Add Index.validate_dataarray_coord

doc/api-hidden.rst

@@ -520,6 +520,7 @@
    Index.stack
    Index.unstack
    Index.create_variables
+   Index.should_add_coord_to_array
    Index.to_pandas_index
    Index.isel
    Index.sel

doc/api.rst

@@ -1645,6 +1645,7 @@ Exceptions
    :toctree: generated/
 
    AlignmentError
+   CoordinateValidationError
    MergeError
    SerializationWarning
 

doc/whats-new.rst

@@ -34,6 +34,10 @@ New Features
 - Improved compatibility with OPeNDAP DAP4 data model for backend engine ``pydap``. This
   includes ``datatree`` support, and removing slashes from dimension names. By
   `Miguel Jimenez-Urias <https://github.com/Mikejmnez>`_.
+- Allow assigning index coordinates with non-array dimension(s) in a :py:class:`DataArray` by overriding
+  :py:meth:`Index.should_add_coord_to_array`. For example, this enables support for CF boundaries coordinate (e.g.,
+  ``time(time)`` and ``time_bnds(time, nbnd)``) in a DataArray (:pull:`10137`).
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 - Improved support pandas categorical extension as indices (i.e., :py:class:`pandas.IntervalIndex`). (:issue:`9661`, :pull:`9671`)
   By `Ilan Gold <https://github.com/ilan-gold>`_.
 - Improved checks and errors raised when trying to align objects with conflicting indexes.

xarray/__init__.py

@@ -28,7 +28,7 @@
 )
 from xarray.conventions import SerializationWarning, decode_cf
 from xarray.core.common import ALL_DIMS, full_like, ones_like, zeros_like
-from xarray.core.coordinates import Coordinates
+from xarray.core.coordinates import Coordinates, CoordinateValidationError
 from xarray.core.dataarray import DataArray
 from xarray.core.dataset import Dataset
 from xarray.core.datatree import DataTree
@@ -129,6 +129,7 @@
     "Variable",
     # Exceptions
     "AlignmentError",
+    "CoordinateValidationError",
     "InvalidTreeError",
     "MergeError",
     "NotFoundInTreeError",

xarray/core/coordinates.py

@@ -486,7 +486,7 @@ def identical(self, other: Self) -> bool:
         return self.to_dataset().identical(other.to_dataset())
 
     def _update_coords(
-        self, coords: dict[Hashable, Variable], indexes: Mapping[Any, Index]
+        self, coords: dict[Hashable, Variable], indexes: dict[Hashable, Index]
     ) -> None:
         # redirect to DatasetCoordinates._update_coords
         self._data.coords._update_coords(coords, indexes)
@@ -780,7 +780,7 @@ def to_dataset(self) -> Dataset:
         return self._data._copy_listed(names)
 
     def _update_coords(
-        self, coords: dict[Hashable, Variable], indexes: Mapping[Any, Index]
+        self, coords: dict[Hashable, Variable], indexes: dict[Hashable, Index]
     ) -> None:
         variables = self._data._variables.copy()
         variables.update(coords)
@@ -880,7 +880,7 @@ def to_dataset(self) -> Dataset:
         return self._data.dataset._copy_listed(self._names)
 
     def _update_coords(
-        self, coords: dict[Hashable, Variable], indexes: Mapping[Any, Index]
+        self, coords: dict[Hashable, Variable], indexes: dict[Hashable, Index]
     ) -> None:
         from xarray.core.datatree import check_alignment
 
@@ -964,22 +964,14 @@ def __getitem__(self, key: Hashable) -> T_DataArray:
         return self._data._getitem_coord(key)
 
     def _update_coords(
-        self, coords: dict[Hashable, Variable], indexes: Mapping[Any, Index]
+        self, coords: dict[Hashable, Variable], indexes: dict[Hashable, Index]
     ) -> None:
-        coords_plus_data = coords.copy()
-        coords_plus_data[_THIS_ARRAY] = self._data.variable
-        dims = calculate_dimensions(coords_plus_data)
-        if not set(dims) <= set(self.dims):
-            raise ValueError(
-                "cannot add coordinates with new dimensions to a DataArray"
-            )
-        self._data._coords = coords
+        validate_dataarray_coords(
+            self._data.shape, Coordinates._construct_direct(coords, indexes), self.dims
+        )
 
-        # TODO(shoyer): once ._indexes is always populated by a dict, modify
-        # it to update inplace instead.
-        original_indexes = dict(self._data.xindexes)
-        original_indexes.update(indexes)
-        self._data._indexes = original_indexes
+        self._data._coords = coords
+        self._data._indexes = indexes
 
     def _drop_coords(self, coord_names):
         # should drop indexed coordinates only
@@ -1154,9 +1146,58 @@ def create_coords_with_default_indexes(
     return new_coords
 
 
-def _coordinates_from_variable(variable: Variable) -> Coordinates:
-    from xarray.core.indexes import create_default_index_implicit
+class CoordinateValidationError(ValueError):
+    """Error class for Xarray coordinate validation failures."""
+
+
+def validate_dataarray_coords(
+    shape: tuple[int, ...],
+    coords: Coordinates | Mapping[Hashable, Variable],
+    dim: tuple[Hashable, ...],
+):
+    """Validate coordinates ``coords`` to include in a DataArray defined by
+    ``shape`` and dimensions ``dim``.
+
+    If a coordinate is associated with an index, the validation is performed by
+    the index. By default the coordinate dimensions must match (a subset of) the
+    array dimensions (in any order) to conform to the DataArray model. The index
+    may override this behavior with other validation rules, though.
+
+    Non-index coordinates must all conform to the DataArray model. Scalar
+    coordinates are always valid.
+    """
+    sizes = dict(zip(dim, shape, strict=True))
+    dim_set = set(dim)
+
+    indexes: Mapping[Hashable, Index]
+    if isinstance(coords, Coordinates):
+        indexes = coords.xindexes
+    else:
+        indexes = {}
+
+    for k, v in coords.items():
+        if k in indexes:
+            invalid = not indexes[k].should_add_coord_to_array(k, v, dim_set)
+        else:
+            invalid = any(d not in dim for d in v.dims)
+
+        if invalid:
+            raise CoordinateValidationError(
+                f"coordinate {k} has dimensions {v.dims}, but these "
+                "are not a subset of the DataArray "
+                f"dimensions {dim}"
+            )
+
+        for d, s in v.sizes.items():
+            if d in sizes and s != sizes[d]:
+                raise CoordinateValidationError(
+                    f"conflicting sizes for dimension {d!r}: "
+                    f"length {sizes[d]} on the data but length {s} on "
+                    f"coordinate {k!r}"
+                )
+
 
+def coordinates_from_variable(variable: Variable) -> Coordinates:
     (name,) = variable.dims
     new_index, index_vars = create_default_index_implicit(variable)
     indexes = dict.fromkeys(index_vars, new_index)

xarray/core/dataarray.py

@@ -33,6 +33,7 @@
     DataArrayCoordinates,
     assert_coordinate_consistent,
     create_coords_with_default_indexes,
+    validate_dataarray_coords,
 )
 from xarray.core.dataset import Dataset
 from xarray.core.extension_array import PandasExtensionArray
@@ -124,25 +125,6 @@
     T_XarrayOther = TypeVar("T_XarrayOther", bound="DataArray" | Dataset)
 
 
-def _check_coords_dims(shape, coords, dim):
-    sizes = dict(zip(dim, shape, strict=True))
-    for k, v in coords.items():
-        if any(d not in dim for d in v.dims):
-            raise ValueError(
-                f"coordinate {k} has dimensions {v.dims}, but these "
-                "are not a subset of the DataArray "
-                f"dimensions {dim}"
-            )
-
-        for d, s in v.sizes.items():
-            if s != sizes[d]:
-                raise ValueError(
-                    f"conflicting sizes for dimension {d!r}: "
-                    f"length {sizes[d]} on the data but length {s} on "
-                    f"coordinate {k!r}"
-                )
-
-
 def _infer_coords_and_dims(
     shape: tuple[int, ...],
     coords: (
@@ -206,7 +188,7 @@ def _infer_coords_and_dims(
                 var.dims = (dim,)
                 new_coords[dim] = var.to_index_variable()
 
-    _check_coords_dims(shape, new_coords, dims_tuple)
+    validate_dataarray_coords(shape, new_coords, dims_tuple)
 
     return new_coords, dims_tuple
 

xarray/core/dataset.py

@@ -1159,7 +1159,15 @@ def _construct_dataarray(self, name: Hashable) -> DataArray:
         coords: dict[Hashable, Variable] = {}
         # preserve ordering
         for k in self._variables:
-            if k in self._coord_names and set(self._variables[k].dims) <= needed_dims:
+            if k in self._indexes:
+                add_coord = self._indexes[k].should_add_coord_to_array(
+                    k, self._variables[k], needed_dims
+                )
+            else:
+                var_dims = set(self._variables[k].dims)
+                add_coord = k in self._coord_names and var_dims <= needed_dims
+
+            if add_coord:
                 coords[k] = self._variables[k]
 
         indexes = filter_indexes_from_coords(self._indexes, set(coords))

xarray/core/groupby.py

@@ -23,7 +23,7 @@
     DatasetGroupByAggregations,
 )
 from xarray.core.common import ImplementsArrayReduce, ImplementsDatasetReduce
-from xarray.core.coordinates import Coordinates, _coordinates_from_variable
+from xarray.core.coordinates import Coordinates, coordinates_from_variable
 from xarray.core.duck_array_ops import where
 from xarray.core.formatting import format_array_flat
 from xarray.core.indexes import (
@@ -1147,7 +1147,7 @@ def _flox_reduce(
                 new_coords.append(
                     # Using IndexVariable here ensures we reconstruct PandasMultiIndex with
                     # all associated levels properly.
-                    _coordinates_from_variable(
+                    coordinates_from_variable(
                         IndexVariable(
                             dims=grouper.name,
                             data=output_index,

xarray/core/indexes.py

@@ -196,6 +196,49 @@ def create_variables(
         else:
             return {}
 
+    def should_add_coord_to_array(
+        self,
+        name: Hashable,
+        var: Variable,
+        dims: set[Hashable],
+    ) -> bool:
+        """Define whether or not an index coordinate variable should be added to
+        a new DataArray.
+
+        This method is called repeatedly for each Variable associated with this
+        index when creating a new DataArray (via its constructor or from a
+        Dataset) or updating an existing one. The variables associated with this
+        index are the ones passed to :py:meth:`Index.from_variables` and/or
+        returned by :py:meth:`Index.create_variables`.
+
+        By default returns ``True`` if the dimensions of the coordinate variable
+        are a subset of the array dimensions and ``False`` otherwise (DataArray
+        model). This default behavior may be overridden in Index subclasses to
+        bypass strict conformance with the DataArray model. This is useful for
+        example to include the (n+1)-dimensional cell boundary coordinate
+        associated with an interval index.
+
+        Returning ``False`` will either:
+
+        - raise a :py:class:`CoordinateValidationError` when passing the
+          coordinate directly to a new or an existing DataArray, e.g., via
+          ``DataArray.__init__()`` or ``DataArray.assign_coords()``
+
+        - drop the coordinate (and therefore drop the index) when a new
+          DataArray is constructed by indexing a Dataset
+
+        Parameters
+        ----------
+        name : Hashable
+            Name of a coordinate variable associated to this index.
+        var : Variable
+            Coordinate variable object.
+        dims: tuple
+            Dimensions of the new DataArray object being created.
+
+        """
+        return all(d in dims for d in var.dims)
+
     def to_pandas_index(self) -> pd.Index:
         """Cast this xarray index to a pandas.Index object or raise a
         ``TypeError`` if this is not supported.

xarray/groupers.py

@@ -18,7 +18,7 @@
 
 from xarray.coding.cftime_offsets import BaseCFTimeOffset, _new_to_legacy_freq
 from xarray.computation.apply_ufunc import apply_ufunc
-from xarray.core.coordinates import Coordinates, _coordinates_from_variable
+from xarray.core.coordinates import Coordinates, coordinates_from_variable
 from xarray.core.dataarray import DataArray
 from xarray.core.duck_array_ops import array_all, isnull
 from xarray.core.groupby import T_Group, _DummyGroup
@@ -115,7 +115,7 @@ def __init__(
 
         if coords is None:
             assert not isinstance(self.unique_coord, _DummyGroup)
-            self.coords = _coordinates_from_variable(self.unique_coord)
+            self.coords = coordinates_from_variable(self.unique_coord)
         else:
             self.coords = coords
 
@@ -252,7 +252,7 @@ def _factorize_unique(self) -> EncodedGroups:
             codes=codes,
             full_index=full_index,
             unique_coord=unique_coord,
-            coords=_coordinates_from_variable(unique_coord),
+            coords=coordinates_from_variable(unique_coord),
         )
 
     def _factorize_dummy(self) -> EncodedGroups:
@@ -280,7 +280,7 @@ def _factorize_dummy(self) -> EncodedGroups:
             else:
                 if TYPE_CHECKING:
                     assert isinstance(unique_coord, Variable)
-                coords = _coordinates_from_variable(unique_coord)
+                coords = coordinates_from_variable(unique_coord)
 
         return EncodedGroups(
             codes=codes,
@@ -417,7 +417,7 @@ def factorize(self, group: T_Group) -> EncodedGroups:
             codes=codes,
             full_index=full_index,
             unique_coord=unique_coord,
-            coords=_coordinates_from_variable(unique_coord),
+            coords=coordinates_from_variable(unique_coord),
         )
 
 
@@ -551,7 +551,7 @@ def factorize(self, group: T_Group) -> EncodedGroups:
             group_indices=group_indices,
             full_index=full_index,
             unique_coord=unique_coord,
-            coords=_coordinates_from_variable(unique_coord),
+            coords=coordinates_from_variable(unique_coord),
         )
 
 

xarray/testing/assertions.py

@@ -401,12 +401,12 @@ def _assert_dataarray_invariants(da: DataArray, check_default_indexes: bool):
 
     assert isinstance(da._coords, dict), da._coords
     assert all(isinstance(v, Variable) for v in da._coords.values()), da._coords
-    assert all(set(v.dims) <= set(da.dims) for v in da._coords.values()), (
-        da.dims,
-        {k: v.dims for k, v in da._coords.items()},
-    )
 
     if check_default_indexes:
+        assert all(set(v.dims) <= set(da.dims) for v in da._coords.values()), (
+            da.dims,
+            {k: v.dims for k, v in da._coords.items()},
+        )
         assert all(
             isinstance(v, IndexVariable)
             for (k, v) in da._coords.items()