如何用Python语言实现观察者模式

# 观察对象基类class Observable:    def __init__(self):        self.observers= []    def attach(self, observer):        print(type(observer), 'has beenattached!')        self.observers.append(observer)    def detach(self, observer):        print(type(observer), 'has beendetached!')        self.observers.remove(observer)    def notify(self):        for oin self.observers:            o.update()

# 观察对象具象类class ConcreteObservable(Observable):    def __init__(self):        Observable.__init__(self)        self.state= None
    def set_state(self, state):        self.state= state        self.notify()
    def get_state(self):        return self.state

# 观察者具象类class ConcreteObserver(Observer):    def __init__(self,observable):        Observer.__init__(self)        self.observable= observable    def update(self):        print(type(self), self.observable.get_state())

# 主程序if __name__ == '__main__':    observable = ConcreteObservable()    # 构建观察者    observer= ConcreteObserver(observable)    # 观察者注册    observable.attach(observer)    # 改变观察对象状态，产生输出    observable.set_state('State Changed1!')    # 观察者删除    observable.detach(observer)    # 再次改变观察对象状态，不产生输出    observable.set_state('State Changed2!')

 has been attached! has been attached! State Changed1! State Changed1! has been detached! State Changed2!