code: def keep_alive(self):
          
          it = (side.keep_alive for (_, (side, _)) in self.poller.readers)
          return sum(it, 0)
docs: Return :data:`True` if any reader's :attr:`Side.keep_alive` attribute
          is :data:`True`, or any :class:`Context` is still registered that is
          not the master. Used to delay shutdown while some important work is in
          progress (e.g. log draining).