使用ThreadPoolExecutor实现合并排序

Question

我是python的初学者，我已经开始尝试使用python3.4。以下是我尝试进行多线程合并排序。但似乎当执行程序用完线程时，提交将阻塞（我知道这个设计开始时是错误的）。我的问题是，如果没有空闲线程可以从池中使用，是否存在继续使用当前线程的安全非阻塞替代方案。

from concurrent.futures.thread import ThreadPoolExecutor

def merge(left_slice, right_slice):
    lindex = 0
    rindex = 0
    result = []
    while lindex != len(left_slice) or rindex != len(right_slice):
        if lindex == len(left_slice):
            result = result + right_slice[rindex:len(right_slice)]
            return result
        elif rindex == len(right_slice):
            result = result + left_slice[lindex:len(left_slice)]
            return result
        elif left_slice[lindex] < right_slice[rindex]:
            result.append(left_slice[lindex])
            lindex = lindex + 1
        elif left_slice[lindex] > right_slice[rindex]:
            result.append(right_slice[rindex])
            rindex = rindex + 1
        else:
            result.append(right_slice[rindex])
            rindex = rindex + 1
            result.append(left_slice[lindex])
            lindex = lindex + 1
    return result

def sort(numbers):
    """merge sort"""
    if len(numbers) == 1:
        return numbers

    left_slice = numbers[0:len(numbers)//2]
    right_slice = numbers[len(numbers)//2:len(numbers)] 
    left_future = executor.submit(sort, left_slice)
    right_future = executor.submit(sort, right_slice)
    left_slice = left_future.result()
    right_slice = right_future.result()
    return merge(left_slice, right_slice)

if __name__ == '__main__':
    import random
    unsorted = random.sample(range(1000000), 10000 ) 
    import time
    print ("starting to sort...")
    start = time.time();
    with ThreadPoolExecutor(max_workers=5) as executor:
        future = executor.submit(sort, unsorted)       
        print(future.result()[-5:])
    print("done sorting: " + time.time() - start)