class Observable(object):
    def __init__(self):
        self.callbacks = []

    def register(self, callback):
        if callback not in self.callbacks:
            self.callbacks.append(callback)

class Observer(list):
    def callback(self, *args):
        pass

    def observe(self, observable):
        observable.register(self.callback)


observable = Observable()
observer1 = Observer()
observer1.observe(observable)
observer2 = Observer()
observer2.observe(observable)
print observable.callbacks