BUG: MultiIndex.append not checking names for equality (#48288)

* BUG: MultiIndex.append not checking names for equality

* BUG: MultiIndex.append not checking names for equality

* Update doc/source/whatsnew/v1.6.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Add test and fix bug

* Improve name checking

Co-authored-by: Matthew Roeschke <[email protected]>

Author: phofl

doc/source/whatsnew/v1.6.0.rst

@@ -161,7 +161,7 @@ Missing
 
 MultiIndex
 ^^^^^^^^^^
--
+- Bug in :meth:`MultiIndex.append` not checking names for equality (:issue:`48288`)
 -
 
 I/O

pandas/core/indexes/multi.py

@@ -2204,19 +2204,23 @@ def append(self, other):
         if all(
             (isinstance(o, MultiIndex) and o.nlevels >= self.nlevels) for o in other
         ):
-            arrays = []
+            arrays, names = [], []
             for i in range(self.nlevels):
                 label = self._get_level_values(i)
                 appended = [o._get_level_values(i) for o in other]
                 arrays.append(label.append(appended))
-            return MultiIndex.from_arrays(arrays, names=self.names)
+                single_label_name = all(label.name == x.name for x in appended)
+                names.append(label.name if single_label_name else None)
+            return MultiIndex.from_arrays(arrays, names=names)
 
         to_concat = (self._values,) + tuple(k._values for k in other)
         new_tuples = np.concatenate(to_concat)
 
         # if all(isinstance(x, MultiIndex) for x in other):
         try:
-            return MultiIndex.from_tuples(new_tuples, names=self.names)
+            # We only get here if other contains at least one index with tuples,
+            # setting names to None automatically
+            return MultiIndex.from_tuples(new_tuples)
         except (TypeError, IndexError):
             return Index._with_infer(new_tuples)
 

pandas/core/reshape/pivot.py

@@ -391,7 +391,13 @@ def _all_key(key):
                 # GH31016 this is to calculate margin for each group, and assign
                 # corresponded key as index
                 transformed_piece = DataFrame(piece.apply(aggfunc)).T
-                transformed_piece.index = Index([all_key], name=piece.index.name)
+                if isinstance(piece.index, MultiIndex):
+                    # We are adding an empty level
+                    transformed_piece.index = MultiIndex.from_tuples(
+                        [all_key], names=piece.index.names + [None]
+                    )
+                else:
+                    transformed_piece.index = Index([all_key], name=piece.index.name)
 
                 # append piece for margin into table_piece
                 table_pieces.append(transformed_piece)

pandas/tests/indexes/multi/test_reshape.py

@@ -150,6 +150,25 @@ def test_append_index():
     tm.assert_index_equal(result, expected)
 
 
+@pytest.mark.parametrize("name, exp", [("b", "b"), ("c", None)])
+def test_append_names_match(name, exp):
+    # GH#48288
+    midx = MultiIndex.from_arrays([[1, 2], [3, 4]], names=["a", "b"])
+    midx2 = MultiIndex.from_arrays([[3], [5]], names=["a", name])
+    result = midx.append(midx2)
+    expected = MultiIndex.from_arrays([[1, 2, 3], [3, 4, 5]], names=["a", exp])
+    tm.assert_index_equal(result, expected)
+
+
+def test_append_names_dont_match():
+    # GH#48288
+    midx = MultiIndex.from_arrays([[1, 2], [3, 4]], names=["a", "b"])
+    midx2 = MultiIndex.from_arrays([[3], [5]], names=["x", "y"])
+    result = midx.append(midx2)
+    expected = MultiIndex.from_arrays([[1, 2, 3], [3, 4, 5]], names=None)
+    tm.assert_index_equal(result, expected)
+
+
 def test_repeat():
     reps = 2
     numbers = [1, 2, 3]