python单例模式的实现

有些情况下我们需要单例模式来减少程序资源的浪费，在python语言中单例模式的实现同样是方便的。

我现在以tornado框架中IOLoop类单例模式的实现来举例，有兴趣的可以自己看一下源码

 1 class IOLoop(Configurable):
 2 ……
 3
 4     @staticmethod
 5     def instance():
 6         """Returns a global `IOLoop` instance.
 7
 8         Most applications have a single, global `IOLoop` running on the
 9         main thread.  Use this method to get this instance from
10         another thread.  To get the current thread‘s `IOLoop`, use `current()`.
11         """
12         if not hasattr(IOLoop, "_instance"):
13             with IOLoop._instance_lock:
14                 if not hasattr(IOLoop, "_instance"):
15                     # New instance after double check
16                     IOLoop._instance = IOLoop()
17         return IOLoop._instance
18
19     @staticmethod
20     def initialized():
21         """Returns true if the singleton instance has been created."""
22         return hasattr(IOLoop, "_instance")
23
24     def install(self):
25         """Installs this `IOLoop` object as the singleton instance.
26
27         This is normally not necessary as `instance()` will create
28         an `IOLoop` on demand, but you may want to call `install` to use
29         a custom subclass of `IOLoop`.
30         """
31         assert not IOLoop.initialized()
32         IOLoop._instance = self
33
34     @staticmethod
35     def clear_instance():
36         """Clear the global `IOLoop` instance.
37
38         .. versionadded:: 4.0
39         """
40         if hasattr(IOLoop, "_instance"):
41             del IOLoop._instance

其中实现单例模式的代码是

if not hasattr(IOLoop, "_instance"):
            with IOLoop._instance_lock:
                if not hasattr(IOLoop, "_instance"):
                    # New instance after double check
                    IOLoop._instance = IOLoop()
        return IOLoop._instance

而保证单例模式调用的方法也就很简单了：

tornado.ioloop.IOLoop.instance().start()

上面贴的源码类中其余的几个方法是和单例模式配套使用的，他们一起实现了单例的功能