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Queue示例 – 一个并发的网络爬虫

Tornado的tornado.queues模块为协程实现异步生产者/消费者模式,类似于Python标准库的队列模块为线程实现的模式。

一个yieldQueue.get的协程直到队列中有元素之前都会暂停。如果队列设置了最大容量,一个yieldQueue.put的协程在队列有空间之前都会暂停。

一个Queue维护一个从零开始的未完成任务的计数。put增加计数; task_done减少计数。

在此处的web-spider示例中,队列开始仅包含base_url。当一个worker获取一个页面时,它会解析链接并将新的链接放入队列中,然后调用task_done来减少一次计数器。 最终,一个worker获取一个之前URL已经被访问过的页面,并且队列中也没有剩余的工作。 因此,该worker对task_done的调用将计数器减少为零。 正在等待join的主协程将取消暂停然后结束。

#!/usr/bin/env python3

import time
from datetime import timedelta

from html.parser import HTMLParser
from urllib.parse import urljoin, urldefrag

from tornado import gen, httpclient, ioloop, queues

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


async 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'.
    """
    response = await httpclient.AsyncHTTPClient().fetch(url)
    print('fetched %s' % url)

    html = response.body.decode(errors='ignore')
    return [urljoin(url, remove_fragment(new_url))
            for new_url in get_links(html)]


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


async def main():
    q = queues.Queue()
    start = time.time()
    fetching, fetched = set(), set()

    async def fetch_url(current_url):
        if current_url in fetching:
            return

        print('fetching %s' % current_url)
        fetching.add(current_url)
        urls = await 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):
                await q.put(new_url)

    async def worker():
        async for url in q:
            if url is None:
                return
            try:
                await fetch_url(url)
            except Exception as e:
                print('Exception: %s %s' % (e, url))
            finally:
                q.task_done()

    await q.put(base_url)

    # Start workers, then wait for the work queue to be empty.
    workers = gen.multi([worker() for _ in range(concurrency)])
    await q.join(timeout=timedelta(seconds=300))
    assert fetching == fetched
    print('Done in %d seconds, fetched %s URLs.' % (
        time.time() - start, len(fetched)))

    # Signal all the workers to exit.
    for _ in range(concurrency):
        await q.put(None)
    await workers


if __name__ == '__main__':
    io_loop = ioloop.IOLoop.current()
    io_loop.run_sync(main)

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