From b50dcb507bb1c635aff302d700b18f35daefbee9 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 16:17:24 +0200
Subject: [PATCH 01/17] update fix ignored sort in api.py and add test

---
 pandas/core/indexes/api.py                 | 7 +++++--
 pandas/tests/arrays/integer/test_concat.py | 8 ++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 8efc07a2ef148..522fcd40bd13d 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -184,7 +184,10 @@ def union_indexes(indexes, sort: bool = True) -> Index:
     if len(indexes) == 1:
         result = indexes[0]
         if isinstance(result, list):
-            result = Index(sorted(result))
+            if sort:
+                result = Index(sorted(result))
+            else:
+                result = Index(result)
         return result
 
     indexes, kind = _sanitize_and_check(indexes)
@@ -219,7 +222,7 @@ def conv(i):
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:
-                result = result.union(other)
+                result = result.union(other, sort=None if sort else False)
             return result
     elif kind == "array":
         index = indexes[0]
diff --git a/pandas/tests/arrays/integer/test_concat.py b/pandas/tests/arrays/integer/test_concat.py
index 2e8ef506140af..069c9323d0e8a 100644
--- a/pandas/tests/arrays/integer/test_concat.py
+++ b/pandas/tests/arrays/integer/test_concat.py
@@ -63,3 +63,11 @@ def test_concat_series_with_numpy(to_concat_dtypes, result_dtype):
     result = pd.concat([s2, s1], ignore_index=True)
     expected = pd.Series([0, 1, 0, 1, pd.NA], dtype=object).astype(result_dtype)
     tm.assert_series_equal(result, expected)
+
+def test_concat_frame_with_sort_false():
+    result = pd.concat([pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False)
+    expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2,1], columns=[2,1])
+    import ipdb; ipdb.set_trace()
+
+    tm.assert_frame_equal(result, expected)
+

From 623a48d6df7c1a19733736106a7fbc9e0f8211fd Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 16:29:05 +0200
Subject: [PATCH 02/17] update test_concat.py, remove debug statement and
 improve formatting

---
 pandas/tests/arrays/integer/test_concat.py | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/arrays/integer/test_concat.py b/pandas/tests/arrays/integer/test_concat.py
index 069c9323d0e8a..5786530d3100f 100644
--- a/pandas/tests/arrays/integer/test_concat.py
+++ b/pandas/tests/arrays/integer/test_concat.py
@@ -64,10 +64,12 @@ def test_concat_series_with_numpy(to_concat_dtypes, result_dtype):
     expected = pd.Series([0, 1, 0, 1, pd.NA], dtype=object).astype(result_dtype)
     tm.assert_series_equal(result, expected)
 
+
 def test_concat_frame_with_sort_false():
-    result = pd.concat([pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False)
-    expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2,1], columns=[2,1])
-    import ipdb; ipdb.set_trace()
+    # GH 43375
+    result = pd.concat(
+        [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
+    )
+    expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
 
     tm.assert_frame_equal(result, expected)
-

From ef9549da5871c4426e07e04b5d4d197dbccfc34a Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 22:02:10 +0200
Subject: [PATCH 03/17] update api.py, to fix tests

---
 pandas/core/indexes/api.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 522fcd40bd13d..0a3fc8375207b 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -147,7 +147,7 @@ def _get_combined_index(
         for other in indexes[1:]:
             index = index.intersection(other)
     else:
-        index = union_indexes(indexes, sort=sort)
+        index = union_indexes(indexes, sort=None)
         index = ensure_index(index)
 
     if sort:
@@ -184,10 +184,7 @@ def union_indexes(indexes, sort: bool = True) -> Index:
     if len(indexes) == 1:
         result = indexes[0]
         if isinstance(result, list):
-            if sort:
-                result = Index(sorted(result))
-            else:
-                result = Index(result)
+            result = Index(sorted(result))
         return result
 
     indexes, kind = _sanitize_and_check(indexes)
@@ -222,7 +219,7 @@ def conv(i):
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:
-                result = result.union(other, sort=None if sort else False)
+                result = result.union(other, sort=None if sort else sort)
             return result
     elif kind == "array":
         index = indexes[0]

From 5d769337db038a38545897764e127bde95d7f33c Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 22:32:15 +0200
Subject: [PATCH 04/17] go back to original idea

---
 pandas/core/indexes/api.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 0a3fc8375207b..4db068af37023 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -219,7 +219,8 @@ def conv(i):
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:
-                result = result.union(other, sort=None if sort else sort)
+                import pdb; pdb.set_trace()
+                result = result.union(other, sort=None if sort else False)
             return result
     elif kind == "array":
         index = indexes[0]

From f09ee73b09ff60dbbb57bb02567dacd9c9708951 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 22:36:35 +0200
Subject: [PATCH 05/17] move new test from arrays/integer to reshape/concat;
 remove debug statement

---
 pandas/core/indexes/api.py                 |  1 -
 pandas/tests/arrays/integer/test_concat.py |  8 --------
 pandas/tests/reshape/concat/test_sort.py   | 12 ++++++++++++
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 4db068af37023..64ab9df0a88c1 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -219,7 +219,6 @@ def conv(i):
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:
-                import pdb; pdb.set_trace()
                 result = result.union(other, sort=None if sort else False)
             return result
     elif kind == "array":
diff --git a/pandas/tests/arrays/integer/test_concat.py b/pandas/tests/arrays/integer/test_concat.py
index 5786530d3100f..fc260a63eafe4 100644
--- a/pandas/tests/arrays/integer/test_concat.py
+++ b/pandas/tests/arrays/integer/test_concat.py
@@ -65,11 +65,3 @@ def test_concat_series_with_numpy(to_concat_dtypes, result_dtype):
     tm.assert_series_equal(result, expected)
 
 
-def test_concat_frame_with_sort_false():
-    # GH 43375
-    result = pd.concat(
-        [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
-    )
-    expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
-
-    tm.assert_frame_equal(result, expected)
diff --git a/pandas/tests/reshape/concat/test_sort.py b/pandas/tests/reshape/concat/test_sort.py
index 865f696b7a73a..305551d04327d 100644
--- a/pandas/tests/reshape/concat/test_sort.py
+++ b/pandas/tests/reshape/concat/test_sort.py
@@ -1,3 +1,5 @@
+import numpy as np
+
 import pandas as pd
 from pandas import DataFrame
 import pandas._testing as tm
@@ -81,3 +83,13 @@ def test_concat_aligned_sort_does_not_raise(self):
         expected = DataFrame({1: [1, 2, 1, 2], "a": [3, 4, 3, 4]}, columns=[1, "a"])
         result = pd.concat([df, df], ignore_index=True, sort=True)
         tm.assert_frame_equal(result, expected)
+
+    def test_concat_frame_with_sort_false(self):
+        # GH 43375
+        result = pd.concat(
+            [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
+        )
+        expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
+
+        tm.assert_frame_equal(result, expected)
+

From 109cd47ec347356db06c0c8d6ea5f9cb8811b33d Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 1 Oct 2021 22:38:38 +0200
Subject: [PATCH 06/17] reformat tests

---
 pandas/tests/arrays/integer/test_concat.py | 2 --
 pandas/tests/reshape/concat/test_sort.py   | 5 +++--
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/arrays/integer/test_concat.py b/pandas/tests/arrays/integer/test_concat.py
index fc260a63eafe4..2e8ef506140af 100644
--- a/pandas/tests/arrays/integer/test_concat.py
+++ b/pandas/tests/arrays/integer/test_concat.py
@@ -63,5 +63,3 @@ def test_concat_series_with_numpy(to_concat_dtypes, result_dtype):
     result = pd.concat([s2, s1], ignore_index=True)
     expected = pd.Series([0, 1, 0, 1, pd.NA], dtype=object).astype(result_dtype)
     tm.assert_series_equal(result, expected)
-
-
diff --git a/pandas/tests/reshape/concat/test_sort.py b/pandas/tests/reshape/concat/test_sort.py
index 305551d04327d..12ee19d7a4ecb 100644
--- a/pandas/tests/reshape/concat/test_sort.py
+++ b/pandas/tests/reshape/concat/test_sort.py
@@ -89,7 +89,8 @@ def test_concat_frame_with_sort_false(self):
         result = pd.concat(
             [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
         )
-        expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
+        expected = pd.DataFrame(
+            [[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1]
+        )
 
         tm.assert_frame_equal(result, expected)
-

From c910835a511786b4a3249ac11338256936f954e8 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Sun, 3 Oct 2021 09:40:48 +0200
Subject: [PATCH 07/17] adjust test_constructors to new logic in concat

---
 pandas/tests/frame/test_constructors.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/test_constructors.py b/pandas/tests/frame/test_constructors.py
index aa4b734411a58..d9fb97c8ada6f 100644
--- a/pandas/tests/frame/test_constructors.py
+++ b/pandas/tests/frame/test_constructors.py
@@ -2355,9 +2355,10 @@ def test_construct_with_two_categoricalindex_series(self):
         result = DataFrame([s1, s2])
         expected = DataFrame(
             np.array(
-                [[np.nan, 39.0, np.nan, 6.0, 4.0], [2.0, 152.0, 2.0, 242.0, 150.0]]
+                [[39, 6, 4, np.nan, np.nan], [152.0, 242.0, 150.0, 2.0, 2.0]]
             ),
-            columns=["f", "female", "m", "male", "unknown"],
+            # columns=["f", "female", "m", "male", "unknown"],
+            columns=['female', 'male', 'unknown', 'f', 'm'],
         )
         tm.assert_frame_equal(result, expected)
 

From 3be2b559ba1c06af059ee89ec14c2c3ef42d90c2 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Sun, 3 Oct 2021 09:48:51 +0200
Subject: [PATCH 08/17] update test

---
 pandas/tests/frame/test_constructors.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/pandas/tests/frame/test_constructors.py b/pandas/tests/frame/test_constructors.py
index d9fb97c8ada6f..859f5171a6f04 100644
--- a/pandas/tests/frame/test_constructors.py
+++ b/pandas/tests/frame/test_constructors.py
@@ -2354,11 +2354,8 @@ def test_construct_with_two_categoricalindex_series(self):
         )
         result = DataFrame([s1, s2])
         expected = DataFrame(
-            np.array(
-                [[39, 6, 4, np.nan, np.nan], [152.0, 242.0, 150.0, 2.0, 2.0]]
-            ),
-            # columns=["f", "female", "m", "male", "unknown"],
-            columns=['female', 'male', 'unknown', 'f', 'm'],
+            np.array([[39, 6, 4, np.nan, np.nan], [152.0, 242.0, 150.0, 2.0, 2.0]]),
+            columns=["female", "male", "unknown", "f", "m"],
         )
         tm.assert_frame_equal(result, expected)
 

From 809b0c80cd4647b3de6f611b099bbf69f0a0d176 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Sun, 3 Oct 2021 18:43:53 +0200
Subject: [PATCH 09/17] adjust tests

---
 pandas/tests/reshape/test_melt.py | 26 +++++++++++++-------------
 pandas/tests/strings/test_cat.py  |  6 +++++-
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/pandas/tests/reshape/test_melt.py b/pandas/tests/reshape/test_melt.py
index 4972cb34aac69..af9d6dd83bee3 100644
--- a/pandas/tests/reshape/test_melt.py
+++ b/pandas/tests/reshape/test_melt.py
@@ -746,11 +746,11 @@ def test_unbalanced(self):
         )
         df["id"] = df.index
         exp_data = {
-            "X": ["X1", "X1", "X2", "X2"],
-            "A": [1.0, 3.0, 2.0, 4.0],
-            "B": [5.0, np.nan, 6.0, np.nan],
-            "id": [0, 0, 1, 1],
-            "year": [2010, 2011, 2010, 2011],
+            "X": ["X1", "X2", "X1", "X2"],
+            "A": [1.0, 2.0, 3.0, 4.0],
+            "B": [5.0, 6.0, np.nan, np.nan],
+            "id": [0, 1, 0, 1],
+            "year": [2010, 2010, 2011, 2011],
         }
         expected = DataFrame(exp_data)
         expected = expected.set_index(["id", "year"])[["X", "A", "B"]]
@@ -993,10 +993,10 @@ def test_nonnumeric_suffix(self):
         )
         expected = DataFrame(
             {
-                "A": ["X1", "X1", "X2", "X2"],
-                "colname": ["placebo", "test", "placebo", "test"],
-                "result": [5.0, np.nan, 6.0, np.nan],
-                "treatment": [1.0, 3.0, 2.0, 4.0],
+                "A": ["X1", "X2", "X1", "X2"],
+                "colname": ["placebo", "placebo", "test", "test"],
+                "result": [5.0, 6.0, np.nan, np.nan],
+                "treatment": [1.0, 2.0, 3.0, 4.0],
             }
         )
         expected = expected.set_index(["A", "colname"])
@@ -1040,10 +1040,10 @@ def test_float_suffix(self):
         )
         expected = DataFrame(
             {
-                "A": ["X1", "X1", "X1", "X1", "X2", "X2", "X2", "X2"],
-                "colname": [1, 1.1, 1.2, 2.1, 1, 1.1, 1.2, 2.1],
-                "result": [0.0, np.nan, 5.0, np.nan, 9.0, np.nan, 6.0, np.nan],
-                "treatment": [np.nan, 1.0, np.nan, 3.0, np.nan, 2.0, np.nan, 4.0],
+                "A": ["X1", "X2", "X1", "X2", "X1", "X2", "X1", "X2"],
+                "colname": [1.2, 1.2, 1.0, 1.0, 1.1, 1.1, 2.1, 2.1],
+                "result": [5.0, 6.0, 0.0, 9.0, np.nan, np.nan, np.nan, np.nan],
+                "treatment": [np.nan, np.nan, np.nan, np.nan, 1.0, 2.0, 3.0, 4.0],
             }
         )
         expected = expected.set_index(["A", "colname"])
diff --git a/pandas/tests/strings/test_cat.py b/pandas/tests/strings/test_cat.py
index 48f853cfdcb10..8abbc59343e78 100644
--- a/pandas/tests/strings/test_cat.py
+++ b/pandas/tests/strings/test_cat.py
@@ -278,7 +278,11 @@ def test_str_cat_align_mixed_inputs(join):
     expected_outer = Series(["aaA", "bbB", "c-C", "ddD", "-e-"])
     # joint index of rhs [t, u]; u will be forced have index of s
     rhs_idx = (
-        t.index.intersection(s.index) if join == "inner" else t.index.union(s.index)
+        t.index.intersection(s.index)
+        if join == "inner"
+        else t.index.union(s.index)
+        if join == "outer"
+        else t.index.append(s.index.difference(t.index))
     )
 
     expected = expected_outer.loc[s.index.join(rhs_idx, how=join)]

From 16e9c63b7cd53986ece75577b578b0717d32c1b4 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Tue, 5 Oct 2021 18:55:50 +0200
Subject: [PATCH 10/17] adapt type hints in api.py

---
 pandas/core/indexes/api.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 64ab9df0a88c1..49c3a6b9de6c1 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import textwrap
+from typing import Union
 
 from pandas._libs import (
     NaT,
@@ -163,7 +164,7 @@ def _get_combined_index(
     return index
 
 
-def union_indexes(indexes, sort: bool = True) -> Index:
+def union_indexes(indexes, sort: Union[bool, None] = True) -> Index:
     """
     Return the union of indexes.
 

From 78f699dc650a56284d6c49fcf71d4d3dd9f7331f Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Tue, 5 Oct 2021 19:10:36 +0200
Subject: [PATCH 11/17] update test_sort.py

---
 pandas/tests/reshape/concat/test_sort.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/reshape/concat/test_sort.py b/pandas/tests/reshape/concat/test_sort.py
index 12ee19d7a4ecb..fdbe59ece2771 100644
--- a/pandas/tests/reshape/concat/test_sort.py
+++ b/pandas/tests/reshape/concat/test_sort.py
@@ -89,7 +89,7 @@ def test_concat_frame_with_sort_false(self):
         result = pd.concat(
             [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
         )
-        expected = pd.DataFrame(
+        expected = DataFrame(
             [[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1]
         )
 

From bf64737c7140bcdd7a18d4fc951417e7012a48eb Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 12:37:49 +0200
Subject: [PATCH 12/17] add optional type option for sort parameter

---
 pandas/_libs/lib.pyi       | 3 ++-
 pandas/_libs/lib.pyx       | 3 ++-
 pandas/core/indexes/api.py | 6 +++---
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/pandas/_libs/lib.pyi b/pandas/_libs/lib.pyi
index b88a2e4c28cfb..8aa1a33a05cf2 100644
--- a/pandas/_libs/lib.pyi
+++ b/pandas/_libs/lib.pyi
@@ -8,6 +8,7 @@ from typing import (
     Hashable,
     Literal,
     overload,
+    Optional
 )
 
 import numpy as np
@@ -55,7 +56,7 @@ def is_integer_array(values: np.ndarray, skipna: bool = ...): ...
 def is_bool_array(values: np.ndarray, skipna: bool = ...): ...
 def fast_multiget(mapping: dict, keys: np.ndarray, default=...) -> np.ndarray: ...
 def fast_unique_multiple_list_gen(gen: Generator, sort: bool = ...) -> list: ...
-def fast_unique_multiple_list(lists: list, sort: bool = ...) -> list: ...
+def fast_unique_multiple_list(lists: list, sort: Optional[bool] = ...) -> list: ...
 def fast_unique_multiple(arrays: list, sort: bool = ...) -> list: ...
 def map_infer(
     arr: np.ndarray,
diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index 4cc8d1ac2f60e..234251917c05c 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -1,6 +1,7 @@
 from collections import abc
 from decimal import Decimal
 from enum import Enum
+from typing import Optional
 import warnings
 
 import cython
@@ -350,7 +351,7 @@ def fast_unique_multiple(list arrays, sort: bool = True):
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def fast_unique_multiple_list(lists: list, sort: bool = True) -> list:
+def fast_unique_multiple_list(lists: list, sort: Optional[bool] = True) -> list:
     cdef:
         list buf
         Py_ssize_t k = len(lists)
diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 49c3a6b9de6c1..959c9d6045ba1 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import textwrap
-from typing import Union
+from typing import Optional
 
 from pandas._libs import (
     NaT,
@@ -148,7 +148,7 @@ def _get_combined_index(
         for other in indexes[1:]:
             index = index.intersection(other)
     else:
-        index = union_indexes(indexes, sort=None)
+        index = union_indexes(indexes, sort=False)
         index = ensure_index(index)
 
     if sort:
@@ -164,7 +164,7 @@ def _get_combined_index(
     return index
 
 
-def union_indexes(indexes, sort: Union[bool, None] = True) -> Index:
+def union_indexes(indexes, sort: Optional[bool] = True) -> Index:
     """
     Return the union of indexes.
 

From 39a5746d4c0e6c93cb57e440197c9264f8046953 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 13:07:22 +0200
Subject: [PATCH 13/17] remove optional type hint in favor of piped types

---
 pandas/_libs/lib.pyi       | 3 +--
 pandas/_libs/lib.pyx       | 3 +--
 pandas/core/indexes/api.py | 2 +-
 3 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/pandas/_libs/lib.pyi b/pandas/_libs/lib.pyi
index 8aa1a33a05cf2..dd1fa0780520c 100644
--- a/pandas/_libs/lib.pyi
+++ b/pandas/_libs/lib.pyi
@@ -8,7 +8,6 @@ from typing import (
     Hashable,
     Literal,
     overload,
-    Optional
 )
 
 import numpy as np
@@ -56,7 +55,7 @@ def is_integer_array(values: np.ndarray, skipna: bool = ...): ...
 def is_bool_array(values: np.ndarray, skipna: bool = ...): ...
 def fast_multiget(mapping: dict, keys: np.ndarray, default=...) -> np.ndarray: ...
 def fast_unique_multiple_list_gen(gen: Generator, sort: bool = ...) -> list: ...
-def fast_unique_multiple_list(lists: list, sort: Optional[bool] = ...) -> list: ...
+def fast_unique_multiple_list(lists: list, sort: bool | None = ...) -> list: ...
 def fast_unique_multiple(arrays: list, sort: bool = ...) -> list: ...
 def map_infer(
     arr: np.ndarray,
diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index 234251917c05c..c9d703718683e 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -1,7 +1,6 @@
 from collections import abc
 from decimal import Decimal
 from enum import Enum
-from typing import Optional
 import warnings
 
 import cython
@@ -351,7 +350,7 @@ def fast_unique_multiple(list arrays, sort: bool = True):
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def fast_unique_multiple_list(lists: list, sort: Optional[bool] = True) -> list:
+def fast_unique_multiple_list(lists: list, sort: bool | None = True) -> list:
     cdef:
         list buf
         Py_ssize_t k = len(lists)
diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 959c9d6045ba1..61b151f5e4bbc 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -164,7 +164,7 @@ def _get_combined_index(
     return index
 
 
-def union_indexes(indexes, sort: Optional[bool] = True) -> Index:
+def union_indexes(indexes, sort: bool | None = True) -> Index:
     """
     Return the union of indexes.
 

From 5bd801cd9d85babc785409c712f2b28630721dda Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 11:25:48 +0000
Subject: [PATCH 14/17] Fixes from pre-commit [automated commit]

---
 pandas/tests/reshape/concat/test_sort.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/pandas/tests/reshape/concat/test_sort.py b/pandas/tests/reshape/concat/test_sort.py
index fdbe59ece2771..0b949511cf56e 100644
--- a/pandas/tests/reshape/concat/test_sort.py
+++ b/pandas/tests/reshape/concat/test_sort.py
@@ -89,8 +89,6 @@ def test_concat_frame_with_sort_false(self):
         result = pd.concat(
             [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
         )
-        expected = DataFrame(
-            [[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1]
-        )
+        expected = DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
 
         tm.assert_frame_equal(result, expected)

From 16d8871461021742ca8eda5fda47730a0c163887 Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 15:53:26 +0200
Subject: [PATCH 15/17] update whatsnew

---
 doc/source/whatsnew/v1.4.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.4.0.rst b/doc/source/whatsnew/v1.4.0.rst
index 722d0dcc10041..e68a71c49e6bb 100644
--- a/doc/source/whatsnew/v1.4.0.rst
+++ b/doc/source/whatsnew/v1.4.0.rst
@@ -512,6 +512,7 @@ Reshaping
 - Bug in :func:`concat` of ``bool`` and ``boolean`` dtypes resulting in ``object`` dtype instead of ``boolean`` dtype (:issue:`42800`)
 - Bug in :func:`crosstab` when inputs are are categorical Series, there are categories that are not present in one or both of the Series, and ``margins=True``. Previously the margin value for missing categories was ``NaN``. It is now correctly reported as 0 (:issue:`43505`)
 - Bug in :func:`concat` would fail when the ``objs`` argument all had the same index and the ``keys`` argument contained duplicates (:issue:`43595`)
+- Bug in :func:`concat` which resulted in ignored the ``sort`` parameter (:issue:`43375`)
 
 Sparse
 ^^^^^^

From 24392f6a712415cfc6d62daf554e95e5926228fb Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 15:55:20 +0200
Subject: [PATCH 16/17] update whatsnew

---
 doc/source/whatsnew/v1.4.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.4.0.rst b/doc/source/whatsnew/v1.4.0.rst
index e68a71c49e6bb..8c69820a54bce 100644
--- a/doc/source/whatsnew/v1.4.0.rst
+++ b/doc/source/whatsnew/v1.4.0.rst
@@ -512,7 +512,7 @@ Reshaping
 - Bug in :func:`concat` of ``bool`` and ``boolean`` dtypes resulting in ``object`` dtype instead of ``boolean`` dtype (:issue:`42800`)
 - Bug in :func:`crosstab` when inputs are are categorical Series, there are categories that are not present in one or both of the Series, and ``margins=True``. Previously the margin value for missing categories was ``NaN``. It is now correctly reported as 0 (:issue:`43505`)
 - Bug in :func:`concat` would fail when the ``objs`` argument all had the same index and the ``keys`` argument contained duplicates (:issue:`43595`)
-- Bug in :func:`concat` which resulted in ignored the ``sort`` parameter (:issue:`43375`)
+- Bug in :func:`concat` which ignored the ``sort`` parameter (:issue:`43375`)
 
 Sparse
 ^^^^^^

From 95580bff53d97efd3898aa37dcda301acc7603cf Mon Sep 17 00:00:00 2001
From: Tobias Pitters <tobias.pitters@gmail.com>
Date: Fri, 8 Oct 2021 16:56:14 +0200
Subject: [PATCH 17/17] remove unnecessary import and correct false naming of
 DataFrame

---
 pandas/core/indexes/api.py               | 1 -
 pandas/tests/reshape/concat/test_sort.py | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 61b151f5e4bbc..e497012f23b68 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import textwrap
-from typing import Optional
 
 from pandas._libs import (
     NaT,
diff --git a/pandas/tests/reshape/concat/test_sort.py b/pandas/tests/reshape/concat/test_sort.py
index 0b949511cf56e..3d362ef42d276 100644
--- a/pandas/tests/reshape/concat/test_sort.py
+++ b/pandas/tests/reshape/concat/test_sort.py
@@ -87,7 +87,7 @@ def test_concat_aligned_sort_does_not_raise(self):
     def test_concat_frame_with_sort_false(self):
         # GH 43375
         result = pd.concat(
-            [pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
+            [DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
         )
         expected = DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])