docs: def available_partitions_for_topic(self, topic):
          """Return set of partitions with known leaders
  
          Arguments:
              topic (str): topic to check for partitions
  
          Returns:
              set: {partition (int), ...}
              None if topic not found.
          """
          if topic not in self._partitions:
              return None
          return set([partition for partition, metadata
                                in six.iteritems(self._partitions[topic])
                                if metadata.leader != -1])