译者说
Tornado 4.3于2015年11月6日发布，该版本正式支持Python3.5的async/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Python2.6和Python3.2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)
