After some searching and asking around I didn’t find any good explanation of the simplest way to implement plugins in Python. So, for posterity’s sake, here’s my solution. I’m sure there’s a better way: I’d love to hear your suggestions.
First, the requirements. The code cannot have knowledge of how the plugins are named (.so files, .py, package dirs, etc.). I don’t want to hard-code the list of plugins, as this defeats its dynamic nature altogether. I have to be able to iterate across all plugins. Any user should be able to use the module without knowing that it’s got plugins. Finally, it’s got to be as simple as possible.
First up, we have whatever/__init__.py:
import pkgutil
import imp
import os
plugin_path = [os.path.join(__path__[0], "plugins/")]
for loader, name, ispkg in pkgutil.iter_modules(plugin_path):
file, pathname, desc = imp.find_module(name, plugin_path)
imp.load_module(name, file, pathname, desc)
This basically uses Python’s module search to find all contents of the
plugins sub-directory and load them. Now we have some more scaffolding
in the same directory, as whatever/whatever.py:
plugins = []
class plugin(object):
"""Abstract plugin base class."""
...
def register_plugin(plugin)
global plugins
plugins += [ plugin ]
# utility functions to iterate across and use plugins...
Finally, each plugin looks something like this, in plugins/foo.py:
from whatever/whatever import *
class fooplugin(plugin):
"""Concrete class for foo plugin."""
...
register_plugin(fooplugin)
Simple enough, but it took a while to work it out. Unfortunately, it
doesn’t seem possible to merge whatever.py into __init__.py as we
have a recursive import problem.