Add Index.should_add_coord_to_array

doc/api-hidden.rst

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

doc/whats-new.rst

@@ -34,6 +34,10 @@ New Features
   By `Benoit Bovy <https://github.com/benbovy>`_.
 - Support reading to `GPU memory with Zarr <https://zarr.readthedocs.io/en/stable/user-guide/gpu.html>`_ (:pull:`10078`).
   By `Deepak Cherian <https://github.com/dcherian>`_.
+- Allow assigning index coordinates with non-array dimension(s) in a :py:class:`DataArray` by overriding
+  :py:meth:`Index.validate_dataarray_coord`. 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>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

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 +964,16 @@ 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
+        from xarray.core.dataarray import check_dataarray_coords
 
-        # 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
+        check_dataarray_coords(
+            self._data.shape, Coordinates._construct_direct(coords, indexes), self.dims
+        )
+
+        self._data._coords = coords
+        self._data._indexes = indexes
 
     def _drop_coords(self, coord_names):
         # should drop indexed coordinates only

xarray/core/dataarray.py

@@ -132,18 +132,33 @@
     T_XarrayOther = TypeVar("T_XarrayOther", bound="DataArray" | Dataset)
 
 
-def _check_coords_dims(shape, coords, dim):
+def check_dataarray_coords(
+    shape: tuple[int, ...],
+    coords: Coordinates | Mapping[Hashable, Variable],
+    dim: tuple[Hashable, ...],
+):
     sizes = dict(zip(dim, shape, strict=True))
+
+    indexes: Mapping[Hashable, Index]
+    if isinstance(coords, Coordinates):
+        indexes = coords.xindexes
+    else:
+        indexes = {}
+
+    dim_set = set(dim)
+
     for k, v in coords.items():
-        if any(d not in dim for d in v.dims):
+        if k in indexes:
+            indexes[k].validate_dataarray_coord(k, v, dim_set)
+        elif 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]:
+            if d in sizes and s != sizes[d]:
                 raise ValueError(
                     f"conflicting sizes for dimension {d!r}: "
                     f"length {sizes[d]} on the data but length {s} on "
@@ -214,7 +229,7 @@ def _infer_coords_and_dims(
                 var.dims = (dim,)
                 new_coords[dim] = var.to_index_variable()
 
-    _check_coords_dims(shape, new_coords, dims_tuple)
+    check_dataarray_coords(shape, new_coords, dims_tuple)
 
     return new_coords, dims_tuple
 

xarray/core/dataset.py

@@ -1212,7 +1212,18 @@ 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:
+            var_dims = set(self._variables[k].dims)
+            if k in self._indexes:
+                try:
+                    self._indexes[k].validate_dataarray_coord(
+                        k, self._variables[k], needed_dims
+                    )
+                    coords[k] = self._variables[k]
+                except ValueError:
+                    # failback to strict DataArray model check (index may be dropped later)
+                    if var_dims <= needed_dims:
+                        coords[k] = self._variables[k]
+            elif k in self._coord_names and var_dims <= needed_dims:
                 coords[k] = self._variables[k]
 
         indexes = filter_indexes_from_coords(self._indexes, set(coords))

xarray/core/indexes.py

@@ -195,6 +195,53 @@ def create_variables(
         else:
             return {}
 
+    def validate_dataarray_coord(
+        self,
+        name: Hashable,
+        var: Variable,
+        dims: set[Hashable],
+    ):
+        """Validate an index coordinate to be included in a DataArray.
+
+        This method is called repeatedly for each coordinate associated with
+        this index when creating a new DataArray (via its constructor or from a
+        Dataset) or updating an existing one.
+
+        By default raises an error if the dimensions of the coordinate variable
+        do conflict with the array dimensions (DataArray model).
+
+        This method may be overridden in Index subclasses, e.g., to include index
+        coordinates that does not strictly conform with the DataArray model. This
+        is useful for example to include (n+1)-dimensional cell boundary
+        coordinates attached to an index.
+
+        When a DataArray is constructed from a Dataset, if the validation fails
+        Xarray will fail back to propagating the coordinate according to the
+        default rules for DataArray (i.e., depending on its dimensions), which
+        may drop this index.
+
+        Parameters
+        ----------
+        name : Hashable
+            Name of a coordinate associated to this index.
+        var : Variable
+            Coordinate variable object.
+        dims: tuple
+            Dataarray's dimensions.
+
+        Raises
+        ------
+        ValueError
+            When validation fails.
+
+        """
+        if any(d not in dims for d in var.dims):
+            raise ValueError(
+                f"coordinate {name} has dimensions {var.dims}, but these "
+                "are not a subset of the DataArray "
+                f"dimensions {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/testing/assertions.py

@@ -330,10 +330,13 @@ def _assert_indexes_invariants_checks(
         k: type(v) for k, v in indexes.items()
     }
 
-    index_vars = {
-        k for k, v in possible_coord_variables.items() if isinstance(v, IndexVariable)
-    }
-    assert indexes.keys() <= index_vars, (set(indexes), index_vars)
+    if check_default:
+        index_vars = {
+            k
+            for k, v in possible_coord_variables.items()
+            if isinstance(v, IndexVariable)
+        }
+        assert indexes.keys() <= index_vars, (set(indexes), index_vars)
 
     # check pandas index wrappers vs. coordinate data adapters
     for k, index in indexes.items():
@@ -395,13 +398,18 @@ 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()},
-    )
-    assert all(
-        isinstance(v, IndexVariable) for (k, v) in da._coords.items() if v.dims == (k,)
-    ), {k: type(v) 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()
+            if v.dims == (k,)
+        ), {k: type(v) for k, v in da._coords.items()}
+
     for k, v in da._coords.items():
         _assert_variable_invariants(v, k)
 

xarray/tests/test_dataarray.py

@@ -529,6 +529,26 @@ class CustomIndex(Index): ...
         # test coordinate variables copied
         assert da.coords["x"] is not coords.variables["x"]
 
+    def test_constructor_extra_dim_index_coord(self) -> None:
+        class AnyIndex(Index):
+            def validate_dataarray_coord(self, name, var, dims):
+                # pass all index coordinates
+                pass
+
+        idx = AnyIndex()
+        coords = Coordinates(
+            coords={
+                "x": ("x", [1, 2]),
+                "x_bounds": (("x", "x_bnds"), [(0.5, 1.5), (1.5, 2.5)]),
+            },
+            indexes={"x": idx, "x_bounds": idx},
+        )
+
+        actual = DataArray([1.0, 2.0], coords=coords, dims="x")
+
+        assert_identical(actual.coords, coords, check_default_indexes=False)
+        assert "x_bnds" not in actual.dims
+
     def test_equals_and_identical(self) -> None:
         orig = DataArray(np.arange(5.0), {"a": 42}, dims="x")
 
@@ -1634,6 +1654,28 @@ def test_assign_coords_no_default_index(self) -> None:
         assert_identical(actual.coords, coords, check_default_indexes=False)
         assert "y" not in actual.xindexes
 
+    def test_assign_coords_extra_dim_index_coord(self) -> None:
+        class AnyIndex(Index):
+            def validate_dataarray_coord(self, name, var, dims):
+                # pass all index coordinates
+                pass
+
+        idx = AnyIndex()
+        coords = Coordinates(
+            coords={
+                "x": ("x", [1, 2]),
+                "x_bounds": (("x", "x_bnds"), [(0.5, 1.5), (1.5, 2.5)]),
+            },
+            indexes={"x": idx, "x_bounds": idx},
+        )
+
+        da = DataArray([1.0, 2.0], dims="x")
+        actual = da.assign_coords(coords)
+        expected = DataArray([1.0, 2.0], coords=coords, dims="x")
+
+        assert_identical(actual, expected, check_default_indexes=False)
+        assert "x_bnds" not in actual.dims
+
     def test_coords_alignment(self) -> None:
         lhs = DataArray([1, 2, 3], [("x", [0, 1, 2])])
         rhs = DataArray([2, 3, 4], [("x", [1, 2, 3])])

xarray/tests/test_dataset.py

@@ -4206,6 +4206,27 @@ def test_getitem_multiple_dtype(self) -> None:
         dataset = Dataset({key: ("dim0", range(1)) for key in keys})
         assert_identical(dataset, dataset[keys])
 
+    def test_getitem_extra_dim_index_coord(self) -> None:
+        class AnyIndex(Index):
+            def validate_dataarray_coord(self, name, var, dims):
+                # pass all index coordinates
+                pass
+
+        idx = AnyIndex()
+        coords = Coordinates(
+            coords={
+                "x": ("x", [1, 2]),
+                "x_bounds": (("x", "x_bnds"), [(0.5, 1.5), (1.5, 2.5)]),
+            },
+            indexes={"x": idx, "x_bounds": idx},
+        )
+
+        ds = Dataset({"foo": (("x"), [1.0, 2.0])}, coords=coords)
+        actual = ds["foo"]
+
+        assert_identical(actual.coords, coords, check_default_indexes=False)
+        assert "x_bnds" not in actual.dims
+
     def test_virtual_variables_default_coords(self) -> None:
         dataset = Dataset({"foo": ("x", range(10))})
         expected1 = DataArray(range(10), dims="x", name="x")