TST: Add regression test for DataFrame.where with nullable dtype (#49052)

Author: zhan7236

pandas/tests/frame/indexing/test_where.py

@@ -1063,3 +1063,13 @@ def test_where_inplace_no_other():
     assert result is df
     expected = DataFrame({"a": [1, np.nan], "b": [np.nan, "y"]})
     tm.assert_frame_equal(df, expected)
+
+
+def test_where_other_nullable_dtype():
+    # GH#49052 DataFrame.where should return nullable dtype when
+    # other is a Series with nullable dtype, matching Series.where behavior
+    df = Series([1, 2, 3], dtype="int64").to_frame()
+    other = Series([pd.NA, pd.NA, pd.NA], dtype="Int64")
+    result = df.where(df > 1, other, axis=0)
+    expected = DataFrame({0: Series([pd.NA, 2, 3], dtype="Int64")})
+    tm.assert_frame_equal(result, expected)