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

Author: another-green

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 +186,18 @@ 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 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)
+
+        msg = ("Cannot compare a Categorical for op __{}__ with a scalar,"
+               " which is not a category")
+
+        tm.assert_numpy_array_equal(cat <= 2,
+                                    np.array([True,  True, False, False]))
+
     @pytest.mark.parametrize('data,reverse,base', [
         (list("abc"), list("cba"), list("bbb")),
         ([1, 2, 3], [3, 2, 1], [2, 2, 2])]