class Observer(object):
    def __init__(self, subject):
        self.Subject = subject
        self.Subject.Attach(self)
    def __del__(self):
        self.Detach()
    def Detach(self):
        self.Subject.Detach(self)
    def Update(self):
        raise NotImplementedError

class Subject(object):
    def __init__(self):
        self.Observers = {}
    def Attach(self, observer):
        self.Observers[observer] = None
    def Detach(self, observer):
        if observer in self.Observers:
            del self.Observers[observer]
    # FIXME: how to handle different classes of observers?
    def NotifyObservers(self):
        # for o in self.Observers: # <- not robust iteration!
        observers = self.Observers.keys()
        for o in observers:
            o.Update()

class Prototype(object):
    def Clone(self):
        raise NotImplementedError

class CallbackObserver(Observer):
    def __init__(self, subject, callback):
        Observer.__init__(self, subject)
        self.callback = callback
        self.args = None
    def Update(self):
        self.callback(self)