Raise a TypeError when a Coordinates object is used as a coordinate value

doc/whats-new.rst

@@ -69,6 +69,11 @@ Bug Fixes
   for zarr writes. Existing zarr stores written with the old ``int8`` encoding
   are still read correctly. (:issue:`2937`, :pull:`11318`)
   By `Evan Lyall <https://github.com/elyall>`_.
+- Raise an informative ``TypeError`` when a :py:class:`~xarray.Coordinates` object is
+  passed as a coordinate value, e.g. ``ds.assign_coords({"x": coords})``, instead
+  of silently creating a broken coordinate. Pass the object directly with
+  ``ds.assign_coords(coords)`` (:issue:`10194`).
+  By `NoiceHex <https://github.com/NoiceHax>`_.
 
 
 Documentation

pyproject.toml

@@ -424,6 +424,7 @@ Claus = "Claus"
 Celles = "Celles"
 slowy = "slowy"
 Commun = "Commun"
+Noice = "Noice"
 
 # Tests
 Ome = "Ome"

xarray/core/variable.py

@@ -127,6 +127,7 @@ def as_variable(
         The newly created variable.
 
     """
+    from xarray.core.coordinates import Coordinates
     from xarray.core.dataarray import DataArray
 
     # TODO: consider extending this method to automatically handle Iris and
@@ -160,6 +161,12 @@ def as_variable(
         obj = Variable([], obj)
     elif isinstance(obj, pd.Index | IndexVariable) and obj.name is not None:
         obj = Variable(obj.name, obj)
+    elif isinstance(obj, Coordinates):
+        raise TypeError(
+            f"Variable {name!r}: Using a Coordinates object to construct a variable is "
+            "ambiguous, please pass the Coordinates object directly instead, e.g., "
+            "`obj.assign_coords(coords)` instead of `obj.assign_coords({name: coords})`."
+        )
     elif isinstance(obj, set | dict):
         raise TypeError(f"variable {name!r} has invalid type {type(obj)!r}")
     elif name is not None:

xarray/tests/test_coordinates.py

@@ -289,6 +289,26 @@ def test_rename_vars(self) -> None:
         assert set(actual.dims) == {"x", "y"}
         assert set(actual.variables) == {"a", "u", "v"}
 
+    def test_coordinates_as_coord_value(self) -> None:
+        # a Coordinates object must be passed directly, not as a value of a
+        # mapping of coordinate names (GH10194)
+        ds = Dataset({"foo": ("x", [1, 2, 3])})
+        coords = Coordinates({"x": [4, 5, 6]})
+
+        with pytest.raises(TypeError, match=r"Using a Coordinates object"):
+            ds.assign_coords({"x": coords})
+        with pytest.raises(TypeError, match=r"Using a Coordinates object"):
+            ds.foo.assign_coords({"x": coords})
+        with pytest.raises(TypeError, match=r"Using a Coordinates object"):
+            Dataset({"foo": ("x", [1, 2, 3])}, coords={"x": coords})
+        with pytest.raises(TypeError, match=r"Using a Coordinates object"):
+            DataArray([1, 2, 3], dims="x", coords={"x": coords})
+
+        # passing it directly still works and keeps the index
+        actual = ds.assign_coords(coords)
+        assert_identical(actual, Dataset({"foo": ("x", [1, 2, 3])}, coords=coords))
+        assert "x" in actual.xindexes
+
     def test_operator_merge(self) -> None:
         coords1 = Coordinates({"x": ("x", [0, 1, 2])})
         coords2 = Coordinates({"y": ("y", [3, 4, 5])})

xarray/tests/test_dataset.py

@@ -5021,7 +5021,11 @@ def test_drop_attrs(self) -> None:
         mx = xr.Coordinates.from_pandas_multiindex(
             pd.MultiIndex.from_tuples([(1, 2), (3, 4)], names=["d", "e"]), "z"
         )
-        ds = Dataset(dict(var1=var), coords=dict(y=idx, z=mx)).assign_attrs(a=1, b=2)
+        ds = (
+            Dataset(dict(var1=var), coords=dict(y=idx))
+            .assign_coords(mx)
+            .assign_attrs(a=1, b=2)
+        )
         assert ds.attrs != {}
         assert ds["var1"].attrs != {}
         assert ds["y"].attrs != {}

xarray/tests/test_variable.py

@@ -12,7 +12,14 @@
 import pytest
 import pytz
 
-from xarray import DataArray, Dataset, IndexVariable, Variable, set_options
+from xarray import (
+    Coordinates,
+    DataArray,
+    Dataset,
+    IndexVariable,
+    Variable,
+    set_options,
+)
 from xarray.core import dtypes, duck_array_ops, indexing
 from xarray.core.common import full_like, ones_like, zeros_like
 from xarray.core.extension_array import PandasExtensionArray
@@ -1273,6 +1280,10 @@ def test_as_variable(self):
         with pytest.raises(TypeError):
             as_variable(("x", DataArray([])))
 
+        # GH10194
+        with pytest.raises(TypeError, match=r"Using a Coordinates object"):
+            as_variable(Coordinates({"x": [1, 2, 3]}), name="x")
+
     def test_repr(self):
         v = Variable(["time", "x"], [[1, 2, 3], [4, 5, 6]], {"foo": "bar"})
         v = v.astype(np.uint64)