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fix list.copy to use the expected type #14755

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fix list.copy to use the expected type #14755

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5 changes: 5 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 +19,8 @@ def asd(self) -> int:
assert_type(combined, List[Union[Foo, Bar]])
for item in combined:
assert_type(item.asd(), int)

l1: list[int] = [1]
l2: list[object] = l1.copy()
# this is an error, because a list of ints can't be a list of strs
l3: list[str] = l1.copy() # type: ignore
3 changes: 2 additions & 1 deletion stdlib/builtins.pyi
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Expand Up @@ -1104,7 +1104,8 @@ class list(MutableSequence[_T]):
def __init__(self) -> None: ...
@overload
def __init__(self, iterable: Iterable[_T], /) -> None: ...
def copy(self) -> list[_T]: ...
# `copy` returns a new object, so capture the expected return type here using a type var
def copy(self) -> list[_S | _T]: ...
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_S has no meaning, as it's only used once in this context. It's just interpreted as Any in this context.

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@srittau srittau Sep 21, 2025 •
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I see what you're trying to do, but we should probably use:

Suggested change
def copy(self) -> list[_S | _T]: ...
# The new list can allow broader types that the old one. `Any` allows that.
def copy(self) -> list[_T | Any]: ...

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_S has no meaning, as it's only used once in this context. It's just interpreted as Any in this context.

that is demonstrably not true

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basedpyright is incorrect there. A[int | Any] should be assignable to A[object].

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@KotlinIsland KotlinIsland Sep 21, 2025 •
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consider this example:

def empty_list[T=Never]() -> list[T]:
    return []

a = empty_list()
b: list[int] = empty_list()

if we were to use Any here, then the type of a would be list[Any]

additionally, there would be absolutely no safety within empty_list to return the correct type. i'm not really sure where this idea that type variables only in return positions being invalid came from

this is literally the meaning of a type variable, to use Any is a mistake

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basedpyright is incorrect there. A[int | Any] should be assignable to A[object].

yes: microsoft/pyright#10713 (comment)

i was a little hasty with the first example, i think it's clear though that this case is the intended use of a type variable, not Any

def append(self, object: _T, /) -> None: ...
def extend(self, iterable: Iterable[_T], /) -> None: ...
def pop(self, index: SupportsIndex = -1, /) -> _T: ...
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