Best practices for dynamically loading plugins at startup

[Christoph Haas]

Dear coders...

I'm working on an application that is supposed to support "plugins".
The idea is to use the plugins as packages like this:

Plugins/
  __init__.py
  Plugin1.py
  Plugin2.py
  Plugin3.py

When the application starts up I want to have these modules loaded
dynamically. Users can put their own plugin modules into the
Plugins/ directory and the application should know about it.

Since I don't know which plugins have been put into that directory
I cannot just "import Plugin1, Plugin2, Plugin3" in the "__init__.py".
So I need to find out the *.py there and load them during startup.
I could do that with a "walk" over that directory.

Each plugin is supposed to be a class derived from a general
"Plugin" superclass. I just don't know how to 'register' every
plugin. The main application needs to know which plugin classes
there are. On IRC I was recommended walking through all objects
and finding out if the class is a subclass of "Plugin". Another
person recommended using metaclasses to automatically register
the plugin in a global list.

Since I have only little real-life Python knowledge I wonder what the
best practice for this kind of problem is.

I looked at the "supybot" IRC bot to get an idea how plugins are handled
there. Unfortunately it was still a bit over my (python) head.

Regards
 Christoph
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