Return set of partitions with known leaders

        Arguments:
            topic (str): topic to check for partitions

        Returns:
            set: {partition (int), ...}
            None if topic not found.

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])