Dec 2016: pypubsub 4.0.0 released!
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.
Install most recent stable with "pip install pypubsub".