BUG: None comparison evaluates to True pandas-dev#26504

Author: another-green

doc/source/whatsnew/v0.25.0.rst

@@ -305,7 +305,7 @@ Categorical
 ^^^^^^^^^^^
 
 - Bug in :func:`DataFrame.at` and :func:`Series.at` that would raise exception if the index was a :class:`CategoricalIndex` (:issue:`20629`)
--
+- Bug in :func:`_cat_compare_op` that would valuate comparison with None to True (:issue:`26504`)
 -
 
 Datetimelike

pandas/tests/arrays/categorical/test_operators.py

@@ -188,15 +188,16 @@ def test_comparison_with_unknown_scalars(self):
 
     def test_comparison_with_known_scalars(self):
         # https://github.com/pandas-dev/pandas/issues/26504
-        # and following comparisons with scalars not in categories should raise
-        # for unequal comps, but not for equal/not equal
-        cat = Categorical([1, 2, 3, None], categories=[1, 2, 3], ordered=True)
+        # and following comparisons with scalars in categories with None should
+        # be evaluated as False
 
-        msg = ("Cannot compare a Categorical for op __{}__ with a scalar,"
-               " which is not a category")
+        cat1 = Categorical([1, 2, 3, None], categories=[1, 2, 3], ordered=True)
+        cat2 = Categorical([None, 1, 2, 3], categories=[1, 2, 3], ordered=True)
 
-        tm.assert_numpy_array_equal(cat <= 2,
-                                    np.array([True,  True, False, False]))
+        tm.assert_numpy_array_equal(cat1 <= 2,
+                                    np.array([True, True, False, False]))
+        tm.assert_numpy_array_equal(cat1 <= 2,
+                                    np.array([False, True, True, False]))
 
     @pytest.mark.parametrize('data,reverse,base', [
         (list("abc"), list("cba"), list("bbb")),