BUG: Fix inconsistent ordering between left and right in merge

doc/source/whatsnew/v1.2.0.rst

@@ -547,6 +547,7 @@ Reshaping
 - Bug in :func:`join` returned a non deterministic level-order for the resulting :class:`MultiIndex` (:issue:`36910`)
 - Bug in :meth:`DataFrame.combine_first()` caused wrong alignment with dtype ``string`` and one level of ``MultiIndex`` containing only ``NA`` (:issue:`37591`)
 - Fixed regression in :func:`merge` on merging DatetimeIndex with empty DataFrame (:issue:`36895`)
+- Bug in :meth:`df.merge() <pandas.DataFrame.merge>` returned inconsistent ordering in result for ``how=right`` and ``how=left`` (:issue:`35382`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/merge.py

@@ -1339,7 +1339,7 @@ def get_join_indexers(
     lkey, rkey, count = _factorize_keys(lkey, rkey, sort=sort, how=how)
     # preserve left frame order if how == 'left' and sort == False
     kwargs = copy.copy(kwargs)
-    if how == "left":
+    if how in ("left", "right"):
         kwargs["sort"] = sort
     join_func = {
         "inner": libjoin.inner_join,
@@ -1864,8 +1864,8 @@ def _left_join_on_index(left_ax: Index, right_ax: Index, join_keys, sort: bool =
     return left_ax, None, right_indexer
 
 
-def _right_outer_join(x, y, max_groups):
-    right_indexer, left_indexer = libjoin.left_outer_join(y, x, max_groups)
+def _right_outer_join(x, y, max_groups, **kwargs):
+    right_indexer, left_indexer = libjoin.left_outer_join(y, x, max_groups, **kwargs)
     return left_indexer, right_indexer
 
 

pandas/tests/reshape/merge/test_merge.py

@@ -601,6 +601,18 @@ def test_merge_nosort(self):
 
         assert (df.var3.unique() == result.var3.unique()).all()
 
+    @pytest.mark.parametrize(
+        ("sort", "values"), [(False, [1, 1, 0, 1, 1]), (True, [0, 1, 1, 1, 1])]
+    )
+    @pytest.mark.parametrize("how", ["left", "right"])
+    def test_merge_same_order_left_right(self, sort, values, how):
+        # GH: 35382
+        df = DataFrame({"a": [1, 0, 1]})
+
+        result = df.merge(df, on="a", how=how, sort=sort)
+        expected = DataFrame(values, columns=["a"])
+        tm.assert_frame_equal(result, expected)
+
     def test_merge_nan_right(self):
         df1 = DataFrame({"i1": [0, 1], "i2": [0, 1]})
         df2 = DataFrame({"i1": [0], "i3": [0]})