1.py

from typing import Any, Dict


class Singleton(type):
    _instances: Dict[type, "Singleton"] = {}

    def instance(cls: Any, *args: Any, **kwargs: Any) -> Any:
        return cls(*args, **kwargs)

    def __call__(cls, *args: Any, **kwargs: Any) -> Any:
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class FooSingleton(metaclass=Singleton):
    def __init__(self, num) -> None:
        self.num = num

    def __enter__(self):
        return FooSingleton.instance()

    def __exit__(self, exc_type, exc_val, exc_tb):
        FooSingleton.clear()

    def add(self, x):
        return self.num + x

    @classmethod
    def clear(cls):
        del Singleton._instances[FooSingleton]

    @staticmethod
    def instance(*args: Any, **kwargs: Any) -> "FooSingleton":
        return Singleton.instance(FooSingleton, *args, **kwargs)  # type: ignore


if __name__ == "__main__":

    print("== context call:")
    with FooSingleton.instance(20) as s:
        print(s.add(5))

    print("== standalone call:")
    s = FooSingleton.instance(10)
    print(s.add(5))

== context call:
25
== standalone call:
15