### Navigation - [index](# "General Index") - [modules](# "Python Module Index") | - [next](# "Structure of a Tornado web application") | - [previous](# "Coroutines") | - [Tornado 4.4.dev1 documentation](#) » - [User's guide](#) » # [`Queue`](# "tornado.queues.Queue") example - a concurrent web spider Tornado's [`tornado.queues`](# "tornado.queues") module implements an asynchronous producer /consumer pattern for coroutines, analogous to the pattern implemented forthreads by the Python standard library's [`queue`](https://docs.python.org/3.4/library/queue.html#module-queue "(in Python v3.4)") [https://docs.python.org/3.4/library/queue.html#module-queue] module. A coroutine that yields [`Queue.get`](# "tornado.queues.Queue.get") pauses until there is an item in the queue.If the queue has a maximum size set, a coroutine that yields [`Queue.put`](# "tornado.queues.Queue.put") pausesuntil there is room for another item. A [`Queue`](# "tornado.queues.Queue") maintains a count of unfinished tasks, which begins at zero.[`put`](# "tornado.queues.Queue.put") increments the count; [`task_done`](# "tornado.queues.Queue.task_done") decrements it. In the web-spider example here, the queue begins containing only base_url. Whena worker fetches a page it parses the links and puts new ones in the queue,then calls [`task_done`](# "tornado.queues.Queue.task_done") to decrement the counter once. Eventually, aworker fetches a page whose URLs have all been seen before, and there is alsono work left in the queue. Thus that worker's call to [`task_done`](# "tornado.queues.Queue.task_done")decrements the counter to zero. The main coroutine, which is waiting for[`join`](# "tornado.queues.Queue.join"), is unpaused and finishes. ~~~ 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) ~~~ © Copyright 2009-2016, The Tornado Authors. Created using [Sphinx](http://sphinx-doc.org/) 1.3.5.