TST: check inequality by comparing categorical with NaN ( pandas-dev#28384 ) (pandas-dev#36520)

Author: junjunjunk

pandas/tests/arrays/categorical/test_missing.py

@@ -148,3 +148,24 @@ def test_use_inf_as_na_outside_context(self, values, expected):
             result = pd.isna(DataFrame(cat))
             expected = DataFrame(expected)
             tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "a1, a2, categories",
+        [
+            (["a", "b", "c"], [np.nan, "a", "b"], ["a", "b", "c"]),
+            ([1, 2, 3], [np.nan, 1, 2], [1, 2, 3]),
+        ],
+    )
+    def test_compare_categorical_with_missing(self, a1, a2, categories):
+        # GH 28384
+        cat_type = CategoricalDtype(categories)
+
+        # !=
+        result = Series(a1, dtype=cat_type) != Series(a2, dtype=cat_type)
+        expected = Series(a1) != Series(a2)
+        tm.assert_series_equal(result, expected)
+
+        # ==
+        result = Series(a1, dtype=cat_type) == Series(a2, dtype=cat_type)
+        expected = Series(a1) == Series(a2)
+        tm.assert_series_equal(result, expected)