convert shutil._PathReturn to a type parameter?

[dpinol]

Since shutil._PathReturn is defined as Any, it may hide type errors to mypy-like tools. E.g in the following code, mypy will not detect that f function is invoked with an str argument instead of a Path.

import shutil
from pathlib import Path


def f(p: Path) -> None:
    pass


def cp(target: str | Path) -> None:
    p = shutil.copy("/src", target)
    return f(p)

[srittau]

Unfortunately, changing this to a union in the general case would mean that users would need to add isinstance checks to the returned value in many cases. That said, looking at the implementation of copy(), I think we could do better for some cases: For example, passing str or bytes as dst will always return the same type. We could reflect that and keep the Any return type only when passing in a PathLike.

The best solution would be to fix the functions in the standard library to return consistent types, but that's out of scope for typeshed.

[dpinol]

I understand, thanks