Programming Python, Second Edition

Registering Callback Handler Objects

In examples thus far, C has been running and calling Python code from a standard main program flow of control. That’s not always the way programs work, though; in some cases, programs are modeled on an event-driven architecture where code is executed only in response to some sort of event. The event might be an end user clicking a button in a GUI, the operating system delivering a signal, or simply software running an action associated with an entry in a table.

In any event (pun accidental), program code in such an architecture is typically structured as callback handlers -- chunks of code dispatched by event-processing logic. It’s easy to use embedded Python code to implement callback handlers in such a system; in fact, the event-processing layer can simply use the embedded-call API tools we saw earlier in this chapter to run Python handlers.

The only new trick in this model is how to make the C layer know what code should be run for each event. Handlers must somehow be registered to C to associate them with future events. In general, there is a wide variety of ways to achieve this code/event association; for instance, C programs can:

• Fetch and call functions by event name from one or more module files

• Fetch and run code strings associated with event names in a database

• Extract and run code associated with event tags in HTML or XML^[156]

• Run Python code that calls back to C to tell it what should be run

And so on. Really, any place you can ...