sentinels.py

from typing import Type, TypeVar


class Sentinel:
    def __new__(cls):
        instance = super().__new__(cls)
        cls.__new__ = lambda _: instance
        return instance


S = TypeVar("S", bound=Sentinel)


def create_sentinel(name: str) -> tuple[Type[S], S]:
    sentinel_type = type(name, (Sentinel,), {})
    return sentinel_type, sentinel_type()


DemoSentinel, demo_sentinel = create_sentinel("DemoSentinel")


def example(value: int | DemoSentinel = demo_sentinel):
    if value is demo_sentinel:
        print("Sentinel set")

    else:
        print("Value", value)


example(10)
example(DemoSentinel())
example()