BUG: MultiIndex.append not checking names for equality

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

@@ -2203,21 +2203,19 @@ 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)
+                level_names = {label.name}.union({x.name for x in appended})
+                names.append(None if len(level_names) > 1 else label.name)
+            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)
-        except (TypeError, IndexError):
-            return Index._with_infer(new_tuples)
+        return Index._with_infer(new_tuples)
 
     def argsort(self, *args, **kwargs) -> npt.NDArray[np.intp]:
         return self._values.argsort(*args, **kwargs)

pandas/tests/indexes/multi/test_reshape.py

@@ -150,6 +150,16 @@ 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_repeat():
     reps = 2
     numbers = [1, 2, 3]