) -> str:
    """Human-readable summary of the dataset."""
    lines = [
        f"UgridDataset: {self._file_name or '(in-memory)'}",
        f"  Mesh: {self.mesh_name}",
        f"  Nodes: {self.n_node}, Faces: {self.n_face}, Edges: {self.n_edge}",
        f"  Bounds: {self.bounds}",
        # A mesh may carry a projected WKT with no EPSG code; reporting
        # 'unknown' for a CRS it plainly has was misleading.
        f"  CRS: {self.epsg or (self.crs.name if self.crs is not None else 'unknown')}",
        f"  Data variables ({len(self._data_variables)}):",
    ]
    for name, var in self._data_variables.items():
        lines.append(f"    {name}: location={var.location}, shape={var.shape}")
    result = "\n".join(lines)
    return result

__repr__()

Repr string for the dataset.

Source code in src/pyramids/netcdf/ugrid/dataset.py

def __repr__(self) -> str:
    """Repr string for the dataset."""
    result = (
        f"UgridDataset(mesh='{self.mesh_name}', "
        f"n_node={self.n_node}, n_face={self.n_face}, n_edge={self.n_edge}, "
        f"variables={self.data_variable_names})"
    )
    return result