How to implement redis's pubsub timeout feature?

I want to use Redis's pubsub feature to implement comet, but pubsub doesn't have timeout, so if I use ps.listen(), it will block, even if client closes browser.

Greenlet has a timeout feature when spawn process, but I don't know how to combine them.

Flask's pseudo code:

@app.route('/')
def comet():
    rc = redis.Redis()
    ps = rc.pubsub()
    ps.subscribe('foo')
    for item in ps.listen():
        if item['type'] == 'message':
            return item['data']
    # ps.listen() will block, so how to make it timeout after 30 s?