APIs related to `importlib.abc.PathEntryFinder` should use protocol instead

[abravalheri]

Although importlib.abc.PathEntryFinder is defined as an abstract class in the standard library, path entry finders are intentionally loosely defined in the Python docs (and originated PEPs), so that the only thing that needs to be implemented is one method (find_spec). This matches the functioning of a protocol.

The reason why the abstract class is defined is probably informational (for documentation purposes) + extra code re-use internally in the stdlib. This situation is similar to importlib.abc.MetaPathFinder, and other importlib.abc classes (like loaders).

This affects

typeshed/stdlib/sys/__init__.pyi

Line 54 in af84d2f

path_hooks: list[Callable[[str], PathEntryFinder]]

and probably other places.

For example, I would expect the following snippet to type check with no problems:

import sys
from types import ModuleType
from importlib.machinery import ModuleSpec
from typing_extensions import Self


class Finder:
    @classmethod
    def path_hook(cls, path_entry: str) -> type[Self]:
        return cls  # simplified mock for demonstration purposes only

    @classmethod
    def find_spec(cls, fullname: str, target: ModuleType | None = None)  -> ModuleSpec | None:
        return None  # simplified mock for demonstration purposes only
    
        
sys.path_hooks.append(Finder.path_hook)

But instead I am having an error:

main.py:9: error: Missing return statement  [empty-body]
main.py:17: error: Argument 1 to "append" of "list" has incompatible type "Callable[[str], type[Finder]]"; expected "Callable[[str], PathEntryFinder]"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

https://mypy-play.net/?mypy=latest&python=3.12&gist=dfd3a0dcf277afc43cf59c8eb07afc5b