Example of a plugin architecture using metaclasses and class decorator

registry2.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)

def plugin(cls):
    """Class decorator for adding plugins to the registry"""
    PluginRegistry().add(cls())
    return cls


# Example plugin class
@plugin
class MyPlugin(object):
    __metaclass__ = Singleton

    def __init__(self):
        pass