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Fix: proper return types for MultiIndex.swaplevel & MultiIndex.union #1437

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Fix: proper return types for MultiIndex.swaplevel & MultiIndex.union #1437

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8d51763
Fix: proper return types for MultiIndex.swaplevel & MultiIndex.union
realzachbrownjohn Oct 21, 2025
4552867
Index of and List of tuples in MultiIndex overriding
realzachbrownjohn Oct 22, 2025
d3f5757
MultiIndex union tests
realzachbrownjohn Oct 22, 2025
d401e0c
MultiIndex-Index union overload simplified
realzachbrownjohn Oct 22, 2025
7739015
MultiIndex test amended and swaplevel test added
realzachbrownjohn Oct 22, 2025
2a77d79
Simplified MultiIndex-Index union overload
realzachbrownjohn Oct 22, 2025
2f5c338
Reverted S1 changes
realzachbrownjohn Oct 22, 2025
5e82ee5
amended MultiIndex union test and added swaplevel test
realzachbrownjohn Oct 22, 2025
a606e96
Removed runtime-failing test
realzachbrownjohn Oct 22, 2025
23b603f
Merge branch 'pandas-dev:main' into bugfix
realzachbrownjohn Oct 22, 2025
46e8050
Pre-commit refactoring
realzachbrownjohn Oct 22, 2025
092eec2
Removed a MultiIndex-Index test
realzachbrownjohn Oct 22, 2025
6b676bf
Removed MultiIndex union overloading
realzachbrownjohn Oct 22, 2025
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1 change: 1 addition & 0 deletions pandas-stubs/_typing.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -908,6 +908,7 @@ SeriesDType: TypeAlias = (
| datetime.datetime # includes pd.Timestamp
| datetime.timedelta # includes pd.Timedelta
)

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This extra empty line can also be removed

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Would you?

S1 = TypeVar("S1", bound=SeriesDType, default=Any)
# Like S1, but without `default=Any`.
S2 = TypeVar("S2", bound=SeriesDType)
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7 changes: 6 additions & 1 deletion pandas-stubs/core/indexes/multi.pyi
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Expand Up @@ -135,7 +135,7 @@ 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,
Expand Down Expand Up @@ -163,3 +163,8 @@ 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 = ...,
Comment on lines +167 to +169
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I suppose Index should be removed. It is not tested, either.

Suggested change
self,
other: Self | Index | list[HashableT],
sort: bool | None = ...,
self, other: list[HashableT] | Self, sort: bool | None = ...

) -> Self: ...
Empty file removed pandas-stubs/py.typed
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19 changes: 19 additions & 0 deletions tests/indexes/test_indexes.py
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Expand Up @@ -1601,3 +1601,22 @@ def test_to_series() -> None:
np.complexfloating,
)
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"""
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,
)
check(
assert_type(mi.union(mi2), "pd.MultiIndex"),
pd.MultiIndex,
)
check(
assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
pd.MultiIndex,
)
Comment on lines +1606 to +1622
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Please make two tests, since swaplevel and union are two independent functionalities:

Suggested change
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,
)
check(
assert_type(mi.union(mi2), "pd.MultiIndex"),
pd.MultiIndex,
)
check(
assert_type(mi.union([("c", 3), ("d", 4)]), "pd.MultiIndex"),
pd.MultiIndex,
)
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.union(mi2), "pd.MultiIndex"), pd.MultiIndex
)
check(
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.swaplevel(0, 1), "pd.MultiIndex"), pd.MultiIndex
)