BUG: Fix inconsistent ordering between left and right in merge

doc/source/whatsnew/v1.2.0.rst

@@ -507,6 +507,7 @@ Reshaping
 - Bug in :meth:`DataFrame.agg` with ``func={'name':<FUNC>}`` incorrectly raising ``TypeError`` when ``DataFrame.columns==['Name']`` (:issue:`36212`)
 - Bug in :meth:`Series.transform` would give incorrect results or raise when the argument ``func`` was dictionary (:issue:`35811`)
 - Bug in :func:`join` returned a non deterministic level-order for the resulting :class:`MultiIndex` (:issue:`36910`)
+- 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

@@ -1336,7 +1336,7 @@ 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,
@@ -1861,8 +1861,8 @@ 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

@@ -2283,3 +2283,13 @@ def test_merge_join_categorical_multiindex():
     expected = expected.drop(["Cat", "Int"], axis=1)
     result = a.join(b, on=["Cat1", "Int1"])
     tm.assert_frame_equal(expected, result)
+
+
+@pytest.mark.parametrize("how", ["left", "right"])
+def test_merge_same_order_left_right(how):
+    # GH: 35382
+    df = DataFrame({"a": [1, 0, 1]})
+
+    result = df.merge(df, on="a", how=how, sort=False)
+    expected = DataFrame([1, 1, 0, 1, 1], columns=["a"])
+    tm.assert_frame_equal(result, expected)