List coordinates in coords __delitem__ error message, update tests

xarray/core/coordinates.py

@@ -624,7 +624,9 @@ def __delitem__(self, key: Hashable) -> None:
         if key in self:
             del self._data[key]
         else:
-            raise KeyError(f"{key!r} is not a coordinate variable.")
+            raise KeyError(
+                f"{key!r} is not in coordinate variables {tuple(self.keys())}"
+            )
 
     def _ipython_key_completions_(self):
         """Provide method for the key-autocompletions in IPython."""
@@ -712,7 +714,9 @@ def to_dataset(self) -> Dataset:
 
     def __delitem__(self, key: Hashable) -> None:
         if key not in self:
-            raise KeyError(f"{key!r} is not a coordinate variable.")
+            raise KeyError(
+                f"{key!r} is not in coordinate variables {tuple(self.keys())}"
+            )
         assert_no_index_corrupted(self._data.xindexes, {key})
 
         del self._data._coords[key]

xarray/tests/test_coordinates.py

@@ -82,12 +82,22 @@ def test_getitem(self) -> None:
             DataArray([0, 1, 2], coords={"x": [0, 1, 2]}, name="x"),
         )
 
-    def test_delitem(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+    @pytest.mark.parametrize("as_dataset", [True, False])
+    def test_delitem(self, as_dataset: bool) -> None:
+        if as_dataset:
+            data = Dataset(coords={"x": [0, 1, 2]})
+        else:
+            data = DataArray([1, 2, 3], coords={"x": [0, 1, 2]})
+
+        coords = Coordinates(coords=data.coords, indexes=data.xindexes)
         del coords["x"]
         assert "x" not in coords
 
+        with pytest.raises(
+            KeyError, match="'nonexistent' is not in coordinate variables"
+        ):
+            del coords["nonexistent"]
+
     def test_update(self) -> None:
         _ds = Dataset(coords={"x": [0, 1, 2]})
         coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)