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isinstance erases type info #12949
Description
Bug Report
Accessing a variable of a generic type after an if isinstance() against a (non-generic) subclass of said type changes the variable's type to <nothing> within the isinstance clause scope:
from typing import List, TypeVar
class IntList(List[int]):
pass
T = TypeVar("T")
def f(s: List[T]) -> T:
reveal_type(s)
if isinstance(s, IntList):
reveal_type(s)
s.pop()
reveal_type(s)
return s.pop()Expected Behavior
No error.
Actual Behavior
note: Revealed type is "builtins.list[T`-1]" note: Revealed type is "<nothing>" error: <nothing> has no attribute "pop" [attr-defined] note: Revealed type is "builtins.list[T`-1]"
Your Environment
- Mypy version used: 0.960
Activity
erictraut commented on Aug 13, 2023
This is a duplicate of #8252.
This bug report is a simpler demonstration of the bug, so it might be worth keeping this one open and closing the other.
ichard26 commented on Aug 22, 2023
#8252, which I just closed, contains some more examples of this issue.
erictraut commented on Aug 22, 2023
I think this might also be the same as #15793.
Do not narrow types to Never with binder (python#18972)
Do not narrow types to Never with binder (#18972)