[Python] Singleton with once initialization

singleton.py

class Singleton(object):
    _instance_lock = threading.Lock()
    __instance = None

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            with Singleton._instance_lock:
                cls.__instance = super(
                    Singleton, cls).__new__(cls)
        else:
            def init_pass(self): pass
            cls.__init__ = init_pass

        return cls.__instance