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/core/arrays/categorical.py

@@ -100,7 +100,15 @@ def f(self, other):
         if is_scalar(other):
             if other in self.categories:
                 i = self.categories.get_loc(other)
-                return getattr(self._codes, op)(i)
+                f = getattr(self._codes, op)
+                ret = f(i)
+
+                # check for NaN in self
+                na_mask = (self._codes == -1)
+                if na_mask.any():
+                    # In other series, the leads to False, so do that here too
+                    ret[na_mask] = False
+                return ret
             else:
                 if op == '__eq__':
                     return np.repeat(False, len(self))

pandas/tests/arrays/categorical/test_operators.py

@@ -186,6 +186,19 @@ def test_comparison_with_unknown_scalars(self):
         tm.assert_numpy_array_equal(cat != 4,
                                     np.array([True, True, True]))
 
+    def test_comparison_with_known_scalars(self):
+        # https://github.com/pandas-dev/pandas/issues/26504
+        # and following comparisons with scalars in categories with None should
+        # be evaluated as False
+
+        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(cat1 <= 2,
+                                    np.array([True, True, False, False]))
+        tm.assert_numpy_array_equal(cat2 <= 2,
+                                    np.array([False, True, True, False]))
+
     @pytest.mark.parametrize('data,reverse,base', [
         (list("abc"), list("cba"), list("bbb")),
         ([1, 2, 3], [3, 2, 1], [2, 2, 2])]