新手刚学 gevent,请帮忙看看我写的代码

麻烦帮我看看下面的代码,我这里在最后会报错,不知什么原因.

gevent.hub.LoopExit: ('This operation would block forever', <Hub at 0x2f62af8 select default pending=0 ref=0>)

还有我这段代码有什么问题吗?有哪些地方可以优化,求赐教.

我是新手,代码可能比较 low,求"教做人".

谢谢. (还请jiandan同学放过....我就是学习下)

# -*- coding:utf-8 -*-

import gevent
import gevent.queue
import requests
from lxml import etree

url_queue = gevent.queue.JoinableQueue(100)
headers = {
    'User-Agent': 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)'
}


def spider():
    while True:
        url = url_queue.get()
        if url is None:
            url_queue.task_done()
            break

        try:
            html = requests.get(url, headers=headers, timeout=1).content
            selector = etree.HTML(html)
            title = selector.xpath('//a[@href="%s"]/text()' % url)[0]
            print(title)
        except Exception as e:
            print(e)


if __name__ == '__main__':
    urls = [
        'http://jandan.net/2016/09/22/migrants-choice.html',
        'http://jandan.net/2016/09/22/farting-really-good.html',
        'http://jandan.net/2016/09/22/special-cleaner.html',
        'http://jandan.net/2016/09/22/hand-mobile-phone.html',
        'http://jandan.net/2016/09/22/beer-you-order.html',
        'http://jandan.net/2016/09/22/pigeons-can-read.html',
        'http://jandan.net/2016/09/22/snake-inter-species.html',
        'http://jandan.net/2016/09/21/north-koreas-internet-2.html',
        'http://jandan.net/2016/09/21/mona-lisa-overrated.html',
        'http://jandan.net/2016/09/21/antikythera-ancient-skeleton.html',
        'http://jandan.net/2016/09/21/mentality-fish.html',
        'http://jandan.net/2016/09/21/things-smuggled-space.html',
        'http://jandan.net/2016/09/21/water-bear.html',
        'http://jandan.net/2016/09/21/oldest-fishing-hooks.html',
        'http://jandan.net/2016/09/21/b-21-raider.html',
        'http://jandan.net/2016/09/21/paper-cuts-hurt.html',
        'http://jandan.net/2016/09/21/cat-ecological-disaster.html',
        'http://jandan.net/2016/09/21/pluto-owns-heart.html',
        'http://jandan.net/2016/09/21/a-teenage-girl.html',
        'http://jandan.net/2016/09/21/light-drive-men.html',
        'http://jandan.net/2016/09/21/ai-analyses-mammograms.html',
        'http://jandan.net/2016/09/21/burnt-cheese.html',
        'http://jandan.net/2016/09/21/black-hole-spaghetti.html',
        'http://jandan.net/2016/09/21/womens-pubic-hair.html'
    ]
    for url in urls:
        url_queue.put(url)
    threads = []
    for i in range(1, 3):
        threads.append(gevent.spawn(spider))
    gevent.joinall(threads)