实现了异步的生产者、调用者模式，和python中的threads的queue一样。

Queue.get会直到队列中有值才会返回，如果队列设置了最大值，那么如果队列满了，则Queue.put会阻塞直到有了空位。Queue中保存的是未完成的任务，初始值是0，put增加，task_done减少。

爬虫例子：

起始，队列中值有一个base url，worker获取到一个页面然后解析，再放一个新的进来，在调用task_done来减少数量，最终所有的页面都爬取完了，队列中数量为0，主循环中获得通知。

# coding: utf-8
import time
from datetime import timedelta

try:
    from HTMLParser import HTMLParser
    from urlparse import urljoin, urldefrag
except ImportError:
    from html.parser import HTMLParser
    from urllib.parse import urljoin, urldefrag

from tornado import httpclient, gen, ioloop, queues

base_url = 'http://www.tornadoweb.org/en/stable/'
concurrency = 10


@gen.coroutine
def get_links_from_url(url):
    """
    从队列中取出一个url 然后解析

    :param url:
    :return:
    """

    try:
        response = yield httpclient.AsyncHTTPClient().fetch(url)
        print('fetched %s' % url)

        html = response.body if isinstance(response.body, str) else response.body.decode()
        urls = [urljoin(url, remove_fragment(new_url))
                for new_url in get_links(html)]
    except Exception as e:
        print('Exception: %s %s' % (e, url))
        raise gen.Return([])

    raise gen.Return(urls)


def remove_fragment(url):
    """
    清除url中的#

    :param url:
    :return:
    """
    pure_url, frag = urldefrag(url)
    return pure_url


def get_links(html):
    """
    获取html页面中的链接

    :param html:
    :return:
    """
    class URLSeeker(HTMLParser):
        def __init__(self):
            HTMLParser.__init__(self)
            self.urls = []

        def handle_starttag(self, tag, attrs):
            href = dict(attrs).get('href')
            if href and tag == 'a':
                self.urls.append(href)

    url_seeker = URLSeeker()
    url_seeker.feed(html)
    return url_seeker.urls


@gen.coroutine
def main():

    q = queues.Queue()
    start = time.time()
    fetching, fetched = set(), set()

    @gen.coroutine
    def fetch_url():
        current_url = yield q.get()  # 队列中取出一个url
        try:
            if current_url in fetching:
                return

            print('fetching %s' % current_url)
            fetching.add(current_url)  # 加入到正在爬取的集合中
            urls = yield get_links_from_url(current_url)  # 启动
            fetched.add(current_url)  # 加入到已经爬取完毕的集合中

            for new_url in urls:
                # 需要以base url开头的 不然有外链就爬的没完了
                if new_url.startswith(base_url):
                    yield q.put(new_url)  # 放入该url到队列中

        finally:
            q.task_done()  # 删除这个url

    @gen.coroutine
    def worker():
        while True:
            yield fetch_url()

    q.put(base_url)  # 放入base url

    for _ in range(concurrency):
        # 启动从currency个数的worker
        worker()

    yield q.join(timeout=timedelta(seconds=300))  # 直到队列空了才返回
    assert fetching == fetched
    print('Done in %d seconds, fetched %s URLs.' % (
        time.time() - start, len(fetched)))


if __name__ == '__main__':

    import logging
    logging.basicConfig()
    io_loop = ioloop.IOLoop.current()
    io_loop.run_sync(main)