singleton_base.py

from threading import Lock, Thread


class SingletonMeta(type):
    _instances = {}
    _lock: Lock = Lock()

    def __call__(cls, *args, **kwargs):
        with cls._lock:
            if cls not in cls._instances:
                instance = super().__call__(*args, **kwargs)
                cls._instances[cls] = instance
        return cls._instances[cls]


class Singleton(metaclass=SingletonMeta):
    value: str = None

    def __init__(self, value: str) -> None:
        self.value = value

    def some_business_logic(self) -> None:
        pass


def test_singleton(value: str) -> None:
    singleton = Singleton(value)
    print(singleton.value)


if __name__ == "__main__":
    print("If you see the same value, then singleton was reused (yay!)\n"
        "If you see different values, "
        "then 2 singletons were created (booo!!)\n\n"
        "RESULT:\n")

    process1 = Thread(target=test_singleton, args=("Singleton 1",))
    process2 = Thread(target=test_singleton, args=("Singleton 2",))
    process1.start()
    process2.start()