import abc
import threading


class Observer(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def work(self):
        pass


class ClientA(Observer):
    def work(self):
        print(type(self))


class ClientB(Observer):
    def work(self):
        print(id(self))


class Subject(object):
    def __init__(self):
        self.observers = list()
        self.lock = threading.RLock()

    def register_observer(self, observer):
        self.lock.acquire()
        if observer not in self.observers:
            self.observers.append(observer)
        self.lock.release()

    def un_register_observer(self, observer):
        self.lock.acquire()
        if observer in self.observers:
            self.observers.remove(observer)
        self.lock.release()

    def notify(self):
        self.lock.acquire()
        for _ in self.observers:
            _.work()
        self.lock.release()


if __name__ == '__main__':
    subject = Subject()
    a = ClientA()
    b = ClientB()
    subject.register_observer(a)
    subject.register_observer(b)
    subject.notify()