Tornado 4.3 文档翻译: 用户指南-并发网络爬虫

jopen 8年前

译者说

Tornado 4.3于2015年11月6日发布,该版本正式支持Python3.5async/await关键字,并且用旧版本CPython编译Tornado同样可以使用这两个关键字,这无疑是一种进步。其次,这是最后一个支持Python2.6Python3.2的版本了,在后续的版本了会移除对它们的兼容。现在网络上还没有Tornado4.3的中文文档,所以为了让更多的朋友能接触并学习到它,我开始了这个翻译项目,希望感兴趣的小伙伴可以一起参与翻译,项目地址是tornado-zh on Github,翻译好的文档在Read the Docs上直接可以看到。欢迎Issues or PR。

示例 - 一个并发网络爬虫

Tornado的 tornado.queues 模块实现了异步生产者/消费者模式的协程, 类似于通过Python 标准库的 queue实现线程模式.

一个yield Queue.get 的协程直到队列中有值的时候才会暂停. 如果队列设置了最大长度yield Queue.put 的协程直到队列中有空间才会暂停.

一个Queue从0开始对完成的任务进行计数. Queue.put 加计数;Queue.task_done减少计数.

这里的网络爬虫的例子, 队列开始的时候只包含base_url. 当一个worker抓取到一个页面它会解析链接并把它添加到队列中, 然后调用Queue.task_done 减少计数一次. 最后, 当一个worker抓取到的页面URL都是之前抓取到过的并且队列中没有任务了.于是worker调用 Queue.task_done 把计数减到0. 等待 Queue.join 的主协程取消暂停并且完成.

    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)

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