Relaxing usage of `type` with `typing.Protocol` in typing spec

Some time ago I opened a thread on the Python discourse about implementing an AbstractType semantic that would allow functions to receive non-instantiable types so to try and conform a bit the situation presented in the the thread.

Consider the snippet:

from abc import abstractmethod
from typing import runtime_checkable, Protocol, TypeVar, TypeGuard

@runtime_checkable
class MyProtocol(Protocol):
    @abstractmethod
    def my_method(self) -> str:
        ...

class MyClass:
    def my_method(self) -> str:
        return "Hello from MyClass!"
    
class MyOtherClass:
    def another_method(self) -> str:
        return "Hello from MyOtherClass!"
    

P = TypeVar('P', bound=MyProtocol)
    
def check_protocol(obj: object, protocol: type[P]) -> TypeGuard[P]:
    return isinstance(obj, protocol)

# Example usage
my_obj = MyClass()
my_other_obj = MyOtherClass()
if check_protocol(my_obj, MyProtocol):
    print(my_obj.my_method())
if check_protocol(my_other_obj, MyProtocol):
    print(my_other_obj.my_method())
else:
    print("Object does not implement MyProtocol.")

Output:

Hello from MyClass!
my_obj does not implement MyOtherClass protocol.

At runtime, this works as intended. At type checking time, mypy complains that check_protocol cannot accept a non-instantiable type, while pyright and ty are accepting this behavior.

According to the typing specification, mypy is technically correct (the best kind of correct), but this code is perfectly acceptable. There are other examples referenced in python/mypy#4717 (which is the 3rd most upvoted issue by 👍 by the way).

I was hoping for more discussion points presented in the thread but not many replied. I didn't want to monopolize it, so I'm trying my luck here.

@JelleZijlstra (sorry for the ping) did point out that it might be worthwhile to change the spec to allow this behavior but I'm guessing there should be some discussion on how to deal with it first. Jelle suggested: "If you want it to be instantiable, use Callable[] instead."

I do agree it makes sense, but this snippet:

from typing import Protocol

class MyProtocol(Protocol):
    def method(self) -> str:
        ...
print(callable(MyProtocol)) # prints True

confuses me.

So my point is: could the spec allow for type to accept non-instantiable types, since it seems to be a pretty common pattern? Is there a way to effectively deal with the separation between instantiable and non-instantiable types?

I tried adding the "typing spec" label but couldn't. At any rate, thank you for the consideration.

Inizia dalla discussione sulla specifica di typing e dal thread di Python Discourse collegato, quindi confronta il comportamento descritto per mypy, pyright e ty. Esamina mypy#4717 per esempi correlati. Il lavoro è completato quando si raggiunge un accordo sul fatto che type[P] possa accettare tipi Protocol non istanziabili e si documenta la regola risultante.