Tornado 4.3文件翻譯: 使用者指南-示例-一個併發網路爬蟲

TaoBeier發表於2016-01-03
爬蟲

譯者說

Tornado 4.3於2015年11月6日釋出，該版本正式支援Python3.5的async/await關鍵字，並且用舊版本CPython編譯Tornado同樣可以使用這兩個關鍵字，這無疑是一種進步。其次，這是最後一個支援Python2.6和Python3.2的版本了，在後續的版本了會移除對它們的相容。現在網路上還沒有Tornado4.3的中文文件，所以為了讓更多的朋友能接觸並學習到它，我開始了這個翻譯專案，希望感興趣的小夥伴可以一起參與翻譯，專案地址是tornado-zh on Github，翻譯好的文件在Read the Docs上直接可以看到。歡迎Issues or PR。

示例 – 一個併發網路爬蟲

Tornado的 tornado.queues 模組實現了非同步生產者/消費者模式的協程, 類似於通過Python 標準庫的 queue實現執行緒模式.

一個yield Queue.get 的協程直到佇列中有值的時候才會暫停. 如果佇列設定了最大長度yield Queue.put 的協程直到佇列中有空間才會暫停.

一個Queue從0開始對完成的任務進行計數. Queue.put 加計數;Queue.task_done減少計數.

這裡的網路爬蟲的例子, 佇列開始的時候只包含base_url. 當一個worker抓取到一個頁面它會解析連結並把它新增到佇列中, 然後呼叫Queue.task_done 減少計數一次. 最後, 當一個worker抓取到的頁面URL都是之前抓取到過的並且佇列中沒有任務了.於是worker呼叫 Queue.task_done 把計數減到0. 等待 Queue.join 的主協程取消暫停並且完成.


    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)

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