code: def _select_nonblock(sockets, remain=None):
      
      # pcap sockets aren't selectable, so we return all of them
      # and ask the selecting functions to use nonblock_recv instead of recv
      def _sleep_nonblock_recv(self):
          try:
              res = self.nonblock_recv()
              if res is None:
                  time.sleep(conf.recv_poll_rate)
              return res
          except TimeoutElapsed:
              return None
      return sockets, _sleep_nonblock_recv
docs: This function is called during sendrecv() routine to select
      the available sockets.