Python线程-动态更改并行线程的数量

我正在尝试并发.futures.ThreadPoolExecutor并行运行一些作业。它使我可以在初始化时设置一次max_workers。但是出于某种原因，我的要求是即时更改活动线程的数量。我找不到任何办法。试图即时重置“ _max_workers”，但这没有影响。

这里是代码。

from concurrent.futures import ThreadPoolExecutor, as_completed
import urllib.request
from random import randint
from time import sleep

URLS = ['http://www.foxnews.com/',
        'http://www.cnn.com/',
        'http://europe.wsj.com/',
        'http://www.bbc.co.uk/',
        'http://some-made-up-domain.com/']

# Retrieve a single page and report the url and contents
def load_url(url, timeout):
    with urllib.request.urlopen(url, timeout=timeout) as conn:
        return conn.read()

if __name__ == "__main__":
    pool = ThreadPoolExecutor(1)
    fs = []
    for url in URLS:
        fs.append(pool.submit(load_url, url, 60))

    r = randint(1,5)
    print ("Changing max_workers to: {}".format(r))
    pool._max_workers = r
    print ("Current queue size: {}. Number of threads in parallel: {}".format(pool._work_queue.qsize(), len(pool._threads)))

    for f in as_completed(fs):
        r = randint(1,5)
        print ("Changing max_workers to: {}".format(r))
        pool._max_workers = r
        print ("Current queue size: {}. Number of threads in parallel: {}".format(pool._work_queue.qsize(), len(pool._threads)))

        try:
            data = f.result()
        except Exception as exc:
            print('%r generated an exception: %s' % (url, exc))
        else:
            print('Page is %d bytes' % (len(data)))

    print ("done")

这里是打印内容：

Changing max_workers to: 3  
Current queue size: 4. Number of threads in parallel: 1  
Changing max_workers to: 1  
Current queue size: 3. Number of threads in parallel: 1  
Page is 233315 bytes  
Changing max_workers to: 4  
Current queue size: 2. Number of threads in parallel: 1  
Page is 1127911 bytes  
Changing max_workers to: 2  
Current queue size: 1. Number of threads in parallel: 1  
Page is 2741410 bytes  
Changing max_workers to: 4  
Current queue size: 0. Number of threads in parallel: 1  
Page is 296361 bytes  
done  

有没有一种方法可以更改max_worker，它将更改pool._threads中的活动线程？