BUG: Fixed __join_multi always returning indexers (#34074)

doc/source/whatsnew/v1.1.0.rst

@@ -665,7 +665,7 @@ MultiIndex
         # Common elements are now guaranteed to be ordered by the left side
         left.intersection(right, sort=False)
 
--
+- Bug when joining 2 Multi-indexes, without specifying level with different columns. Return-indexers parameter is ignored. (:issue:`34074`)
 
 I/O
 ^^^

pandas/core/indexes/base.py

@@ -3529,7 +3529,10 @@ def _join_multi(self, other, how, return_indexers=True):
 
             multi_join_idx = multi_join_idx.remove_unused_levels()
 
-            return multi_join_idx, lidx, ridx
+            if return_indexers:
+                return multi_join_idx, lidx, ridx
+            else:
+                return multi_join_idx
 
         jl = list(overlap)[0]
 

pandas/tests/indexes/multi/test_join.py

@@ -96,10 +96,19 @@ def test_join_multi_wrong_order():
     midx1 = pd.MultiIndex.from_product([[1, 2], [3, 4]], names=["a", "b"])
     midx2 = pd.MultiIndex.from_product([[1, 2], [3, 4]], names=["b", "a"])
 
-    join_idx, lidx, ridx = midx1.join(midx2, return_indexers=False)
+    join_idx, lidx, ridx = midx1.join(midx2, return_indexers=True)
 
     exp_ridx = np.array([-1, -1, -1, -1], dtype=np.intp)
 
     tm.assert_index_equal(midx1, join_idx)
     assert lidx is None
     tm.assert_numpy_array_equal(ridx, exp_ridx)
+
+
+def test_join_multi_return_indexers():
+    # GH 34074
+
+    midx1 = pd.MultiIndex.from_product([[1, 2], [3, 4], [5, 6]], names=["a", "b", "c"])
+    midx2 = pd.MultiIndex.from_product([[1, 2], [3, 4]], names=["a", "b"])
+
+    assert type(midx1.join(midx2, return_indexers=False)) != tuple