from multiprocessing import Lock, Queue

class Observer:
    def __init__(self):
        self.__receivers_list = {}
        self.__lock = Lock()
    def listen(self, identifier):
        with self.__lock:
            queue = Queue()
            self.__receivers_list.setdefault(identifier, set())
            self.__receivers_list[identifier].add(queue)
            return Subscription(self, identifier, queue)
    def publish(self, identifier, message):
        if identifier in self.__receivers_list:
            for queue in self.__receivers_list[identifier]:
                queue.put(message)
    def quit(self, identifer, queue):
        with self.__lock:
            self.__receivers_list[identifer].remove(queue)
            if len(self.__receivers_list[identifer]) == 0:
                del self.__receivers_list[identifer]

class Subscription:
    def __init__(self, observer, identifier, queue):
        self.__observer = observer
        self.__identifier = identifier
        self.__queue = queue
    def receive(self, timeout):
        return self.__queue.get(timeout=timeout)
    def __del__(self):
        self.__observer.quit(self.__identifier, self.__queue)