my_observer.py

class NewsPublisher:
    def __init__(self):
        self.__subscribers = []
        self.__latestNews = None
    def attach(self, subscriber):
        self.__subscribers.append(subscriber)
    def detach(self):
        return self.__subscribers.pop()
    def subscribers(self):
        return [type(x).__name__ for x in self.__subscribers]
    def notifySubscribers(self):
        for sub in self.__subscribers:
            sub.update()
    def addNews(self, news):
        self.__latestNews = news
    def getNews(self):
        return 'News: {}'.format(self.__latestNews)

class PidgeonSubscriber:
    def __init__(self, publisher):
        self.publisher = publisher
        self.publisher.attach(self) # 
    def update(self):
        print(type(self).__name__, self.publisher.getNews())

class SMSSubscriber:
    def __init__(self, publisher):
        self.publisher = publisher
        self.publisher.attach(self) # 
    def update(self):
        print(type(self).__name__, self.publisher.getNews())

class EmailSubscriber:
    def __init__(self, publisher):
        self.publisher = publisher
        self.publisher.attach(self) # 
    def update(self):
        print(type(self).__name__, self.publisher.getNews())

if __name__ == '__main__':
    news_publisher = NewsPublisher()
    for Subscriber in [SMSSubscriber, EmailSubscriber, PidgeonSubscriber]:
        Subscriber(news_publisher)
        print( '\nSubscribers: {}'.format( news_publisher.subscribers() ) )
        news_publisher.addNews('something interesting occured')
        # lets tell all the subscribers about the news
        news_publisher.notifySubscribers()