PyPubSub provides a publish - subscribe API that facilitates the development of
event-based / message-based applications. PyPubSub supports sending and
receiving messages between objects of an application. It is centered on the
notion of a topic; senders publish messages of a given topic, and listeners
subscribe to messages of a given topic. The package also supports a variety of
advanced features that facilitate debugging and maintaining pypubsub topics and
messages in larger applications.