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mypy thinks value is <nothing> when it isn't #15793
Description
Mypy believes that it is impossible for an isinstance to succeed if the known type has a generic and the subtype has specified that generic.
To Reproduce
from typing import Generic, TypeVar, cast
T = TypeVar("T")
class A(Generic[T]):
pass
class B(A[str]):
something: int = 0
def f(b: A[T]) -> None:
if isinstance(b, B):
print(b.something)Expected Behavior
Mypy should have no problems with this code
Actual Behavior
dumb_test.py:16: error: <nothing> has no attribute "something" [attr-defined]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.4.1. This bug reproduces at least back to 1.2.0
- Mypy command-line flags: None. Just run with
mypy FILE - Mypy configuration options from
mypy.ini(and other config files): - Python version used: 3.10.0
Activity
[-]mypy thinks value is `<nothing>` when it clearly isn't[/-][+]mypy thinks value is `<nothing>` when it isn't[/+]erictraut commentedon Aug 14, 2023
The code sample contains a bug. The
ffunction is using type variableTin an incorrect manner. A type variable scoped to a function should always appear in the return type annotation. In your example, the correct annotation for parameterbisT[Any]. If you make this change, the code type checks without error.It may be a good idea for mypy to emit an error when a type variable is used in an incorrect manner like this. There was a discussion about this in the typing forums a while back. If I remember correctly, the authors of pyright, pytype and pyre all decided to add such an error (or warning), but mypy does not emit an error in this case.
jack-tutor commentedon Aug 14, 2023
I don't think that is the issue here. Edited to return
T(T[Any]doesn't make sense as far as I understand).but this still fails:
Do not narrow types to Never with binder (python#18972)
Do not narrow types to Never with binder (#18972)