import threading
import time

class Singleton(object):
    _instance_lock = threading.Lock()
    def __init__(self):
        time.sleep(1)

    @classmethod
    def instance(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    print ("Init Singleton")
                    Singleton._instance = Singleton(*args, **kwargs)
        return Singleton._instance


class MySingleton(Singleton):
    def __init__(self):
        import time
        time.sleep(1)
        print ("MySingleton")


def task(arg):
    obj = Singleton.instance()
    print(obj)

for i in range(20):
    t = threading.Thread(target=task, args=[i,])
    t.start()

time.sleep(20)
obj = Singleton.instance()
print(obj)