Example of a plugin architecture with metaclasses

registry1.py

from application.python.util import Singleton

class PluginRegistry(object):
    __metaclass__ = Singleton

    def __init__(self):
        self.plugins = []

    def __iter__(self):
        return iter(self.plugins)

    def add(self, cls):
        if cls not in self.plugins:
            self.plugins.append(cls)

class Plugin(type):
    """Metaclass for adding a plugin to the registry"""
    def __init__(cls, name, bases, dic):
        super(Plugin, cls).__init__(name, bases, dic)
        PluginRegistry().add(cls)


# Example plugin class
class MyPlugin(object):
    __metaclass__ = Plugin

    def __init__(self):
        pass