new_contents: 
  class Event(object):
      def __init__(self, **kwargs):
          for k, v in kwargs.iteritems():
              setattr(self, k, v)
  
  
  class Observable(object):
      def __init__(self):
          self.callbacks = set()
  
      def subscribe(self, callback):
          self.callbacks.add(callback)
          
      def unsubscribe(self, callback):
          self.callbacks.discard(callback)
  
      def fire(self, **attrs):
          e = Event(source=self, **attrs)
          for fn in self.callbacks:
              fn(e)
  
subject: Add observer pattern