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

class IOLoop(Configurable):
……

    @staticmethod
    def instance():
        """Returns a global `IOLoop` instance.

        Most applications have a single, global `IOLoop` running on the
        main thread.  Use this method to get this instance from
        another thread.  To get the current thread's `IOLoop`, use `current()`.
        """
        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

    @staticmethod
    def initialized():
        """Returns true if the singleton instance has been created."""
        return hasattr(IOLoop, "_instance")

    def install(self):
        """Installs this `IOLoop` object as the singleton instance.

        This is normally not necessary as `instance()` will create
        an `IOLoop` on demand, but you may want to call `install` to use
        a custom subclass of `IOLoop`.
        """
        assert not IOLoop.initialized()
        IOLoop._instance = self

    @staticmethod
    def clear_instance():
        """Clear the global `IOLoop` instance.

        .. versionadded:: 4.0
        """
        if hasattr(IOLoop, "_instance"):
            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