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#encoding=utf8
import threading

class Singleton(object):
    """
    ***Python实现单例模式(类似Tronado.IOLoop)***
    关键点：**双重判断，二层加锁**，各种语言实现方式都类似
    """
    _instance_lock = threading.Lock()

    @staticmethod
    def instance():
        if not hasattr(Singleton, '_instance'):
            with Singleton._instance_lock:
                if not hasattr(Singleton, '_instance'):
                    Singleton._instance = Singleton()
        return Singleton._instance