觀察者模式，被觀察物自身屬性通知觀察者。

class Obj(object):
    def __init__(self, value):
        super().__init__()
        self.observers = []
        self.dynamic_value = value

    def add_observer(self, observer):
        self.observers.append(observer)

    def remove_observer(self, observer):
        self.observers.remove(observer)

    def notify_observer(self):
        for observer in self.observers:
            observer.update()


class Observer(object):
    def __init__(self, target):
        super().__init__()
        self.target = target

    def update(self):
        print(self.target.dynamic_value)


def main():
    obj = Obj("value")

    observer = Observer(obj)

    obj.add_observer(observer)
    obj.dynamic_value = "new_value"
    obj.notify_observer()


if __name__ == '__main__':
    main()