code: def add_callback(self, method):
          
          from_actor = get_current()
          if from_actor is not None:
              callback = (method, from_actor.channel, from_actor.url)
              with self.__condition:
                  if self.__state is not FINISHED:
                      self.__callbacks.append(callback)
                      return
              # Invoke the callback directly
              # msg = TellRequest(TELL, method, [self], from_actor.url)
              msg = {TYPE: TELL, METHOD: method, PARAMS: ([self], {}),
                     TO: from_actor.url}
              from_actor.channel.send(msg)
          else:
              raise FutureError("add_callback only works when called " +
                                "from inside an actor")
docs: Attaches a mehtod that will be called when the future finishes.
  
          :param method: A callable from an actor that will be called
              when the future completes. The only argument for that
              method must be the future itself from wich you can get the
              result though `future.:meth:`result()``. If the future has
              already completed, then the callable will be called
              immediately.
  
          .. note:: This functionallity only works when called from an actor,
              specifying a method from the same actor.