from threading import Lock, Thread


class SingletonMeta(type):
    """
    This is a thread-safe implementation of Singleton.
    """

    _instances = {}

    # We now have a lock object that will be used to synchronize threads during
    # first access to the Singleton.
    _lock = Lock()

    def __call__(cls, *args, **kwargs):
        """
        Possible changes to the value of the `__init__` argument do not affect
        the returned instance.
        """
        # Now, imagine that the program has just been launched. Since there's
        # no Singleton instance yet, multiple threads can simultaneously pass
        # the previous conditional and reach this point almost at the same time
        # The first of them will acquire lock and will proceed further, while
        # the rest will wait here.
        with cls._lock:
            # The first thread to acquire the lock, reaches this conditional,
            # goes inside and creates the Singleton instance. Once it leaves
            # the lock block, a thread that might have been waiting for the
            # lock release may then enter this section. But since the Singleton
            # field is already initialized the thread won't create a new object
            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
    """
    We'll use this property to prove that our Singleton really works.
    """

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

    def some_business_logic(self):
        """
        Finally, any singleton should define some business logic, which can be
        executed on its instance.
        """


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=("FOO",))
    process2 = Thread(target=test_singleton, args=("BAR",))
    process1.start()
    process2.start()