I agree that a [list] would be a more pythonic API.

From what I can tell, the KazooClient uses the comma-separated connect string because that's the java zookeeper API  (see connectString in http://zookeeper.apache.org/doc/r3.2.2/api/org/apache/zookeeper/ZooKeeper.html)

The current kafka java api also uses a comma-separated string for bootstrapping (metadata.broker.list in http://kafka.apache.org/documentation.html) and it does not appear that that will be changing w/ new Producer API (http://empathybox.com/kafka-javadoc/index.html?kafka/clients/producer/KafkaProducer.html)

How about supporting both w/ a little polymorphism?