plugin system?

[Jeff Epler]

Well, I'd make each plugin be a class (instance).  You would actually name
the plugin with a dotted name, modname.classname (in configuration files,
or what have you).

For instance:

    [Plugins]
    PlugIn1 = spam_plugins.VikingChorus

    # in file spam_plugins.py
    class VikingChorus:
        def __init__(self, app):
            self.app = app
        def sing_refrain(self, text):
            self.app.display(", ".join(text * 4) + "!")
            raise StopPropagating

In the code, you would instantiate the class with a helper function:
    def get_plugin_factory(name):
        # If you have packages (eg monty_python.spam.VikingChorus
        # or spam.spam.spam.spam.baked-beans.spam) this won't work
        # as written
        m, f = name.split(".")
        d = {}
        exec "import m" in d
        return eval(name, d)

    ...
    # Actually create the plugin and put it on the list
    factory = get_plugin_factory("spam_plugins.VikingChorus")
    plugin = factory(app)
    plugins.append(plugin)

If you have an event concept, you can now filter it through the plugins
(this is similar to an observer pattern):
    class App:
        ...
        def plugin_event(self, event, *args):
            for p in self.plugins:
                handler = getattr(p, event, None)
                if handler is None: continue
                try:
                    handler(*args)
                except StopPropagating:
                    # The plugin says the event was completely handled
                    break

    # And here's how you'd do it
    app.plugin_event("sing_refrain", "spam")

A plugin could call back into app or access app's data members, etc by
storing a reference in its initializer.  Or you could choose to pass 'app'
at every call.

There's a lot of stuff here, and I'd say it's well beyond "beginner"
technique.  Of course, I've probably also left a bug in the above
since it's not quite a complete, runnable example... best of luck.

Jeff