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Default values for generic attributes are unsafe #5703
Description
Currently mypy accepts this unsafe code:
from typing import Generic, TypeVar, List
T = TypeVar('T')
class B(Generic[T]):
x: List[T] = []
class C1(B[int]):
pass
class C2(B[str]):
pass
C1().x.append(42)
C2().x.append('Hm...')A potential solution would be to disallow all default values for generic attributes except None. This is however a low priority, since it is a false negative, and a rare corner case.
Activity
andersk commentedon Jul 2, 2022
Same issue as #11538.
We don’t need to go so far as disallowing all defaults other than
None. Any default value is fine as long as the default value has a type that can be written withoutT. For example:x: Sequence[T] = []is fine because[]has typeSequence[Never], which can be written withoutT, but is convertible toSequence[T]by covariance. That is, a single[]may simultaneously have typeSequence[T]for allT.x: List[T] = []must be disallowed; no single[]may simultaneously have typeList[T]for allT.