concurrent futures with FIFO

# requires python 3.2.3 or higher
import concurrent.futures

def f(x):
    print ("working on task: %s" %{x})
    if x-1 > 0 :
        return x-1 # I want this task back in the queue.

def main():
    Queue = [x for x in range(4)]
    with concurrent.futures.ThreadPoolExecutor(max_workers=3) as executor:
        while len(Queue)>0:
            task = Queue.pop(0)
            future = executor.submit(f, task)
            result = future.result()
            if result is not None:
                Queue.append(result) # here the f may return a task is to be added to the queue.
        print ("processing completed.")

if __name__ == "__main__":
    main()

    # expected output:
    # "working on task: 1"
    # "working on task: 2" (returns 1 to the Queue)
    # "working on task: 3" (returns 2 to the Queue)
    # "working on task: 4" (returns 3 to the Queue)
    # "working on task: 1" 
    # "working on task: 2" (returns 1 to the Queue)
    # "working on task: 3" (returns 2 to the Queue)
    # "working on task: 1"
    # "working on task: 2" (returns 1 to the Queue)
    # "working on task: 1"