TST: add test for df.where() with category dtype (pandas-dev#29454)

xref pandas-dev#16979

Author: ganevgv

pandas/tests/frame/test_dtypes.py

@@ -815,6 +815,22 @@ def test_astype_extension_dtypes_duplicate_col(self, dtype):
         expected = concat([a1.astype(dtype), a2.astype(dtype)], axis=1)
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("kwargs", [dict(), dict(other=None)])
+    def test_df_where_with_category(self, kwargs):
+        # GH 16979
+        df = DataFrame(np.arange(2 * 3).reshape(2, 3), columns=list("ABC"))
+        mask = np.array([[True, False, True], [False, True, True]])
+
+        # change type to category
+        df.A = df.A.astype("category")
+        df.B = df.B.astype("category")
+        df.C = df.C.astype("category")
+
+        result = df.A.where(mask[:, 0], **kwargs)
+        expected = Series(pd.Categorical([0, np.nan], categories=[0, 3]), name="A")
+
+        tm.assert_series_equal(result, expected)
+
     @pytest.mark.parametrize(
         "dtype", [{100: "float64", 200: "uint64"}, "category", "float64"]
     )