TST: Add regression tests for enlarging MultiIndex with None keys (GH#59153)

Author: wdyy20041223

pandas/tests/indexing/multiindex/test_setitem.py

@@ -533,3 +533,24 @@ def test_frame_setitem_partial_multiindex():
     expected = df.copy()
     expected["d"] = 8
     tm.assert_frame_equal(result, expected)
+
+def test_setitem_enlargement_multiindex_with_none(self):
+        # GH#59153
+        # test enlarge a DataFrame with a MultiIndex
+        index = MultiIndex.from_tuples(
+            [("A", "a1"), ("A", "a2"), ("B", "b1"), ("B", None)]
+        )
+        df = DataFrame([(0.0, 6.0), (1.0, 5.0), (2.0, 4.0), (3.0, 7.0)], index=index)
+        df.loc[("A", None), :] = [12.0, 13.0]
+        expected_index = MultiIndex.from_tuples(
+            [("A", "a1"), ("A", "a2"), ("B", "b1"), ("B", None), ("A", None)]
+        )
+        expected = DataFrame(
+            [[0.0, 6.0], [1.0, 5.0], [2.0, 4.0], [3.0, 7.0], [12.0, 13.0]],
+            index=expected_index,
+            columns=[0, 1],
+        )
+        tm.assert_frame_equal(df, expected, check_index_type=False)
+        result = df.loc[("A", None), :]
+        expected_row = Series([12.0, 13.0], index=[0, 1], name=("A", np.nan))
+        tm.assert_series_equal(result, expected_row)