`Queue` 示例 - 一个并发web爬虫¶

Tornado的 tornado.queues 模块实现了异步生产者 / 消费者模式的协程，类似于Python标准库的 queue 模块为线程实现的模式.

一个协程会使用yield 暂停 Queue.get 直到队列中有值的时候才会继续执行。如果队列设置了最大长度, 协程会yield暂停 Queue.put 直到队列中有空间才会继续。

一个 Queue 会监控未完成任务的计数, 直到未完成任务计数为零。 put 增加未完成任务计数; task_done 减少未完成任务计数。

这个网络爬虫的例子，队列开始的时候只包含base_url。当一个worker抓取一个页面并解析链接把链接加入队列中，然后调用 task_done 减少计数一次。最后，当一个worker抓到的页面URL都是之前已经抓取过的并且队列中没有任务了。于是worker 调用 task_done 把计数减到0。主协程 join 等待挂起的执行完成。

#!/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)

Queue 示例 - 一个并发web爬虫¶

`Queue` 示例 - 一个并发web爬虫¶