在开发过程中，时常需要某些对象在软件整个运行生命周期只需存在一个副本，比如服务器加载的参数配置等等，有种现成的设计模式专干这种活---单例模式，能很好的适用这种需求场景。而由于Python动态语言的灵活性，相对于其它编程语言的实现尤为优雅、简洁、方便，实现代码如下:
class Application(object):
     def __init__(self):
         self.args = "Hello world"

     @classmethod
     def instance(cls):
         if not hasattr(cls, "_instance"):
             cls._instance = cls()
         return cls._instance
以下是多线程测试代码：
class Work(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)
        self.start()

    def run(self):
        app = Application.instance()
        print threading.current_thread(), id(app)
        time.sleep(.1)

if __name__ == "__main__":
     for i in range(10):
         Work()
测试结果如下：
<Work(Thread-1, started -1218360464)> 3076741900
<Work(Thread-2, started -1226753168)> 3076741900
<Work(Thread-3, started -1235145872)> 3076741900
<Work(Thread-4, started -1243538576)> 3076741900
<Work(Thread-5, started -1251931280)> 3076741900
<Work(Thread-6, started -1260323984)> 3076741900
<Work(Thread-7, started -1268716688)> 3076741900
<Work(Thread-8, started -1277109392)> 3076741900
<Work(Thread-9, started -1285502096)> 3076741900
<Work(Thread-10, started -1293894800)> 3076741900