import threading
class SingletonType(type):
    _instance_lock = threading.Lock()

    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, "_instance"):
            with SingletonType._instance_lock:
                if not hasattr(cls, "_instance"):
                    cls._instance = super(SingletonType, cls).__call__(*args, **kwargs)
        return cls._instance

    @classmethod
    def clear_instance(cls):
        if hasattr(cls, "_instance"):
            delattr(cls, "_instance")