Andy Piper Messages: 15 Registered: November 2011 Location: Kingston upon Thames, UK
This would tend to be something that a broker would maintain, but I don't know of any brokers with an API that would enable you to obtain such a list.
In many ways the publish/subscribe model is aimed at completely decoupling publishers and subscribers - you don't know whether your publication is passed on to 0, 1 or many subscribers, and it "should not" matter. On the subscriber side, you make the choice which topics to listen to.
MQTT doesn't provide any method to obtain this and therefore Paho does not implement it. Per Nick's suggestion you could maintain a list or modify the client. It is probably simplest just to remember which topics you subscribe to as you go.