code: def _on_del_route(self, msg):
          
          LOG.error('%r._on_del_route() %r', self, msg)
          if msg.is_dead:
              return
  
          target_id_s, _, name = bytes_partition(msg.data, b(':'))
          target_id = int(target_id_s, 10)
          context = self._context_by_id.get(target_id)
          if context:
              fire(context, 'disconnect')
          else:
              LOG.debug('DEL_ROUTE for unknown ID %r: %r', target_id, msg)
docs: Stub :data:`DEL_ROUTE` handler; fires 'disconnect' events on the
          corresponding :attr:`_context_by_id` member. This is replaced by
          :class:`mitogen.parent.RouteMonitor` in an upgraded context.