How do I correctly add type-hints to Mixin classes?

For reference, mypy recommends to implement mixins through a Protocol (documentation here).

It works with mypy >= 750.

from typing import Protocol


class HasValueProtocol(Protocol):
    @property
    def value(self) -> int: ...


class MultiplicationMixin:

    def multiply(self: HasValueProtocol, m: int) -> int:
        return self.value * m


class AdditionMixin:

    def add(self: HasValueProtocol, b: int) -> int:
        return self.value + b


class MyClass(MultiplicationMixin, AdditionMixin):

    def __init__(self, value: int) -> None:
        self.value = value

The Protocol base class is provided in the typing_extensions package for Python 2.7 and 3.4-3.7.

One approach I saw in this question is type hinting the self attribute. Together with Union from the typing package, you are able to use the attributes from a class which is used together with your mixin, while still having correct type hinting for own attributes:

from typing import Union

class AdditionMixin:

    def add(self: Union[MyBaseClass, 'AdditionMixin'], b: int) -> int:
        return self.value + b


class MyBaseClass:

    def __init__(self, value: int):
        self.value = value

Downside is that you have to add the hint to every method, which is kind of cumbersome.