code: def target_key(self, key):
          
          router = self.connection_pool.cluster.get_router()
          host_id = router.get_host_for_key(key)
          rv = self.target([host_id])
          rv.__resolve_singular_result = True
          return rv
docs: Temporarily retarget the client for one call to route
          specifically to the one host that the given key routes to.  In
          that case the result on the promise is just the one host's value
          instead of a dictionary.
  
          .. versionadded:: 1.3