BUG: Fix sorting behavior of DataFrame.join on list arguments

Author: rhshadrach

pandas/core/frame.py

@@ -11389,16 +11389,21 @@ def join(
             # "Iterable[Union[DataFrame, Series]]" due to the if statements
             frames = [cast("DataFrame | Series", self)] + list(other)
 
-            can_concat = all(df.index.is_unique for df in frames)
+            can_concat = (how != "right") and all(df.index.is_unique for df in frames)
 
             # join indexes only using concat
             if can_concat:
                 if how == "left":
                     res = concat(
                         frames, axis=1, join="outer", verify_integrity=True, sort=sort
                     )
-                    return res.reindex(self.index)
+                    result = res.reindex(self.index)
+                    if sort:
+                        result = result.sort_index()
+                    return result
                 else:
+                    if how == "outer":
+                        sort = True
                     return concat(
                         frames, axis=1, join=how, verify_integrity=True, sort=sort
                     )
@@ -11409,6 +11414,7 @@ def join(
                 joined = merge(
                     joined,
                     frame,
+                    sort=sort,
                     how=how,
                     left_index=True,
                     right_index=True,

pandas/tests/frame/methods/test_join.py

@@ -396,11 +396,8 @@ def test_join_list_series(float_frame):
 
 
 def test_suppress_future_warning_with_sort_kw(sort):
-    sort_kw = sort
     a = DataFrame({"col1": [1, 2]}, index=["c", "a"])
-
     b = DataFrame({"col2": [4, 5]}, index=["b", "a"])
-
     c = DataFrame({"col3": [7, 8]}, index=["a", "b"])
 
     expected = DataFrame(
@@ -410,11 +407,7 @@ def test_suppress_future_warning_with_sort_kw(sort):
             "col3": {"a": 7.0, "b": 8.0, "c": float("nan")},
         }
     )
-    if sort_kw is False:
-        expected = expected.reindex(index=["c", "a", "b"])
-
-    with tm.assert_produces_warning(None):
-        result = a.join([b, c], how="outer", sort=sort_kw)
+    result = a.join([b, c], how="outer", sort=sort)
     tm.assert_frame_equal(result, expected)
 
 

pandas/tests/reshape/merge/test_join.py

@@ -671,6 +671,20 @@ def _check_diff_index(df_list, result, exp_index):
         with pytest.raises(ValueError, match=msg):
             df_list[0].join(df_list[1:], on="a")
 
+    @pytest.mark.parametrize("how", ["left", "right", "inner", "outer"])
+    def test_join_many_sort(self, how, sort):
+        df = DataFrame({"a": [1, 2, 3]}, index=[1, 0, 2])
+        df2 = DataFrame({"b": [4, 5, 6]}, index=[2, 0, 1])
+        if how == "right":
+            expected = DataFrame({"a": [3, 2, 1], "b": [4, 5, 6]}, index=[2, 0, 1])
+        else:
+            expected = DataFrame({"a": [1, 2, 3], "b": [6, 5, 4]}, index=[1, 0, 2])
+        if how == "outer" or sort:
+            # outer always sorts.
+            expected = expected.sort_index()
+        result = df.join([df2], how=how, sort=sort)
+        tm.assert_frame_equal(result, expected)
+
     def test_join_many_mixed(self):
         df = DataFrame(
             np.random.default_rng(2).standard_normal((8, 4)),