That a plugin architecture for a complex app is a good idea is one of those things that most people kinda agree on. One thing we don't quite agree is how the heck are we going to make out app modular?
One way to do it (if you are coding python) is using Yapsy.
Yapsy is awesome. Also, yapsy is a bit underdocumented. Let's see if this post fixes that a bit and leaves just the awesome.
Update: I had not seen the new Yapsy docs, released a few days ago. They are much better than what was there before :-)
Here's the general idea behind yapsy:
You create a Plugin Manager that can find and load plugins from a list of places (for example, from ["/usr/share/appname/plugins", "~/.appname/plugins"]).
A plugin category is a class.
There is a mapping between category names and category classes.
A plugin is a module and a metadata file. The module defines a class that inherits from a category class, and belongs to that category.
The metadata file has stuff like the plugin's name, description, URL, version, etc.
One of the great things about Yapsy is that it doesn't specify too much. A plugin will be just a python object, you can put whatever you want there, or you can narrow it down by specifying the category class.
In fact, the way I have been doing the category classes is:
Start with an empty class
Implement two plugins of that category
If there is a chunk that's much alike in both, move it into the category class.
But trust me, this will all be clearer with an example :-)
I will be doing it with a graphical PyQt app, but Yapsy works just as well for headless of CLI apps.
Let's start with a simple app: an HTML editor with a preview widget.
Here's the code for the app, which is really simple (it doesn't save or do anything, really, it's just an example):
But this application has an obvious limit: you have to type HTML in it. Why not type python code in it and have it convert to HTML for display? Or Wiki markup, or restructured text?
You could, in principle, just implement all those modes, but then you are assuming the responsability of supporting every thing-that-can-be-turned-into-HTML. Your app would be a monolith. That's where yapsy enters the scene.
So, let's create a plugin category, called "Formatter" which takes plain text and returns HTML. Then we add stuff in the UI so the user can choose what formatter he wants, and implement two of those.
Here's our plugin category class:
Of course what good is a plugin architecture without any plugins for it? So, let's create two plugins.
First: a plugin that takes python code and returns HTML, thanks to pygments.
See how it goes into a plugins folder? Later on we will tell yapsy to search there for plugins.
To be recognized as a plugin, it needs a metadata file, too:
And really, that's all there is to making a plugin. Here's another one for comparison, which uses docutils to format reStructured Text:
And here they are in action:
Of course using categories you can do things like a "Tools" category, where the plugins get added to a Tools menu, too.
And here's the application code:
In short: this is easy to do, and it leads to fixing your application's internal structure, so it helps you write better code.