Improve implementation & add another test

Author: rhshadrach

doc/source/whatsnew/v3.0.0.rst

@@ -1183,6 +1183,7 @@ Groupby/resample/rolling
 
 Reshaping
 ^^^^^^^^^
+- Bug in :func:`DataFrame.join` not producing the correct row order when joining with a list of Series/DataFrames (:issue:`62954)
 - Bug in :func:`concat` with mixed integer and bool dtypes incorrectly casting the bools to integers (:issue:`45101`)
 - Bug in :func:`qcut` where values at the quantile boundaries could be incorrectly assigned (:issue:`59355`)
 - Bug in :meth:`DataFrame.combine_first` not preserving the column order (:issue:`60427`)

pandas/core/frame.py

@@ -11389,17 +11389,18 @@ def join(
             # "Iterable[Union[DataFrame, Series]]" due to the if statements
             frames = [cast("DataFrame | Series", self)] + list(other)
 
-            can_concat = (how != "right") and all(df.index.is_unique for df in frames)
+            can_concat = all(df.index.is_unique for df in frames)
 
             # join indexes only using concat
             if can_concat:
-                if how == "left":
+                if how == "left" or how == "right":
                     res = concat(
                         frames, axis=1, join="outer", verify_integrity=True, sort=sort
                     )
-                    result = res.reindex(self.index)
+                    index = self.index if how == "left" else frames[-1].index
                     if sort:
-                        result = result.sort_index()
+                        index = index.sort_values()
+                    result = res.reindex(index)
                     return result
                 else:
                     if how == "outer":

pandas/tests/reshape/merge/test_join.py

@@ -672,7 +672,7 @@ def _check_diff_index(df_list, result, exp_index):
             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):
+    def test_join_many_sort_unique(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":
@@ -685,6 +685,33 @@ def test_join_many_sort(self, how, sort):
         result = df.join([df2], how=how, sort=sort)
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("how", ["left", "right", "inner", "outer"])
+    def test_join_many_sort_nonunique(self, how, sort):
+        df = DataFrame({"a": [1, 2, 3]}, index=[3, 0, 0])
+        df2 = DataFrame({"b": [4, 5, 6]}, index=[2, 0, 1])
+        if how == "inner":
+            expected = DataFrame({"a": [2, 3], "b": [5, 5]}, index=[0, 0])
+        elif how == "left":
+            expected = DataFrame(
+                {"a": [1, 2, 3], "b": [np.nan, 5.0, 5.0]}, index=[3, 0, 0]
+            )
+        elif how == "right":
+            expected = DataFrame(
+                {"a": [np.nan, 2.0, 3.0, np.nan], "b": [4, 5, 5, 6]}, index=[2, 0, 0, 1]
+            )
+        else:
+            expected = DataFrame(
+                {
+                    "a": [2.0, 3.0, np.nan, np.nan, 1.0],
+                    "b": [5.0, 5.0, 6.0, 4.0, np.nan],
+                },
+                index=[0, 0, 1, 2, 3],
+            )
+        if sort:
+            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)),