code: def get_outputs(self):
          
          end_p = [p for p in self.postorder() if not self.incoming_edges(p)]
          self.log.debug('%s got output pipers %s' % (repr(self), end_p))
          return end_p
docs: Returns ``Piper`` instances, which are outputs to the pipeline i.e. have
          no outgoing pipes (incoming dependency edges).