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

Author: wdyy20041223

pandas/tests/indexing/multiindex/test_setitem.py

@@ -490,6 +490,64 @@ def test_setitem_enlargement_keep_index_names(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    def test_setitem_enlargement_multiindex_with_none(self):
+        # GH#59153
+        # Test that we can enlarge a DataFrame with a MultiIndex
+        # when one or more level keys are None
+        index = MultiIndex.from_tuples(
+            [("A", "a1"), ("A", "a2"), ("B", "b1"), ("B", None)]
+        )
+        df = DataFrame([(0, 6), (1, 5), (2, 4), (3, 7)], index=index)
+
+        # Enlarge with a new index entry where one key is None
+        df.loc[("A", None), :] = [12, 13]
+
+        expected_index = MultiIndex.from_tuples(
+            [("A", "a1"), ("A", "a2"), ("B", "b1"), ("B", None), ("A", None)]
+        )
+        expected = DataFrame(
+            [[0, 6], [1, 5], [2, 4], [3, 7], [12, 13]],
+            index=expected_index,
+            columns=[0, 1],
+        )
+        tm.assert_frame_equal(df, expected)
+
+        # Test retrieval of the newly added row
+        result = df.loc[("A", None), :]
+        expected_row = Series([12, 13], index=[0, 1], name=("A", np.nan))
+        tm.assert_series_equal(result, expected_row)
+
+    def test_setitem_enlargement_multiindex_multiple_none(self):
+        # GH#59153
+        # Test enlarging with multiple None keys in different levels
+        index = MultiIndex.from_tuples([("A", "a1"), ("B", "b1")])
+        df = DataFrame([[1, 2], [3, 4]], index=index, columns=["x", "y"])
+
+        # Add row with None in first level
+        df.loc[(None, "c1"), :] = [5, 6]
+
+        # Add row with None in second level
+        df.loc[("C", None), :] = [7, 8]
+
+        # Add row with None in both levels
+        df.loc[(None, None), :] = [9, 10]
+
+        expected_index = MultiIndex.from_tuples(
+            [
+                ("A", "a1"),
+                ("B", "b1"),
+                (None, "c1"),
+                ("C", None),
+                (None, None),
+            ]
+        )
+        expected = DataFrame(
+            [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]],
+            index=expected_index,
+            columns=["x", "y"],
+        )
+        tm.assert_frame_equal(df, expected)
+
 
 def test_frame_setitem_view_direct(multiindex_dataframe_random_data):
     # this works because we are modifying the underlying array