Fix a derp.

Author: sstanovnik

pandas/tests/types/test_cast.py

@@ -231,9 +231,10 @@ def test_numpy_dtypes(self):
 
     def test_pandas_dtypes(self):
         with self.assertRaises(TypeError):
-            self.assertEqual(_find_common_type([CategoricalDtype]),
+            self.assertEqual(_find_common_type([CategoricalDtype()]),
                              CategoricalDtype)
-            self.assertEqual(_find_common_type([DatetimeTZDtype]),
+        with self.assertRaises(TypeError):
+            self.assertEqual(_find_common_type([DatetimeTZDtype()]),
                              DatetimeTZDtype)
 
 if __name__ == '__main__':

pandas/types/cast.py

@@ -867,8 +867,7 @@ def _possibly_cast_to_datetime(value, dtype, errors='raise'):
 def _find_common_type(types):
     """Find a common data type among the given dtypes."""
     # TODO: enable using pandas-specific types
-    if any(issubclass(t, ExtensionDtype) or isinstance(t, ExtensionDtype)
-           for t in types):
+    if any(isinstance(t, ExtensionDtype) for t in types):
         raise TypeError("Common type discovery is currently only "
                         "supported for pure numpy dtypes.")
     return np.find_common_type(types, [])