Type inference into function

Feature

I tried to search using some keywords and couldn't find anything obvious. But I was trying to do type validation of a class that I don't control (external sdk) and it has a class with optional fields. I'd like to write a function to validate that the instance I am passed as the correct field types, instead of doing it all in one.

Pitch

Ability to do type validation of objects you don't have control of in a subfunction, instead of in a main function.

import typing as t


class Test:
    opt: t.Optional[str]


def validate_test(t: Test) -> None:
    assert (t.opt)


def check_test_not_working() -> None:
    t = Test()
    validate_test(t)
    # Expect str, get Union[str, None]
    reveal_type(t.opt)


def check_test_working() -> None:
    t = Test()
    assert (t.opt)
    # Expect str, get str
    reveal_type(t.opt)

Start by reproducing the issue with the shown Test, validate_test, and reveal_type examples, then inspect how mypy propagates narrowing across function calls. Done means a supported approach lets the helper's assertion narrow t.opt at the caller while preserving existing Optional behavior.