import threading

lock = threading.Lock()


class Singleton(type):
    _instances = {}

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