An example of forward reference in mypy.

forward_ref.py

#!/usr/bin/env python
# coding: utf-8

from typing import Generic
from typing import Optional
from typing import TypeVar

T = TypeVar('T')

class Box(Generic[T]):
    def __init__(self, value: Optional[T]) -> None:
        self.__value = value

    # @classmethod
    # def empty(c) -> Box[T]: # NG in Python 3.5.2: NameError: name 'Box' is not defined 
    #     return c(None)

    @classmethod
    def empty(c) -> 'Box[T]': # OK: python/mypy #2020
        return c(None)

    # @classmethod
    # def empty(c) -> 'Box[T]':
    #     return 1 # NG in mypy: Incompatible return value type (got "int", expected Box[T])

    @property
    def value(self) -> Optional[T]:
        return self.__value


b = Box(10)

print(b.value)

Error when referencing a class within its init method · Issue #2020 · python/mypy python/mypy#2020