Raise UserWarning when rename creates a new dimension coord

doc/whats-new.rst

@@ -65,6 +65,10 @@ Bug fixes
   By `András Gunyhó <https://github.com/mgunyho>`_.
 - Avoid use of random numbers in `test_weighted.test_weighted_operations_nonequal_coords` (:issue:`6504`, :pull:`6961`).
   By `Luke Conibear <https://github.com/lukeconibear>`_.
+- Raise a ``UserWarning`` when renaming a coordinate or a dimension creates a
+  non-indexed dimension coordinate, and suggest the user creating an index
+  either with ``swap_dims`` or ``set_index`` (:issue:`6607`, :pull:`6999`). By
+  `Benoît Bovy <https://github.com/benbovy>`_.
 - Use ``keep_attrs=True`` in grouping and resampling operations by default (:issue:`7012`).
   This means :py:attr:`Dataset.attrs` and :py:attr:`DataArray.attrs` are now preserved by default.
   By `Deepak Cherian <https://github.com/dcherian>`_.

xarray/core/dataarray.py

@@ -2032,11 +2032,11 @@ def rename(
         if utils.is_dict_like(new_name_or_name_dict) or new_name_or_name_dict is None:
             # change dims/coords
             name_dict = either_dict_or_kwargs(new_name_or_name_dict, names, "rename")
-            dataset = self._to_temp_dataset().rename(name_dict)
+            dataset = self._to_temp_dataset()._rename(name_dict)
             return self._from_temp_dataset(dataset)
         if utils.hashable(new_name_or_name_dict) and names:
             # change name + dims/coords
-            dataset = self._to_temp_dataset().rename(names)
+            dataset = self._to_temp_dataset()._rename(names)
             dataarray = self._from_temp_dataset(dataset)
             return dataarray._replace(name=new_name_or_name_dict)
         # only change name

xarray/core/dataset.py

@@ -3560,6 +3560,48 @@ def _rename_all(
 
         return variables, coord_names, dims, indexes
 
+    def _rename(
+        self: T_Dataset,
+        name_dict: Mapping[Any, Hashable] | None = None,
+        **names: Hashable,
+    ) -> T_Dataset:
+        """Also used internally by DataArray so that the warning (if any)
+        is raised at the right stack level.
+        """
+        name_dict = either_dict_or_kwargs(name_dict, names, "rename")
+        for k in name_dict.keys():
+            if k not in self and k not in self.dims:
+                raise ValueError(
+                    f"cannot rename {k!r} because it is not a "
+                    "variable or dimension in this dataset"
+                )
+
+            create_dim_coord = False
+            new_k = name_dict[k]
+
+            if k in self.dims and new_k in self._coord_names:
+                coord_dims = self._variables[name_dict[k]].dims
+                if coord_dims == (k,):
+                    create_dim_coord = True
+            elif k in self._coord_names and new_k in self.dims:
+                coord_dims = self._variables[k].dims
+                if coord_dims == (new_k,):
+                    create_dim_coord = True
+
+            if create_dim_coord:
+                warnings.warn(
+                    f"rename {k!r} to {name_dict[k]!r} does not create an index "
+                    "anymore. Try using swap_dims instead or use set_index "
+                    "after rename to create an indexed coordinate.",
+                    UserWarning,
+                    stacklevel=3,
+                )
+
+        variables, coord_names, dims, indexes = self._rename_all(
+            name_dict=name_dict, dims_dict=name_dict
+        )
+        return self._replace(variables, coord_names, dims=dims, indexes=indexes)
+
     def rename(
         self: T_Dataset,
         name_dict: Mapping[Any, Hashable] | None = None,
@@ -3588,18 +3630,7 @@ def rename(
         Dataset.rename_dims
         DataArray.rename
         """
-        name_dict = either_dict_or_kwargs(name_dict, names, "rename")
-        for k in name_dict.keys():
-            if k not in self and k not in self.dims:
-                raise ValueError(
-                    f"cannot rename {k!r} because it is not a "
-                    "variable or dimension in this dataset"
-                )
-
-        variables, coord_names, dims, indexes = self._rename_all(
-            name_dict=name_dict, dims_dict=name_dict
-        )
-        return self._replace(variables, coord_names, dims=dims, indexes=indexes)
+        return self._rename(name_dict=name_dict, **names)
 
     def rename_dims(
         self: T_Dataset,

xarray/tests/test_dataarray.py

@@ -1742,6 +1742,23 @@ def test_rename(self) -> None:
         )
         assert_identical(renamed_all, expected_all)
 
+    def test_rename_dimension_coord_warnings(self) -> None:
+        # create a dimension coordinate by renaming a dimension or coordinate
+        # should raise a warning (no index created)
+        da = DataArray([0, 0], coords={"x": ("y", [0, 1])}, dims="y")
+
+        with pytest.warns(
+            UserWarning, match="rename 'x' to 'y' does not create an index.*"
+        ):
+            da.rename(x="y")
+
+        da = xr.DataArray([0, 0], coords={"y": ("x", [0, 1])}, dims="x")
+
+        with pytest.warns(
+            UserWarning, match="rename 'x' to 'y' does not create an index.*"
+        ):
+            da.rename(x="y")
+
     def test_init_value(self) -> None:
         expected = DataArray(
             np.full((3, 4), 3), dims=["x", "y"], coords=[range(3), range(4)]

xarray/tests/test_dataset.py

@@ -2892,6 +2892,23 @@ def test_rename_dimension_coord(self) -> None:
         actual_2 = original.rename_dims({"x": "x_new"})
         assert "x" in actual_2.xindexes
 
+    def test_rename_dimension_coord_warnings(self) -> None:
+        # create a dimension coordinate by renaming a dimension or coordinate
+        # should raise a warning (no index created)
+        ds = Dataset(coords={"x": ("y", [0, 1])})
+
+        with pytest.warns(
+            UserWarning, match="rename 'x' to 'y' does not create an index.*"
+        ):
+            ds.rename(x="y")
+
+        ds = Dataset(coords={"y": ("x", [0, 1])})
+
+        with pytest.warns(
+            UserWarning, match="rename 'x' to 'y' does not create an index.*"
+        ):
+            ds.rename(x="y")
+
     def test_rename_multiindex(self) -> None:
         mindex = pd.MultiIndex.from_tuples([([1, 2]), ([3, 4])], names=["a", "b"])
         original = Dataset({}, {"x": mindex})