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fix list.__add__ to use the expected type #14801

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fix list.__add__ to use the expected type #14801

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b5ad828
fix `list.__add__` to use the expected type
KotlinIsland Sep 21, 2025
d8db897
better name for the typevar
KotlinIsland Oct 1, 2025
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21 changes: 21 additions & 0 deletions stdlib/@tests/test_cases/builtins/check_list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -19,3 +19,24 @@ def asd(self) -> int:
assert_type(combined, List[Union[Foo, Bar]])
for item in combined:
assert_type(item.asd(), int)

# ignoring this so we can test mypy and pyright separately
# pyright: reportUnnecessaryTypeIgnoreComment=false

# defining separately so that the value is not inferred at the usage
l_int = [1, 2]
l_str = ["a", "b"]
combined1 = l_int + l_str
assert_type(combined1, List[Union[int, str]])

combined2: list[Union[int, str]]
# mypy doesn't support this case
combined2 = l_int + l_str # type: ignore[operator]
assert_type(combined2, List[Union[int, str]])

combined2 = l_int + combined1
assert_type(combined2, List[Union[int, str]])

# mypy doesn't support this case
combined3: List[object] = l_int + l_str # type: ignore[operator]
assert_type(combined3, List[object])
6 changes: 4 additions & 2 deletions stdlib/builtins.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,7 @@ _T2 = TypeVar("_T2")
_T3 = TypeVar("_T3")
_T4 = TypeVar("_T4")
_T5 = TypeVar("_T5")
_Expected = TypeVar("_Expected")
_SupportsNextT_co = TypeVar("_SupportsNextT_co", bound=SupportsNext[Any], covariant=True)
_SupportsAnextT_co = TypeVar("_SupportsAnextT_co", bound=SupportsAnext[Any], covariant=True)
_AwaitableT = TypeVar("_AwaitableT", bound=Awaitable[Any])
Expand Down Expand Up @@ -1137,9 +1138,10 @@ class list(MutableSequence[_T]):
def __delitem__(self, key: SupportsIndex | slice, /) -> None: ...
# Overloading looks unnecessary, but is needed to work around complex mypy problems
@overload
def __add__(self, value: list[_T], /) -> list[_T]: ...
# # `__add__` returns a new object, so we capture the expected result type with a type variable
def __add__(self, value: list[_T], /) -> list[_Expected | _T]: ...
@overload
def __add__(self, value: list[_S], /) -> list[_S | _T]: ...
def __add__(self, value: list[_S], /) -> list[_Expected | _T | _S]: ...
def __iadd__(self, value: Iterable[_T], /) -> Self: ... # type: ignore[misc]
def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...
def __rmul__(self, value: SupportsIndex, /) -> list[_T]: ...
Expand Down