enhance the comprehension of the `__new__` method of a metaclass

[DylannCordel]

Feature

The __new__ method of a Metaclass should be hinted as returning a specific Type.

Pitch

class MyPool:
    registry: Dict[str, Type[MyAbstractClass]] = {}
    
    @classmethod
    def register(cls, the_class):
        cls.registry[the_class.__name__] = the_class

class MyMetaClass(type):
    def __new__(cls, name, bases, attrs) -> Type[MyAbstractClass]:
        if not bases:
            return super().__new__(name, bases, attrs)
        new_class = super().__new__(name, bases, attrs)
        # do something with new_class. in my case:
        MyPool.register(new_class)
        return new_class

class MyAbstractClass(metaclass=MyMetaClass):
    pass

class MyRealClass(MyAbstractClass):
    pass

Currently, mypy will say: "__new__" must return a class instance (got "Type[MyAbstractClass]")

[sobolevn]

Great catch!

Minimal example:

from typing import Type

class MyMetaClass(type):
    def __new__(cls, name, bases, attrs) -> Type['MyClass']:
        pass

class MyClass(metaclass=MyMetaClass):
    pass

Error:

out/ex.py:4: error: "__new__" must return a class instance (got "Type[MyClass]")

PR is on its way.