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)

#用于从一个包含片段的url中提取中真正的url.
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 = []

	#从所有a标签中提取中href属性。
  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)