add unit test for none label joins

Author: matiaslindgren

pandas/tests/reshape/merge/test_join.py

@@ -1098,3 +1098,29 @@ def test_join_multiindex_categorical_output_index_dtype(how, values):
 
     result = df1.join(df2, how=how)
     tm.assert_frame_equal(result, expected)
+
+
+def test_join_multiindex_with_none_as_label():
+    # GH 58721
+    df1 = DataFrame(
+        {"A": [1]},
+        index=MultiIndex.from_tuples([(3, 3)], names=["X", None]),
+    )
+    df2 = DataFrame(
+        {"B": [2]},
+        index=MultiIndex.from_tuples([(3, 3)], names=[None, "X"]),
+    )
+
+    result12 = df1.join(df2)
+    expected12 = DataFrame(
+        {"A": [1], "B": [2]},
+        index=MultiIndex.from_tuples([(3, 3)], names=["X", None]),
+    )
+    tm.assert_frame_equal(result12, expected12)
+
+    result21 = df2.join(df1)
+    expected21 = DataFrame(
+        {"B": [2], "A": [1]},
+        index=MultiIndex.from_tuples([(3, 3)], names=[None, "X"]),
+    )
+    tm.assert_frame_equal(result21, expected21)