Fix np scaler warning in _drop_missing_variables() (#291)

Co-authored-by: dcherian <[email protected]>

Author: tomvothecoder

cf_xarray/accessor.py

@@ -1030,8 +1030,12 @@ def isin(self, test_elements):
         return self._obj.isin(mapped_test_elements)
 
     def _drop_missing_variables(self, variables: list[str]) -> list[str]:
+        if isinstance(self._obj, Dataset):
+            good_names = set(self._obj.variables)
+        elif isinstance(self._obj, DataArray):
+            good_names = set(self._obj.coords)
 
-        return [var for var in variables if var in self._obj or var in self._obj.coords]
+        return [var for var in variables if var in good_names]
 
     def _get_all_cell_measures(self):
         """

doc/whats-new.rst

@@ -5,6 +5,7 @@ What's New
 v0.6.4 (unreleased)
 ===================
 - integrate ``xclim``'s CF-compliant unit formatter. :pr:`278`. By `Justus Magin`_.
+- Fix dropping of bad variable names. :pr:`291`. By `Tom Vo`_.
 
 v0.6.3 (December 16, 2021)
 ==========================
@@ -142,4 +143,5 @@ v0.1.3
 .. _`Kristen Thyng`: https://github.com/kthyng
 .. _`Julius Busecke`: https://github.com/jbusecke
 .. _`Filipe Fernandes`: https://github.com/ocefpaf
+.. _`Tom Vo`: https://github.com/tomvothecoder
 .. _`flag variables`: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#flags