一句话概括：其实核心就是用了回调机制。

1 class PyBus (object): 2  3     def __init__(self,): 4         self.clear() 5          6     def clear(self): 7         self.subscriptions = {} 8      9     def subscribe(self, subject, owner, func):10         if not self.subscriptions.has_key(owner):11             self.subscriptions[owner] = {}12         self.subscriptions[owner][subject] = func13         14     def has_subscription(self, owner, subject):15         return self.subscriptions.has_key(owner) and self.subscriptions[owner].has_key(subject)16         17     def publish(self, subject, *args, **kwargs):18         for owner in self.subscriptions.keys():19             if self.has_subscription(owner, subject):20                 self.subscriptions[owner][subject](*args, **kwargs)