How to type the __new__ method in a Python metaclass so that mypy is happy

I am trying to type the __new__ method in a metaclass in Python so that it pleases mypy. The code would be something like this (taken from pep-3115 - "Metaclasses in Python 3000" and stripped down a bit):

from __future__ import annotations

from typing import Type


# The metaclass
class MetaClass(type):

    # The metaclass invocation
    def __new__(cls: Type[type], name: str, bases: tuple, classdict: dict) -> type:
        result = type.__new__(cls, name, bases, classdict)
        print('in __new__')
        return result


class MyClass(metaclass=MetaClass):
    pass

With this, mypy complains, Incompatible return type for "__new__" (returns "type", but must return a subtype of "MetaClass"), pointing at the line def __new__.

I have also tried with:

def __new__(cls: Type[MetaClass], name: str, bases: tuple, classdict: dict) -> MetaClass:

Then mypy complains (about the return result line): Incompatible return value type (got "type", expected "MetaClass").

I have also tried with a type var (TSubMetaclass = TypeVar('TSubMetaclass', bound='MetaClass')) and the result is the same as using MetaClass.

Using super().__new__ instead of type.__new__ gave similar results.

What would be the correct way to do it?

First, the return type is MetaClass, not type. Second, you need to explicitly cast the return value, since type.__new__ doesn't know it is returning an instance of MetaClass. (Its specific return type is determined by its first argument, which isn't known statically.)

from __future__ import annotations

from typing import Type, cast


# The metaclass
class MetaClass(type):

    # The metaclass invocation
    def __new__(cls: Type[type], name: str, bases: tuple, classdict: dict) -> MetaClass:
        result = type.__new__(cls, name, bases, classdict)
        print('in __new__')
        return cast(MetaClass, result)


class MyClass(metaclass=MetaClass):
    pass

To use super, you need to adjust the static type of the cls parameter.

class MetaClass(type):

    # The metaclass invocation
    def __new__(cls: Type[MetaClass], name: str, bases: tuple, classdict: dict) -> MetaClass:
        result = super().__new__(name, bases, classdict)
        print('in __new__')
        return cast(MetaClass, result)