docs: def highwater(self, partition):
          """Last known highwater offset for a partition.
  
          A highwater offset is the offset that will be assigned to the next
          message that is produced. It may be useful for calculating lag, by
          comparing with the reported position. Note that both position and
          highwater refer to the *next* offset -- i.e., highwater offset is
          one greater than the newest available message.
  
          Highwater offsets are returned in FetchResponse messages, so will
          not be available if no FetchRequests have been sent for this partition
          yet.
  
          Arguments:
              partition (TopicPartition): Partition to check
  
          Returns:
              int or None: Offset if available
          """
          if not isinstance(partition, TopicPartition):
              raise TypeError('partition must be a TopicPartition namedtuple')
          assert self._subscription.is_assigned(partition), 'Partition is not assigned'
          return self._subscription.assignment[partition].highwater