tornado singleton单例模式

class TOLoop:
    @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"):  //避免无意义的锁定， 因为大部分调用时只是简单的返回已经创建的_instance
            with IOLoop._instance_lock:
                if not hasattr(IOLoop, "_instance"):
                    # New instance after double check
                    IOLoop._instance = IOLoop()
        return IOLoop._instance

Borg is good for SUBCLASS

class Borg:  __shared_state = {}  # init internal state variables here  __register = {}  def __init__(self):    self.__dict__ = self.__shared_state    if not self.__register:      self._init_default_register()

for example 

class SubBorg(Borg):

pass