additional review edits

Author: jschendel

doc/source/whatsnew/v0.23.0.txt

@@ -438,7 +438,7 @@ Other API Changes
 - Set operations (union, difference...) on :class:`IntervalIndex` with incompatible index types will now raise a ``TypeError`` rather than a ``ValueError`` (:issue:`19329`)
 - :class:`DateOffset` objects render more simply, e.g. "<DateOffset: days=1>" instead of "<DateOffset: kwds={'days': 1}>" (:issue:`19403`)
 - :func:`pandas.merge` provides a more informative error message when trying to merge on timezone-aware and timezone-naive columns (:issue:`15800`)
-- The default value of the ``ordered`` parameter for :class:`~pandas.api.types.CategoricalDtype` has changed from ``False`` to ``None``.  Behavior should remain consistent for downstream objects, such as :class:`Categorical` (:issue:`18790`)
+- The default value of the ``ordered`` parameter for :class:`~pandas.api.types.CategoricalDtype` has changed from ``False`` to ``None`` to allow updating of ``categories`` without impacting ``ordered``.  Behavior should remain consistent for downstream objects, such as :class:`Categorical` (:issue:`18790`)
 
 .. _whatsnew_0230.deprecations:
 

pandas/tests/dtypes/test_dtypes.py

@@ -644,11 +644,22 @@ def test_same_categories_different_order(self):
     @pytest.mark.parametrize('ordered1', [True, False, None])
     @pytest.mark.parametrize('ordered2', [True, False, None])
     def test_categorical_equality(self, ordered1, ordered2):
-        # same categories
+        # same categories, same order
+        # any combination of None/False are equal
+        # True/True is the only combination with True that are equal
         c1 = CategoricalDtype(list('abc'), ordered1)
         c2 = CategoricalDtype(list('abc'), ordered2)
         result = c1 == c2
-        expected = (ordered1 is ordered2) or not any([ordered1, ordered2])
+        expected = bool(ordered1) is bool(ordered2)
+        assert result is expected
+
+        # same categories, different order
+        # any combination of None/False are equal (order doesn't matter)
+        # any combination with True are not equal (different order of cats)
+        c1 = CategoricalDtype(list('abc'), ordered1)
+        c2 = CategoricalDtype(list('cab'), ordered2)
+        result = c1 == c2
+        expected = (bool(ordered1) is False) and (bool(ordered2) is False)
         assert result is expected
 
         # different categories