class Subscription(object):

    def __init__(self, periodical, subscriber):
        self.periodical = periodical
        self.subscriber = subscriber

    def cancel(self):
        self.periodical.subscriptions.remove(self)

    def issue(self, issue):
        self.subscriber(issue)


class Periodical(object):

    def __init__(self):
        self.subscriptions = []

    def subscribe(self, who):
        subscription = Subscription(self, who)
        self.subscriptions.append(subscription)
        return subscription

    def issue(self, issue):
        for subscr in self.subscriptions:
            subscr.issue(issue)