Class Event

object --+
         |
        Event

Usage:

>>> myEvent = Event() # a, b
>>> callback1 = lambda a, b: dostuff(a, b)
>>> callback2 = lambda a, b: otherstuff(a, b)
>>> myEvent += callback1
>>> myEvent += callback2
>>> myEvent(a, b) # calls both callbacks
>>> myEvent -= callback1
>>> myEvent -= callback2
>>> myEvent(a, b) # no callbacks; does nothing

Note: this is a great source of circular references; see gc.collect()