plugins.py

def write(msg):
  print msg

class Plugins(object):
  plugins = {
    'writer': write,
  }
  
  @classmethod
  def register(cls, name, plugin):
    cls.plugins[name] = plugin
  
  @classmethod
  def get(cls, name):
    return cls.plugins[name]

def double_write(msg):
  print msg
  print msg

Plugins.register('writer', double_write)

class Messenger(object):
  def send(self, msg):
    writer = Plugins.get('writer')
    writer.write(msg)