Backport PR #47946: REGR: fix error caused by deprecation warning in pd.merge when joining two Series

doc/source/whatsnew/v1.4.4.rst

@@ -19,6 +19,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.loc` not updating the cache correctly after values were set (:issue:`47867`)
 - Fixed regression in :meth:`DataFrame.loc` not aligning index in some cases when setting a :class:`DataFrame` (:issue:`47578`)
 - Fixed regression in setting ``None`` or non-string value into a ``string``-dtype Series using a mask (:issue:`47628`)
+- Fixed regression in :func:`merge` throwing an error when passing a :class:`Series` with a multi-level name (:issue:`47946`)
 - Fixed regression in :meth:`DataFrame.eval` creating a copy when updating inplace (:issue:`47449`)
 -
 

pandas/core/reshape/merge.py

@@ -672,8 +672,8 @@ def __init__(
         if _left.columns.nlevels != _right.columns.nlevels:
             msg = (
                 "merging between different levels is deprecated and will be removed "
-                f"in a future version. ({left.columns.nlevels} levels on the left, "
-                f"{right.columns.nlevels} on the right)"
+                f"in a future version. ({_left.columns.nlevels} levels on the left, "
+                f"{_right.columns.nlevels} on the right)"
             )
             # stacklevel chosen to be correct when this is reached via pd.merge
             # (and not DataFrame.join)

pandas/tests/reshape/merge/test_merge.py

@@ -2155,6 +2155,26 @@ def test_merge_series(on, left_on, right_on, left_index, right_index, nm):
             )
 
 
+def test_merge_series_multilevel():
+    # GH#47946
+    a = DataFrame(
+        {"A": [1, 2, 3, 4]},
+        index=MultiIndex.from_product([["a", "b"], [0, 1]], names=["outer", "inner"]),
+    )
+    b = Series(
+        [1, 2, 3, 4],
+        index=MultiIndex.from_product([["a", "b"], [1, 2]], names=["outer", "inner"]),
+        name=("B", "C"),
+    )
+    expected = DataFrame(
+        {"A": [2, 4], ("B", "C"): [1, 3]},
+        index=MultiIndex.from_product([["a", "b"], [1]], names=["outer", "inner"]),
+    )
+    with tm.assert_produces_warning(FutureWarning):
+        result = merge(a, b, on=["outer", "inner"])
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "col1, col2, kwargs, expected_cols",
     [