TST: Add testing to df.where() typecasting as per GH 42295 (#43173)

* Add testing to df.where() typecasting as per GH 42295

* Fix style

* Directly compare data frames

* Fix pre-commit

* Rename dataframe to df

* create expected before calling where

Co-authored-by: Maximilian Carr <[email protected]>
Co-authored-by: Maximilian Carr <[email protected]>

Author: 3 people

pandas/tests/frame/indexing/test_where.py

@@ -771,3 +771,13 @@ def test_where_non_keyword_deprecation():
         result = s.where(s > 1, 10, False)
     expected = DataFrame([10, 10, 2, 3, 4])
     tm.assert_frame_equal(expected, result)
+
+
+def test_where_columns_casting():
+    # GH 42295
+
+    df = DataFrame({"a": [1.0, 2.0], "b": [3, np.nan]})
+    expected = df.copy()
+    result = df.where(pd.notnull(df), None)
+    # make sure dtypes don't change
+    tm.assert_frame_equal(expected, result)