Type safe returns

returns.py

import random
from typing import Generic
from typing import TypeVar

from typing_extensions import reveal_type

V = TypeVar('V', covariant=True)
E = TypeVar('E', covariant=True)


class Success(Generic[V]):
    def __init__(self, value: V):
        self.value = value


class Failure(Generic[E]):
    def __init__(self, error: E):
        self.error = error


Result = Success[V] | Failure[E]


def some_func(number: int) -> Result[int, str]:
    value = random.randint(0, number)
    if value == number:
        return Failure('no luck')
    return Success(value)


def do_stuff() -> None:
    for i in range(5):
        result = some_func(i)
        reveal_type(result)
        # Revealed type is "Union[Success[int], Failure[str]]"

        if isinstance(result, Failure):
            reveal_type(result)
            # Revealed type is "Failure[str]"
            print(f'{i} => failure, {result.error}')
        else:
            reveal_type(result)
            # Revealed type is "Success[int]"
            print(f'{i} => success, {result.value}')


if __name__ == '__main__':
    do_stuff()