TST: add regression test for GH#56853 (insert/delete with MultiIndex datetime)

Author: skalwaghe-56

pandas/tests/frame/indexing/test_insert.py

@@ -10,6 +10,7 @@
 from pandas import (
     DataFrame,
     Index,
+    MultiIndex,
 )
 import pandas._testing as tm
 
@@ -94,3 +95,27 @@ def test_insert_int64_loc(self):
         df = DataFrame({"a": [1, 2]})
         df.insert(np.int64(0), "b", 0)
         tm.assert_frame_equal(df, DataFrame({"b": [0, 0], "a": [1, 2]}))
+
+    def test_insert_delete_mixed_multiindex_columns(self):
+        # GH#56853
+        from pandas import (
+            DataFrame,
+            Timestamp,
+        )
+
+        df = DataFrame({("A", Timestamp("2024-01-01")): [0]})
+
+        # Insert scalar label 'B'; alongside MultiIndex columns
+        df.insert(1, "B", [1])
+
+        # Verify MultiIndex and that 'B' appears in the top level
+        assert isinstance(df.columns, MultiIndex)
+        assert "B" in df.columns.get_level_values(0)
+
+        # Should not raise RecursionError (this was the original bug)
+        del df["B"]
+
+        # Confirm 'B' no longer present and structure restored
+        assert "B" not in df.columns.get_level_values(0)
+        expected = DataFrame({("A", Timestamp("2024-01-01")): [0]})
+        tm.assert_frame_equal(df, expected)