From 8d51763b4fb492a96f14629eef1656fd1c1fc820 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Tue, 21 Oct 2025 21:46:32 +1100
Subject: [PATCH 01/16] Fix: proper return types for MultiIndex.swaplevel &
 MultiIndex.union

---
 pandas-stubs/core/indexes/multi.pyi | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index e66c845ec..5a4b55431 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -7,6 +7,7 @@ from collections.abc import (
 from typing import (
     final,
     overload,
+    Any
 )
 
 import numpy as np
@@ -135,7 +136,7 @@ class MultiIndex(Index):
     def append(self, other): ...
     def repeat(self, repeats, axis=...): ...
     def drop(self, codes, level: Level | None = None, errors: str = "raise") -> Self: ...  # type: ignore[override] # pyright: ignore[reportIncompatibleMethodOverride]
-    def swaplevel(self, i: int = -2, j: int = -1): ...
+    def swaplevel(self, i: int = -2, j: int = -1) -> Self: ...
     def reorder_levels(self, order): ...
     def sortlevel(
         self,
@@ -163,3 +164,7 @@ class MultiIndex(Index):
     def insert(self, loc, item): ...
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
+    @overload
+    def union(self, other: MultiIndex, sort: bool | None = ...) -> MultiIndex: ...
+    @overload
+    def union(self, other: Index | list[HashableT], sort: bool | None = ...) -> Index[Any]: ...

From 4552867c27891ddbc81aa755223734468cc0c468 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Wed, 22 Oct 2025 23:13:15 +1100
Subject: [PATCH 02/16] Index of and List of tuples in MultiIndex overriding

---
 pandas-stubs/_typing.pyi            | 5 ++++-
 pandas-stubs/core/indexes/multi.pyi | 5 ++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/pandas-stubs/_typing.pyi b/pandas-stubs/_typing.pyi
index 7d944d243..3d2b1715b 100644
--- a/pandas-stubs/_typing.pyi
+++ b/pandas-stubs/_typing.pyi
@@ -904,7 +904,10 @@ SeriesDType: TypeAlias = (
     | datetime.datetime  # includes pd.Timestamp
     | datetime.timedelta  # includes pd.Timedelta
 )
-S1 = TypeVar("S1", bound=SeriesDType, default=Any)
+
+IndexKey: TypeAlias = SeriesDType | tuple[Hashable, ...] # to support Indexes of tuples for MultiIndex unions
+
+S1 = TypeVar("S1", bound=IndexKey, default=Any)
 # Like S1, but without `default=Any`.
 S2 = TypeVar("S2", bound=SeriesDType)
 S2_CT = TypeVar("S2_CT", bound=SeriesDType, contravariant=True)
diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index 5a4b55431..6cc71e1d0 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -7,7 +7,6 @@ from collections.abc import (
 from typing import (
     final,
     overload,
-    Any
 )
 
 import numpy as np
@@ -165,6 +164,6 @@ class MultiIndex(Index):
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
     @overload
-    def union(self, other: MultiIndex, sort: bool | None = ...) -> MultiIndex: ...
+    def union(self, other: MultiIndex | Index[tuple[Hashable, ...]] | list[tuple[Hashable, ...]], sort: bool | None = None,) -> Self: ...
     @overload
-    def union(self, other: Index | list[HashableT], sort: bool | None = ...) -> Index[Any]: ...
+    def union(self, other: list[HashableT] | Index, sort: bool | None = None,) -> Index: ...

From d3f575761310e0a62ef93322ca68f9617b8500f3 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Wed, 22 Oct 2025 23:13:31 +1100
Subject: [PATCH 03/16] MultiIndex union tests

---
 tests/indexes/test_indexes.py | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 4db44276d..60cd1f30b 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1613,3 +1613,33 @@ def test_to_series() -> None:
         np.complexfloating,
     )
     check(assert_type(Index(["1"]).to_series(), "pd.Series[str]"), pd.Series, str)
+
+def test_multiindex_union() -> None:
+    """Test that union returns MultiIndex on MultiIndex input"""
+    mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
+    mi2 = pd.MultiIndex.from_product([["a", "b"], [3, 4]], names=["let", "num"])
+
+    check(
+        assert_type(mi.union(mi2), "pd.MultiIndex"),
+        pd.MultiIndex,
+    )
+    check(
+        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.MultiIndex"), 
+        pd.MultiIndex,
+    )
+    check(
+        assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
+        pd.MultiIndex,
+    )
+    check(
+        assert_type(mi.union(pd.Index([1, 2, 3])), "pd.Index"), 
+        pd.Index
+    )
+    check(
+        assert_type(mi.union(pd.Index(["x", "y"])), "pd.Index"), 
+        pd.Index
+    )
+    check(
+        assert_type(mi.union([1, 2, 3]), "pd.Index"), 
+        pd.Index
+    )
\ No newline at end of file

From d401e0ca9e30c32d63c00735f847581290750652 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 00:18:06 +1100
Subject: [PATCH 04/16] MultiIndex-Index union overload simplified

---
 pandas-stubs/core/indexes/multi.pyi | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index 6cc71e1d0..e02521409 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -164,6 +164,6 @@ class MultiIndex(Index):
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
     @overload
-    def union(self, other: MultiIndex | Index[tuple[Hashable, ...]] | list[tuple[Hashable, ...]], sort: bool | None = None,) -> Self: ...
+    def union(self, other: Self | Index | list[tuple[Hashable, ...]], sort: bool | None = ...,) -> Self: ...
     @overload
-    def union(self, other: list[HashableT] | Index, sort: bool | None = None,) -> Index: ...
+    def union(self, other: Index | list[HashableT], sort: bool | None = ...,) -> Index: ...

From 7739015d34c436dd3b1605c801d30e02c306c2e2 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 00:25:05 +1100
Subject: [PATCH 05/16] MultiIndex test amended and swaplevel test added

---
 tests/indexes/test_indexes.py | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 60cd1f30b..0fcd272ff 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1614,6 +1614,14 @@ def test_to_series() -> None:
     )
     check(assert_type(Index(["1"]).to_series(), "pd.Series[str]"), pd.Series, str)
 
+def test_swaplevel_rettype() -> None:
+    """Test that swaplevel returns Self"""
+    mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
+    check(
+        assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"),
+        pd.MultiIndex,
+    )
+
 def test_multiindex_union() -> None:
     """Test that union returns MultiIndex on MultiIndex input"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
@@ -1631,14 +1639,6 @@ def test_multiindex_union() -> None:
         assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
         pd.MultiIndex,
     )
-    check(
-        assert_type(mi.union(pd.Index([1, 2, 3])), "pd.Index"), 
-        pd.Index
-    )
-    check(
-        assert_type(mi.union(pd.Index(["x", "y"])), "pd.Index"), 
-        pd.Index
-    )
     check(
         assert_type(mi.union([1, 2, 3]), "pd.Index"), 
         pd.Index

From 2a77d799aee7c68567590da0ed38ef97e5688394 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 01:05:05 +1100
Subject: [PATCH 06/16] Simplified MultiIndex-Index union overload

---
 pandas-stubs/core/indexes/multi.pyi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index e02521409..c040bea12 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -164,6 +164,6 @@ class MultiIndex(Index):
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
     @overload
-    def union(self, other: Self | Index | list[tuple[Hashable, ...]], sort: bool | None = ...,) -> Self: ...
+    def union(self, other: Self | list[tuple[Hashable, ...]], sort: bool | None = ...,) -> Self: ...
     @overload
     def union(self, other: Index | list[HashableT], sort: bool | None = ...,) -> Index: ...

From 2f5c33854c8676d0778cb0fc2d7d9904c3454b2d Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 01:05:55 +1100
Subject: [PATCH 07/16] Reverted S1 changes

---
 pandas-stubs/_typing.pyi | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/pandas-stubs/_typing.pyi b/pandas-stubs/_typing.pyi
index 3d2b1715b..5a5fe5bbb 100644
--- a/pandas-stubs/_typing.pyi
+++ b/pandas-stubs/_typing.pyi
@@ -905,9 +905,7 @@ SeriesDType: TypeAlias = (
     | datetime.timedelta  # includes pd.Timedelta
 )
 
-IndexKey: TypeAlias = SeriesDType | tuple[Hashable, ...] # to support Indexes of tuples for MultiIndex unions
-
-S1 = TypeVar("S1", bound=IndexKey, default=Any)
+S1 = TypeVar("S1", bound=SeriesDType, default=Any)
 # Like S1, but without `default=Any`.
 S2 = TypeVar("S2", bound=SeriesDType)
 S2_CT = TypeVar("S2_CT", bound=SeriesDType, contravariant=True)

From 5e82ee56b2b03c0290e98e28c3db7d5bf3ba9d63 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 01:06:26 +1100
Subject: [PATCH 08/16] amended MultiIndex union test and added swaplevel test

---
 tests/indexes/test_indexes.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 0fcd272ff..16d528729 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1632,8 +1632,8 @@ def test_multiindex_union() -> None:
         pd.MultiIndex,
     )
     check(
-        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.MultiIndex"), 
-        pd.MultiIndex,
+        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.Index"), 
+        pd.Index,
     )
     check(
         assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),

From a606e96bc83e8fd09ded8a486b095cf689b805b3 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 01:28:16 +1100
Subject: [PATCH 09/16] Removed runtime-failing test

---
 tests/indexes/test_indexes.py | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 16d528729..79bbf0cd2 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1614,6 +1614,7 @@ def test_to_series() -> None:
     )
     check(assert_type(Index(["1"]).to_series(), "pd.Series[str]"), pd.Series, str)
 
+
 def test_swaplevel_rettype() -> None:
     """Test that swaplevel returns Self"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
@@ -1622,6 +1623,7 @@ def test_swaplevel_rettype() -> None:
         pd.MultiIndex,
     )
 
+
 def test_multiindex_union() -> None:
     """Test that union returns MultiIndex on MultiIndex input"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
@@ -1632,14 +1634,10 @@ def test_multiindex_union() -> None:
         pd.MultiIndex,
     )
     check(
-        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.Index"), 
+        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.Index"),
         pd.Index,
     )
     check(
         assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
         pd.MultiIndex,
     )
-    check(
-        assert_type(mi.union([1, 2, 3]), "pd.Index"), 
-        pd.Index
-    )
\ No newline at end of file

From 46e805082a5619df89cdcba84cce0af524c6c2cb Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 09:14:55 +1100
Subject: [PATCH 10/16] Pre-commit refactoring

---
 pandas-stubs/core/indexes/multi.pyi | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index c040bea12..c93926022 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -164,6 +164,14 @@ class MultiIndex(Index):
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
     @overload
-    def union(self, other: Self | list[tuple[Hashable, ...]], sort: bool | None = ...,) -> Self: ...
+    def union(
+        self,
+        other: Self | list[tuple[Hashable, ...]],
+        sort: bool | None = ...,
+    ) -> Self: ...
     @overload
-    def union(self, other: Index | list[HashableT], sort: bool | None = ...,) -> Index: ...
+    def union(
+        self,
+        other: Index | list[HashableT],
+        sort: bool | None = ...,
+    ) -> Index: ...

From 092eec2096dd9d7009f8f83a0d674cea7cc9c0a6 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 09:19:57 +1100
Subject: [PATCH 11/16] Removed a MultiIndex-Index test

---
 tests/indexes/test_indexes.py | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 9ff37f10b..6a427ae2b 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1621,10 +1621,6 @@ def test_multiindex_union() -> None:
         assert_type(mi.union(mi2), "pd.MultiIndex"),
         pd.MultiIndex,
     )
-    check(
-        assert_type(mi.union(pd.Index([("c", 3), ("d", 4)])), "pd.Index"),
-        pd.Index,
-    )
     check(
         assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
         pd.MultiIndex,

From 6b676bf303394bbc39244393b3992f48d80d37d9 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Thu, 23 Oct 2025 09:32:00 +1100
Subject: [PATCH 12/16] Removed MultiIndex union overloading

---
 pandas-stubs/core/indexes/multi.pyi |  9 +--------
 pandas-stubs/py.typed               |  0
 tests/indexes/test_indexes.py       | 13 ++++---------
 3 files changed, 5 insertions(+), 17 deletions(-)
 delete mode 100644 pandas-stubs/py.typed

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index c93926022..519ef92d6 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -163,15 +163,8 @@ class MultiIndex(Index):
     def insert(self, loc, item): ...
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
-    @overload
     def union(
         self,
-        other: Self | list[tuple[Hashable, ...]],
+        other: Self | Index | list[HashableT],
         sort: bool | None = ...,
     ) -> Self: ...
-    @overload
-    def union(
-        self,
-        other: Index | list[HashableT],
-        sort: bool | None = ...,
-    ) -> Index: ...
diff --git a/pandas-stubs/py.typed b/pandas-stubs/py.typed
deleted file mode 100644
index e69de29bb..000000000
diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 6a427ae2b..6fc27f719 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1603,20 +1603,15 @@ def test_to_series() -> None:
     check(assert_type(Index(["1"]).to_series(), "pd.Series[str]"), pd.Series, str)
 
 
-def test_swaplevel_rettype() -> None:
-    """Test that swaplevel returns Self"""
+def test_multiindex_swaplevel_rettype() -> None:
+    """Test that union returns MultiIndex on MultiIndex input and swaplevel returns Self"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
+    mi2 = pd.MultiIndex.from_product([["a", "b"], [3, 4]], names=["let", "num"])
+
     check(
         assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"),
         pd.MultiIndex,
     )
-
-
-def test_multiindex_union() -> None:
-    """Test that union returns MultiIndex on MultiIndex input"""
-    mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
-    mi2 = pd.MultiIndex.from_product([["a", "b"], [3, 4]], names=["let", "num"])
-
     check(
         assert_type(mi.union(mi2), "pd.MultiIndex"),
         pd.MultiIndex,

From 5d8153f66d57365d643e87cf3351845f58edd8eb Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <166975079+realzachbrownjohn@users.noreply.github.com>
Date: Fri, 24 Oct 2025 09:25:21 +1100
Subject: [PATCH 13/16] Separated swaplevel and union tests

Co-authored-by: Yi-Fan Wang <cmp0xff@users.noreply.github.com>
---
 tests/indexes/test_indexes.py | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index 6fc27f719..b5031ea04 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1603,20 +1603,22 @@ def test_to_series() -> None:
     check(assert_type(Index(["1"]).to_series(), "pd.Series[str]"), pd.Series, str)
 
 
-def test_multiindex_swaplevel_rettype() -> None:
-    """Test that union returns MultiIndex on MultiIndex input and swaplevel returns Self"""
+def test_multiindex_union() -> None:
+    """Test that MultiIndex.union returns MultiIndex"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
     mi2 = pd.MultiIndex.from_product([["a", "b"], [3, 4]], names=["let", "num"])
 
     check(
-        assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"),
-        pd.MultiIndex,
+        assert_type(mi.union(mi2), "pd.MultiIndex"), pd.MultiIndex
     )
     check(
-        assert_type(mi.union(mi2), "pd.MultiIndex"),
-        pd.MultiIndex,
+        assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"), pd.MultiIndex
     )
+
+    
+def test_multiindex_swaplevel() -> None:
+    """Test that MultiIndex.swaplevel returns MultiIndex"""
+    mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
     check(
-        assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
-        pd.MultiIndex,
+        assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"), pd.MultiIndex
     )

From 3397d49dc91ca20c0f32766f4351a136c5123b33 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Fri, 24 Oct 2025 09:31:33 +1100
Subject: [PATCH 14/16] test for MultiIndex-Index union

---
 tests/indexes/test_indexes.py | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/tests/indexes/test_indexes.py b/tests/indexes/test_indexes.py
index b5031ea04..d477572ee 100644
--- a/tests/indexes/test_indexes.py
+++ b/tests/indexes/test_indexes.py
@@ -1608,17 +1608,12 @@ def test_multiindex_union() -> None:
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
     mi2 = pd.MultiIndex.from_product([["a", "b"], [3, 4]], names=["let", "num"])
 
-    check(
-        assert_type(mi.union(mi2), "pd.MultiIndex"), pd.MultiIndex
-    )
-    check(
-        assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"), pd.MultiIndex
-    )
+    check(assert_type(mi.union(mi2), "pd.MultiIndex"), pd.MultiIndex)
+    check(assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"), pd.MultiIndex)
+    check(assert_type(mi.union([1, 2, 3]), "pd.MultiIndex"), pd.MultiIndex)
+
 
-    
 def test_multiindex_swaplevel() -> None:
     """Test that MultiIndex.swaplevel returns MultiIndex"""
     mi = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=["let", "num"])
-    check(
-        assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"), pd.MultiIndex
-    )
+    check(assert_type(mi.swaplevel(0, 1), "pd.MultiIndex"), pd.MultiIndex)

From 39877b29c57e3c29a7220f4cd99b43e1356c91f2 Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Fri, 24 Oct 2025 09:51:22 +1100
Subject: [PATCH 15/16] MultiIndex.union overload replaced with change of base
 return type

---
 pandas-stubs/core/indexes/base.pyi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas-stubs/core/indexes/base.pyi b/pandas-stubs/core/indexes/base.pyi
index d6b245983..3ff4db49a 100644
--- a/pandas-stubs/core/indexes/base.pyi
+++ b/pandas-stubs/core/indexes/base.pyi
@@ -405,7 +405,7 @@ class Index(IndexOpsMixin[S1], ElementOpsMixin[S1]):
     __bool__ = ...
     def union(
         self, other: list[HashableT] | Self, sort: bool | None = None
-    ) -> Index: ...
+    ) -> Self: ...
     def intersection(
         self, other: list[S1] | Self, sort: bool | None = False
     ) -> Self: ...

From 9c662991ef5801da220efc8ddc3bafa1909db5ea Mon Sep 17 00:00:00 2001
From: Zach Brownjohn <u7919297@anu.edu.au>
Date: Fri, 24 Oct 2025 09:54:01 +1100
Subject: [PATCH 16/16] removed MultiIndex.union overload

---
 pandas-stubs/core/indexes/multi.pyi | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/pandas-stubs/core/indexes/multi.pyi b/pandas-stubs/core/indexes/multi.pyi
index 519ef92d6..7dc76e7f7 100644
--- a/pandas-stubs/core/indexes/multi.pyi
+++ b/pandas-stubs/core/indexes/multi.pyi
@@ -163,8 +163,3 @@ class MultiIndex(Index):
     def insert(self, loc, item): ...
     def delete(self, loc): ...
     def isin(self, values, level=...) -> np_1darray[np.bool]: ...
-    def union(
-        self,
-        other: Self | Index | list[HashableT],
-        sort: bool | None = ...,
-    ) -> Self: ...