Query示例-并发网络蜘蛛

Tornado 的tornado.queues模块为协程实现了异步生产者/消费者模式，类似于 Python 标准库的queue模块为线程实现的模式。
产生Queue.get暂停的协程直到队列中有一个项目。如果队列设置了最大大小，则产生的协程会Queue.put暂停，直到有空间容纳另一个项目。
一个Queue维护未完成任务的计数，该计数从零开始。 put增加计数；task_done递减它。
在此处的 web-spider 示例中，队列开始时仅包含 base_url。当工作人员获取页面时，它会解析链接并将新链接放入队列中，然后调用task_done以减少计数器一次。最终，一个工作人员获取了一个之前所有 URL 都已看到的页面，并且队列中也没有剩余工作。因此，该工人的调用task_done 将计数器减至零。正在等待加入的主协同程序未暂停并完成。

#!/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, dead = set(), 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))
                dead.add(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 | dead)
    print("Done in %d seconds, fetched %s URLs." % (time.time() - start, len(fetched)))
    print("Unable to fetch %s URLS." % len(dead))

    # 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)