TST: add test for df.where() with int dtype (#29498)

Author: ganevgv

pandas/tests/frame/test_dtypes.py

@@ -850,6 +850,31 @@ def test_astype_column_metadata(self, dtype):
         df = df.astype(dtype)
         tm.assert_index_equal(df.columns, columns)
 
+    def test_df_where_change_dtype(self):
+        # GH 16979
+        df = DataFrame(np.arange(2 * 3).reshape(2, 3), columns=list("ABC"))
+        mask = np.array([[True, False, False], [False, False, True]])
+
+        result = df.where(mask)
+        expected = DataFrame(
+            [[0, np.nan, np.nan], [np.nan, np.nan, 5]], columns=list("ABC")
+        )
+
+        tm.assert_frame_equal(result, expected)
+
+        # change type to category
+        df.A = df.A.astype("category")
+        df.B = df.B.astype("category")
+        df.C = df.C.astype("category")
+
+        result = df.where(mask)
+        A = pd.Categorical([0, np.nan], categories=[0, 3])
+        B = pd.Categorical([np.nan, np.nan], categories=[1, 4])
+        C = pd.Categorical([np.nan, 5], categories=[2, 5])
+        expected = DataFrame({"A": A, "B": B, "C": C})
+
+        tm.assert_frame_equal(result, expected)
+
     @pytest.mark.parametrize("dtype", ["M8", "m8"])
     @pytest.mark.parametrize("unit", ["ns", "us", "ms", "s", "h", "m", "D"])
     def test_astype_from_datetimelike_to_objectt(self, dtype, unit):