Warn when a variable referred to in an attribute was not found

Author: dcherian

cf_xarray/accessor.py

@@ -2,6 +2,7 @@
 import inspect
 import itertools
 import textwrap
+import warnings
 from collections import ChainMap
 from contextlib import suppress
 from typing import (
@@ -776,8 +777,17 @@ def __getitem__(self, key: Union[str, List[str]]):
 
             if scalar_key and len(varnames) == 1:
                 da = ds[varnames[0]]
+                failed = []
                 for k1 in coords:
-                    da.coords[k1] = ds.variables[k1]
+                    if k1 not in ds.variables:
+                        failed.append(k1)
+                    else:
+                        da.coords[k1] = ds.variables[k1]
+                if failed:
+                    warnings.warn(
+                        f"Variables {failed!r} not found in object but are referred to in the CF attributes.",
+                        UserWarning,
+                    )
                 return da
 
             ds = ds.reset_coords()[varnames + coords]

cf_xarray/tests/test_accessor.py

@@ -83,6 +83,9 @@ def test_getitem_ancillary_variables():
     assert_identical(anc.cf["q"], expected)
     assert_identical(anc.cf["specific_humidity"], expected)
 
+    with pytest.warns(UserWarning):
+        anc[["q"]].cf["q"]
+
 
 @pytest.mark.parametrize("obj", objects)
 @pytest.mark.parametrize(