SpiffSignal is a Python module that implements a simple signal/event mechanism:

from SpiffSignal import Trackable

class WatchMe(Trackable):
    def do_something(self):
        self.signal_emit('did-something', 'hello world')

def my_callback(arg):
    print arg

foo = WatchMe()
foo.signal_connect('did-something', my_callback)
foo.do_something()