code: def port_add(self, dpid, port, mac):
          
          old_port = self.mac_to_port[dpid].get(mac, None)
          self.mac_to_port[dpid][mac] = port
  
          if old_port is not None and old_port != port:
              LOG.debug('port_add: 0x%016x 0x%04x %s',
                        dpid, port, haddr_to_str(mac))
  
          return old_port
docs: :returns: old port if learned. (this may be = port)
                    None otherwise