code: def get_unavailable_brokers(zk, partition_metadata):
      
      topic_data = zk.get_topics(partition_metadata.topic)
      topic = partition_metadata.topic
      partition = partition_metadata.partition
      expected_replicas = set(topic_data[topic]['partitions'][str(partition)]['replicas'])
      available_replicas = set(partition_metadata.replicas)
      return expected_replicas - available_replicas
docs: Returns the set of unavailable brokers from the difference of replica
      set of given partition to the set of available replicas.