mypy thinks value is <nothing> when it isn't

[zevbo]

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

[erictraut]

The code sample contains a bug. The f function is using type variable T in 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 parameter b is T[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]

I don't think that is the issue here. Edited to return T (T[Any] doesn't make sense as far as I understand).

from __future__ import annotations

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]) -> T:
    if isinstance(b, B):
        print(b.something)
    raise NotImplementedError()

but this still fails:

error: <nothing> has no attribute "something"  [attr-defined]