Fixup set_categories inplace test

Author: TomAugspurger

pandas/core/categorical.py

@@ -601,13 +601,24 @@ def _get_labels(self):
     labels = property(fget=_get_labels, fset=_set_codes)
 
     def _set_categories(self, categories, fastpath=False):
-        """ Sets new categories
+        """ Sets new categories inplace
 
         Parameters
         ----------
         fastpath : boolean (default: False)
            Don't perform validation of the categories for uniqueness or nulls
 
+        Examples
+        --------
+        >>> c = Categorical(['a', 'b'])
+        >>> c
+        [a, b]
+        Categories (2, object): [a, b]
+
+        >>> c._set_categories(pd.Index(['a', 'c']))
+        >>> c
+        [a, c]
+        Categories (2, object): [a, c]
         """
 
         if fastpath:

pandas/tests/test_categorical.py

@@ -965,13 +965,15 @@ def test_set_dtype_nans(self):
 
     def test_set_categories_private(self):
         cat = Categorical(['a', 'b', 'c'], categories=['a', 'b', 'c', 'd'])
-        result = cat._set_categories(['a', 'b', 'c', 'd', 'e'])
-        expected = Categorical(['a', 'b', 'c'], categories=list('abcde'))
-        tm.assert_categorical_equal(result, expected)
+        cat._set_categories(['a', 'c', 'd', 'e'])
+        expected = Categorical(['a', 'c', 'd'], categories=list('acde'))
+        tm.assert_categorical_equal(cat, expected)
 
         # fastpath
-        result = cat._set_categories(['a', 'b', 'c', 'd', 'e'], fastpath=True)
-        tm.assert_categorical_equal(result, expected)
+        cat = Categorical(['a', 'b', 'c'], categories=['a', 'b', 'c', 'd'])
+        cat._set_categories(['a', 'c', 'd', 'e'], fastpath=True)
+        expected = Categorical(['a', 'c', 'd'], categories=list('acde'))
+        tm.assert_categorical_equal(cat, expected)
 
     @pytest.mark.parametrize('values, categories, new_categories', [
         # No NaNs, same cats, same order