BUG: Fix Categorical comparsion with Series of dtype 'category'

This is a fix attempt for issue #16659.

Author: funnycrab

doc/source/whatsnew/v0.21.0.txt

@@ -130,6 +130,8 @@ Numeric
 Categorical
 ^^^^^^^^^^^
 
+- Bug in ``Categorical.is_dtype_equal()`` where comparison with Series whose dtype is 'category' is not handled correctly (:issue:`16659`)
+
 
 Other
 ^^^^^

pandas/core/categorical.py

@@ -1995,16 +1995,22 @@ def is_dtype_equal(self, other):
 
         Parameters
         ----------
-        other : Categorical
+        other : Categorical, Series
 
         Returns
         -------
         are_equal : boolean
         """
-
         try:
-            return (self.categories.equals(other.categories) and
-                    self.ordered == other.ordered)
+            from pandas.core.series import Series
+
+            if isinstance(other, Series):
+                other_categorical = other.values
+            else:
+                other_categorical = other
+
+            return (self.categories.equals(other_categorical.categories) and
+                    self.ordered == other_categorical.ordered)
         except (AttributeError, TypeError):
             return False
 

pandas/tests/test_categorical.py

@@ -152,6 +152,11 @@ def test_is_equal_dtype(self):
             CategoricalIndex(c1, categories=list('cab'))))
         assert not c1.is_dtype_equal(CategoricalIndex(c1, ordered=True))
 
+        s1 = pd.Series(c1)
+        assert c1.is_dtype_equal(s1)
+        assert not c2.is_dtype_equal(s1)
+        assert not c3.is_dtype_equal(s1)
+
     def test_constructor(self):
 
         exp_arr = np.array(["a", "b", "c", "a", "b", "c"], dtype=np.object_)