code: def list_by_cid(self):
          
          prs = self._processors
          return sorted(((cid, [prs[cid]]) for cid in sorted(prs.keys())),
                        key=operator.itemgetter(0))
docs: :return:
              A list of :class:`Processor` or its children classes grouped by
              each cid, [(cid, [:class:`Processor`)]]