docs: def my_pick_non_system_keyspace(self):
          """
          Find a keyspace in the cluster which is not 'system', for the purpose
          of getting a valid ring view. Can't use 'system' or null.
          """
          d = self.my_describe_keyspaces()
  
          def pick_non_system(klist):
              for k in klist:
                  if k.name not in SYSTEM_KEYSPACES:
                      return k.name
              err = NoKeyspacesAvailable("Can't gather information about the "
                                         "Cassandra ring; no non-system "
                                         "keyspaces available")
              warn(err)
              raise err
          d.addCallback(pick_non_system)
          return d