Never and NoReturn Annotations Python examples

step_0.py

def get_num() -> int:
    return 42


def get_word() -> str:
    return "spam"


def main() -> None:
    num = get_num()
    print(num**2)
    word = get_word()
    print(word.title())


if __name__ == "__main__":
    main()

step_1.py

from typing import NoReturn


def abort(reason: str) -> NoReturn:
    raise ValueError(f"Reason: {reason}")


# def main() -> None:
#     print("Hello World!")
#     res: str = abort("Some error")
#     print(res.title())
#     print("Bye!")


def main() -> None:
    print("Hello World!")
    abort("Some error")
    print("Bye!")


if __name__ == "__main__":
    main()

step_2.py

from typing import Never, NoReturn, assert_never
from enum import StrEnum, auto


class TrafficLight(StrEnum):
    red = auto()
    yellow = auto()
    green = auto()


def log_unexpected_action(action: Never) -> None:
    print("Unexpected action", action)


def raise_on_unexpected_action(action: Never) -> NoReturn:
    raise ValueError(f"Unexpected action: {action}")


def take_action(light: TrafficLight) -> None:
    if light is TrafficLight.red:
        print("Please wait.")
    elif light is TrafficLight.green:
        print("You can go")
    elif light is TrafficLight.yellow:
        print("Be extra cautious!")
    else:
        # log_unexpected_action(light)
        # raise_on_unexpected_action(light)
        assert_never(light)


def main() -> None:
    for light in TrafficLight:
        print("+ Light is", light)
        take_action(light)


if __name__ == "__main__":
    main()

step_3.py

from typing import assert_never


def check_count(count: int) -> None:
    if count > 0:
        print("The count is greater than zero")
    elif count == 0:
        print("The count is equal to zero")
    elif count < 0:
        print("The count is less than zero")
    elif isinstance(count, int):
        print("The count is an integer, this code is never reachable")
    else:
        assert_never(count)


def check_count_via_match(count: int) -> None:
    match count:
        case c if c > 0:
            print("The count is greater than zero")
        case c if c == 0:
            print("The count is equal to zero")
        case c if c < 0:
            print("The count is less than zero")
        case int():
            print("The count is int, but this is never reachable")
        case c:
            assert_never(c)


def main() -> None:
    check_count_via_match(1)
    check_count_via_match(0)
    check_count_via_match(-1)


if __name__ == "__main__":
    main()