def subscribe(self, observer):
        """Subscribe an observer to this subject and return a subscription id

        """
        sid = self._sn
        self.observers[sid] = observer
        self._sn += 1
        return SubscribeID(self, sid)
def subscribe ( self , observer ) : sid = self . _sn self . observers [ sid ] = observer self . _sn += 1 return SubscribeID ( self , sid )
Subscribe an observer to this subject and return a subscription id