BUG: align not adding levels only on one side if intersection is equal

doc/source/whatsnew/v1.5.0.rst

@@ -316,6 +316,7 @@ Indexing
 ^^^^^^^^
 - Bug in :meth:`loc.__getitem__` with a list of keys causing an internal inconsistency that could lead to a disconnect between ``frame.at[x, y]`` vs ``frame[y].loc[x]`` (:issue:`22372`)
 - Bug in :meth:`DataFrame.iloc` where indexing a single row on a :class:`DataFrame` with a single ExtensionDtype column gave a copy instead of a view on the underlying data (:issue:`45241`)
+- Bug in :meth:`Series.align` does not create :class:`MultiIndex` with union of levels when both MultiIndexes intersections are identical (:issue:`45224`)
 - Bug in setting a NA value (``None`` or ``np.nan``) into a :class:`Series` with int-based :class:`IntervalDtype` incorrectly casting to object dtype instead of a float-based :class:`IntervalDtype` (:issue:`45568`)
 - Bug in :meth:`Series.__setitem__` with a non-integer :class:`Index` when using an integer key to set a value that cannot be set inplace where a ``ValueError`` was raised instead of casting to a common dtype (:issue:`45070`)
 - Bug in :meth:`Series.__setitem__` when setting incompatible values into a ``PeriodDtype`` or ``IntervalDtype`` :class:`Series` raising when indexing with a boolean mask but coercing when indexing with otherwise-equivalent indexers; these now consistently coerce, along with :meth:`Series.mask` and :meth:`Series.where` (:issue:`45768`)

pandas/core/series.py

@@ -4502,7 +4502,9 @@ def _reindex_indexer(
     ) -> Series:
         # Note: new_index is None iff indexer is None
         # if not None, indexer is np.intp
-        if indexer is None:
+        if indexer is None and (
+            new_index is None or new_index.names == self.index.names
+        ):
             if copy:
                 return self.copy()
             return self

pandas/tests/series/methods/test_align.py

@@ -184,3 +184,41 @@ def test_align_periodindex(join_type):
 
     # TODO: assert something?
     ts.align(ts[::2], join=join_type)
+
+
+def test_align_left_fewer_levels():
+    # GH#45224
+    left = Series([2], index=pd.MultiIndex.from_tuples([(1, 3)], names=["a", "c"]))
+    right = Series(
+        [1], index=pd.MultiIndex.from_tuples([(1, 2, 3)], names=["a", "b", "c"])
+    )
+    result_left, result_right = left.align(right)
+
+    expected_right = Series(
+        [1], index=pd.MultiIndex.from_tuples([(1, 3, 2)], names=["a", "c", "b"])
+    )
+    expected_left = Series(
+        [2], index=pd.MultiIndex.from_tuples([(1, 3, 2)], names=["a", "c", "b"])
+    )
+    tm.assert_series_equal(result_left, expected_left)
+    tm.assert_series_equal(result_right, expected_right)
+
+
+def test_align_left_different_named_levels():
+    # GH#45224
+    left = Series(
+        [2], index=pd.MultiIndex.from_tuples([(1, 4, 3)], names=["a", "d", "c"])
+    )
+    right = Series(
+        [1], index=pd.MultiIndex.from_tuples([(1, 2, 3)], names=["a", "b", "c"])
+    )
+    result_left, result_right = left.align(right)
+
+    expected_left = Series(
+        [2], index=pd.MultiIndex.from_tuples([(1, 3, 4, 2)], names=["a", "c", "d", "b"])
+    )
+    expected_right = Series(
+        [1], index=pd.MultiIndex.from_tuples([(1, 3, 4, 2)], names=["a", "c", "d", "b"])
+    )
+    tm.assert_series_equal(result_left, expected_left)
+    tm.assert_series_equal(result_right, expected_right)

pandas/tests/series/test_arithmetic.py

@@ -885,8 +885,8 @@ def test_series_varied_multiindex_alignment():
     expected = Series(
         [1000, 2001, 3002, 4003],
         index=pd.MultiIndex.from_tuples(
-            [("a", "x", 1), ("a", "x", 2), ("a", "y", 1), ("a", "y", 2)],
-            names=["ab", "xy", "num"],
+            [("x", 1, "a"), ("x", 2, "a"), ("y", 1, "a"), ("y", 2, "a")],
+            names=["xy", "num", "ab"],
         ),
     )
     tm.assert_series_equal(result, expected)