Would like to get this to work with MyPy

my_classes_example.py

from typing import Generic, TypeVar

L = TypeVar('L')
R = TypeVar('R')

class Either(Generic[L, R]):
    pass

class Left:
    def __init__(self, value):
        self.value = value

    def is_left(self) -> bool:
        return True

    def is_right(self) -> bool:
        return False

class Right:
    def __init__(self, value):
        self.value = value

    def is_left(self) -> bool:
        return False

    def is_right(self) -> bool:
        return True

def either_or() -> Either[int, str]:
    """ Some fn that returns one of two types of data.                                                              
                                                                                                                    
    Would like MyPy to say this is a valid return type if                                                           
    and only if the inner type matches.                                                                             
                                                                                                                    
    Example:                                                                                                        
        return Left(10) # ok                                                                                        
        return Right("string") # ok                                                                                 
        return Left("string") # invalid                                                                             
        return Right(10) # invalid                               
        return "string" # invalid
    """
    return Right("Hello!")

if __name__ == "__main__":
    unknown = either_or()

    if unknown.is_right():
        print("Found a string: {}".format(unknown.value))