gistfile1.py

class Worker(object):
    def __init__(self, tasks):
        self.enabled = True
        self.tasks = set(tasks)

    def is_active(self):
        return self.enabled and self.tasks

    def run(self):
        if self.enabled:
            task = self.tasks.pop()
            print(task)


class Watcher(object):
    def __init__(self, worker):
        self.worker = worker
        self.was_active = worker.is_active()

    def watch(self):
        active = self.worker.is_active()
        if active != self.was_active:
            print("worker status changed")
            self.was_active = active


worker = Worker({"1"})
watcher = Watcher(worker)

watcher.watch()
worker.run()
watcher.watch()