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        while self.running:            self.update_event.clear()            task = None            with self.heap_lock:                if len(self.heap):                    runtime, delay, queue = self.heap[0]                    now = time()                    if now < runtime:                        timeout = runtime - now                    else:                        runtime, delay, queue = heappop(self.heap)                        try:                            task = queue.get_nowait()                        except Queue.Empty:                            pass                        else:                            queue.task_done()                            yield task                            heappush(self.heap, (time() + delay, delay, queue))                        timeout = 0                else :                    timeout = None           if task:                self.update_event.wait(timeout)