I'm using Redis as a simple pubsub broker, managed by the redis-py library, using just the default 'main' channel. Is there a technique, in either Redis itself or the wrapping Python library to count the number of messages in this queue? I don't have deeper conceptual knowledge of Redis (in particular how it implements broker functionality) so am not sure if such a question makes sense
Exact counts, lock avoidance etc. is not necessary; I only need to check periodically (on the order of minutes) whether this queue is empty
Redis Pub/Sub doesn't hold any internal queues of messages see - https://redis.io/topics/pubsub.
If you need a more queue based publish mechanism you might to check Redis Streams. Redis Streams provides two methods that might help you XLEN and XINFO.