Coerce IndexVariable to Variable when assigning to data variables or coordinates

doc/whats-new.rst

@@ -34,6 +34,9 @@ Deprecations
 Bug Fixes
 ~~~~~~~~~
 
+- When assigning an indexed coordinate to a data variable or coordinate, coerce it from
+  ``IndexVariable`` to ``Variable`` (:issue:`9859`, :issue:`10829`, :pull:`10909`)
+  By `Julia Signell <https://github.com/jsignell>`_
 - The NetCDF4 backend will now claim to be able to read any URL except for one that contains
   the substring zarr. This restores backward compatibility after
   :pull:`10804` broke workflows that relied on ``xr.open_dataset("http://...")``

xarray/core/coordinates.py

@@ -1265,7 +1265,7 @@ def create_coords_with_default_indexes(
             variables.update(idx_vars)
             all_variables.update(idx_vars)
         else:
-            variables[name] = variable
+            variables[name] = variable.to_base_variable()
 
     new_coords = Coordinates._construct_direct(coords=variables, indexes=indexes)
 

xarray/structure/merge.py

@@ -22,7 +22,12 @@
     emit_user_level_warning,
     equivalent,
 )
-from xarray.core.variable import Variable, as_variable, calculate_dimensions
+from xarray.core.variable import (
+    IndexVariable,
+    Variable,
+    as_variable,
+    calculate_dimensions,
+)
 from xarray.structure.alignment import deep_align
 from xarray.util.deprecation_helpers import (
     _COMPAT_DEFAULT,
@@ -1206,7 +1211,11 @@ def dataset_update_method(dataset: Dataset, other: CoercibleMapping) -> _MergeRe
                     if c not in value.dims and c in dataset.coords
                 ]
                 if coord_names:
-                    other[key] = value.drop_vars(coord_names)
+                    value = value.drop_vars(coord_names)
+                if isinstance(value.variable, IndexVariable):
+                    variable = value.variable.to_base_variable()
+                    value = value._replace(variable=variable)
+                other[key] = value
 
     return merge_core(
         [dataset, other],

xarray/testing/assertions.py

@@ -3,6 +3,7 @@
 import functools
 import warnings
 from collections.abc import Hashable
+from typing import Any
 
 import numpy as np
 import pandas as pd
@@ -362,6 +363,17 @@ def _assert_indexes_invariants_checks(
             if isinstance(v, IndexVariable)
         }
         assert indexes.keys() <= index_vars, (set(indexes), index_vars)
+        assert all(
+            k in index_vars
+            for k, v in possible_coord_variables.items()
+            if v.dims == (k,)
+        ), {k: type(v) for k, v in possible_coord_variables.items()}
+
+    assert not any(
+        isinstance(v, IndexVariable)
+        for k, v in possible_coord_variables.items()
+        if k not in indexes.keys()
+    ), {k: type(v) for k, v in possible_coord_variables.items()}
 
     # check pandas index wrappers vs. coordinate data adapters
     for k, index in indexes.items():
@@ -401,11 +413,17 @@ def _assert_indexes_invariants_checks(
         )
 
 
-def _assert_variable_invariants(var: Variable, name: Hashable = None):
+def _assert_variable_invariants(
+    var: Variable | Any,
+    name: Hashable = None,
+) -> None:
     if name is None:
         name_or_empty: tuple = ()
     else:
         name_or_empty = (name,)
+
+    assert isinstance(var, Variable), {name: type(var)}
+
     assert isinstance(var._dims, tuple), name_or_empty + (var._dims,)
     assert len(var._dims) == len(var._data.shape), name_or_empty + (
         var._dims,
@@ -418,35 +436,28 @@ def _assert_variable_invariants(var: Variable, name: Hashable = None):
 
 
 def _assert_dataarray_invariants(da: DataArray, check_default_indexes: bool):
-    assert isinstance(da._variable, Variable), da._variable
     _assert_variable_invariants(da._variable)
 
     assert isinstance(da._coords, dict), da._coords
-    assert all(isinstance(v, Variable) for v in da._coords.values()), da._coords
 
     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)
 
-    if da._indexes is not None:
-        _assert_indexes_invariants_checks(
-            da._indexes, da._coords, da.dims, check_default=check_default_indexes
-        )
+    assert da._indexes is not None
+    _assert_indexes_invariants_checks(
+        da._indexes, da._coords, da.dims, check_default=check_default_indexes
+    )
 
 
 def _assert_dataset_invariants(ds: Dataset, check_default_indexes: bool):
     assert isinstance(ds._variables, dict), type(ds._variables)
-    assert all(isinstance(v, Variable) for v in ds._variables.values()), ds._variables
+
     for k, v in ds._variables.items():
         _assert_variable_invariants(v, k)
 
@@ -466,17 +477,10 @@ def _assert_dataset_invariants(ds: Dataset, check_default_indexes: bool):
         ds._dims[k] == v.sizes[k] for v in ds._variables.values() for k in v.sizes
     ), (ds._dims, {k: v.sizes for k, v in ds._variables.items()})
 
-    if check_default_indexes:
-        assert all(
-            isinstance(v, IndexVariable)
-            for (k, v) in ds._variables.items()
-            if v.dims == (k,)
-        ), {k: type(v) for k, v in ds._variables.items() if v.dims == (k,)}
-
-    if ds._indexes is not None:
-        _assert_indexes_invariants_checks(
-            ds._indexes, ds._variables, ds._dims, check_default=check_default_indexes
-        )
+    assert ds._indexes is not None
+    _assert_indexes_invariants_checks(
+        ds._indexes, ds._variables, ds._dims, check_default=check_default_indexes
+    )
 
     assert isinstance(ds._encoding, type(None) | dict)
     assert isinstance(ds._attrs, type(None) | dict)

xarray/tests/test_dataarray.py

@@ -1702,6 +1702,20 @@ def should_add_coord_to_array(self, name, var, dims):
         assert_identical(actual, expected, check_default_indexes=False)
         assert "x_bnds" not in actual.dims
 
+    def test_assign_coords_uses_base_variable_class(self) -> None:
+        a = DataArray([0, 1, 3], dims=["x"], coords={"x": [0, 1, 2]})
+        a = a.assign_coords(foo=a.x)
+
+        # explicit check
+        assert isinstance(a["x"].variable, IndexVariable)
+        assert not isinstance(a["foo"].variable, IndexVariable)
+
+        # test internal invariant checks when comparing the datasets
+        expected = DataArray(
+            [0, 1, 3], dims=["x"], coords={"x": [0, 1, 2], "foo": ("x", [0, 1, 2])}
+        )
+        assert_identical(a, expected)
+
     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

@@ -4311,9 +4311,11 @@ def test_to_stacked_array_preserves_dtype(self) -> None:
         # coordinate created from variables names should be of string dtype
         data = np.array(["a", "a", "a", "b"], dtype="<U1")
         expected_stacked_variable = DataArray(name="variable", data=data, dims="z")
+
+        # coerce from `IndexVariable` to `Variable` before comparing
         assert_identical(
-            stacked.coords["variable"].drop_vars(["z", "variable", "y"]),
-            expected_stacked_variable,
+            stacked["variable"].variable.to_base_variable(),
+            expected_stacked_variable.variable,
         )
 
     def test_to_stacked_array_transposed(self) -> None:
@@ -4779,6 +4781,18 @@ def test_setitem_using_list_errors(self, var_list, data, error_regex) -> None:
         with pytest.raises(ValueError, match=error_regex):
             actual[var_list] = data
 
+    def test_setitem_uses_base_variable_class_even_for_index_variables(self) -> None:
+        ds = Dataset(coords={"x": [1, 2, 3]})
+        ds["y"] = ds["x"]
+
+        # explicit check
+        assert isinstance(ds["x"].variable, IndexVariable)
+        assert not isinstance(ds["y"].variable, IndexVariable)
+
+        # test internal invariant checks when comparing the datasets
+        expected = Dataset(data_vars={"y": ("x", [1, 2, 3])}, coords={"x": [1, 2, 3]})
+        assert_identical(ds, expected)
+
     def test_assign(self) -> None:
         ds = Dataset()
         actual = ds.assign(x=[0, 1, 2], y=2)