譯者說

Tornado 4.3于2015年11月6日發布，該版本正式支持Python3.5的async/await關鍵字，并且用舊版本CPython編譯Tornado同樣可以使用這兩個關鍵字，這無疑是一種進步。其次，這是最后一個支持Python2.6和Python3.2的版本了，在后續的版本了會移除對它們的兼容。現在網絡上還沒有Tornado4.3的中文文檔，所以為了讓更多的朋友能接觸并學習到它，我開始了這個翻譯項目，希望感興趣的小伙伴可以一起參與翻譯，項目地址是tornado-zh on Github，翻譯好的文檔在Read the Docs上直接可以看到。歡迎Issues or PR。

示例 - 一個并發網絡爬蟲

Tornado的 tornado.queues 模塊實現了異步生產者/消費者模式的協程, 類似于通過Python 標準庫的 queue實現線程模式.

一個yield Queue.get 的協程直到隊列中有值的時候才會暫停. 如果隊列設置了最大長度yield Queue.put 的協程直到隊列中有空間才會暫停.

一個Queue從0開始對完成的任務進行計數. Queue.put 加計數;Queue.task_done減少計數.

這里的網絡爬蟲的例子, 隊列開始的時候只包含base_url. 當一個worker抓取到一個頁面它會解析鏈接并把它添加到隊列中, 然后調用Queue.task_done 減少計數一次. 最后, 當一個worker抓取到的頁面URL都是之前抓取到過的并且隊列中沒有任務了.于是worker調用 Queue.task_done 把計數減到0. 等待 Queue.join 的主協程取消暫停并且完成.

    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):
        """Download the page at `url` and parse it for links.

        Returned links have had the fragment after `#` removed, and have been made
        absolute so, e.g. the URL 'gen.html#tornado.gen.coroutine' becomes
        'http://www.tornadoweb.org/en/stable/gen.html'.
        """
        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):
        pure_url, frag = urldefrag(url)
        return pure_url


    def get_links(html):
        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()
            try:
                if current_url in fetching:
                    return

                print('fetching %s' % current_url)
                fetching.add(current_url)
                urls = yield get_links_from_url(current_url)
                fetched.add(current_url)

                for new_url in urls:
                    # Only follow links beneath the base URL
                    if new_url.startswith(base_url):
                        yield q.put(new_url)

            finally:
                q.task_done()

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

        q.put(base_url)

        # Start workers, then wait for the work queue to be empty.
        for _ in range(concurrency):
            worker()
        yield q.join(timeout=timedelta(seconds=300))
        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)

來自： http://www.jianshu.com/p/d2fe3882bdd0