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<div class="section" id="queue-example-a-concurrent-web-spider">
<h1><a class="reference internal" href="../queues.html#tornado.queues.Queue" title="tornado.queues.Queue"><code class="xref py py-class docutils literal"><span class="pre">Queue</span></code></a> example - a concurrent web spider<a class="headerlink" href="#queue-example-a-concurrent-web-spider" title="Permalink to this headline">¶</a></h1>
<p>Tornado’s <a class="reference internal" href="../queues.html#module-tornado.queues" title="tornado.queues"><code class="xref py py-obj docutils literal"><span class="pre">tornado.queues</span></code></a> module implements an asynchronous producer /
consumer pattern for coroutines, analogous to the pattern implemented for
threads by the Python standard library’s <a class="reference external" href="/usr/share/doc/python3-doc/html/library/queue.html#module-queue" title="(in Python v3.5)"><code class="xref py py-obj docutils literal"><span class="pre">queue</span></code></a> module.</p>
<p>A coroutine that yields <a class="reference internal" href="../queues.html#tornado.queues.Queue.get" title="tornado.queues.Queue.get"><code class="xref py py-obj docutils literal"><span class="pre">Queue.get</span></code></a> pauses until there is an item in the queue.
If the queue has a maximum size set, a coroutine that yields <a class="reference internal" href="../queues.html#tornado.queues.Queue.put" title="tornado.queues.Queue.put"><code class="xref py py-obj docutils literal"><span class="pre">Queue.put</span></code></a> pauses
until there is room for another item.</p>
<p>A <a class="reference internal" href="../queues.html#tornado.queues.Queue" title="tornado.queues.Queue"><code class="xref py py-obj docutils literal"><span class="pre">Queue</span></code></a> maintains a count of unfinished tasks, which begins at zero.
<a class="reference internal" href="../queues.html#tornado.queues.Queue.put" title="tornado.queues.Queue.put"><code class="xref py py-obj docutils literal"><span class="pre">put</span></code></a> increments the count; <a class="reference internal" href="../queues.html#tornado.queues.Queue.task_done" title="tornado.queues.Queue.task_done"><code class="xref py py-obj docutils literal"><span class="pre">task_done</span></code></a> decrements it.</p>
<p>In the web-spider example here, the queue begins containing only base_url. When
a worker fetches a page it parses the links and puts new ones in the queue,
then calls <a class="reference internal" href="../queues.html#tornado.queues.Queue.task_done" title="tornado.queues.Queue.task_done"><code class="xref py py-obj docutils literal"><span class="pre">task_done</span></code></a> to decrement the counter once. Eventually, a
worker fetches a page whose URLs have all been seen before, and there is also
no work left in the queue. Thus that worker’s call to <a class="reference internal" href="../queues.html#tornado.queues.Queue.task_done" title="tornado.queues.Queue.task_done"><code class="xref py py-obj docutils literal"><span class="pre">task_done</span></code></a>
decrements the counter to zero. The main coroutine, which is waiting for
<a class="reference internal" href="../queues.html#tornado.queues.Queue.join" title="tornado.queues.Queue.join"><code class="xref py py-obj docutils literal"><span class="pre">join</span></code></a>, is unpaused and finishes.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">from</span> <span class="nn">datetime</span> <span class="k">import</span> <span class="n">timedelta</span>
<span class="k">try</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">HTMLParser</span> <span class="k">import</span> <span class="n">HTMLParser</span>
<span class="kn">from</span> <span class="nn">urlparse</span> <span class="k">import</span> <span class="n">urljoin</span><span class="p">,</span> <span class="n">urldefrag</span>
<span class="k">except</span> <span class="ne">ImportError</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">html.parser</span> <span class="k">import</span> <span class="n">HTMLParser</span>
<span class="kn">from</span> <span class="nn">urllib.parse</span> <span class="k">import</span> <span class="n">urljoin</span><span class="p">,</span> <span class="n">urldefrag</span>
<span class="kn">from</span> <span class="nn">tornado</span> <span class="k">import</span> <span class="n">httpclient</span><span class="p">,</span> <span class="n">gen</span><span class="p">,</span> <span class="n">ioloop</span><span class="p">,</span> <span class="n">queues</span>
<span class="n">base_url</span> <span class="o">=</span> <span class="s1">'http://www.tornadoweb.org/en/stable/'</span>
<span class="n">concurrency</span> <span class="o">=</span> <span class="mi">10</span>
<span class="nd">@gen</span><span class="o">.</span><span class="n">coroutine</span>
<span class="k">def</span> <span class="nf">get_links_from_url</span><span class="p">(</span><span class="n">url</span><span class="p">):</span>
<span class="sd">"""Download the page at `url` and parse it for links.</span>
<span class="sd"> Returned links have had the fragment after `#` removed, and have been made</span>
<span class="sd"> absolute so, e.g. the URL 'gen.html#tornado.gen.coroutine' becomes</span>
<span class="sd"> 'http://www.tornadoweb.org/en/stable/gen.html'.</span>
<span class="sd"> """</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">response</span> <span class="o">=</span> <span class="k">yield</span> <span class="n">httpclient</span><span class="o">.</span><span class="n">AsyncHTTPClient</span><span class="p">()</span><span class="o">.</span><span class="n">fetch</span><span class="p">(</span><span class="n">url</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'fetched </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">url</span><span class="p">)</span>
<span class="n">html</span> <span class="o">=</span> <span class="n">response</span><span class="o">.</span><span class="n">body</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">response</span><span class="o">.</span><span class="n">body</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> \
<span class="k">else</span> <span class="n">response</span><span class="o">.</span><span class="n">body</span><span class="o">.</span><span class="n">decode</span><span class="p">()</span>
<span class="n">urls</span> <span class="o">=</span> <span class="p">[</span><span class="n">urljoin</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">remove_fragment</span><span class="p">(</span><span class="n">new_url</span><span class="p">))</span>
<span class="k">for</span> <span class="n">new_url</span> <span class="ow">in</span> <span class="n">get_links</span><span class="p">(</span><span class="n">html</span><span class="p">)]</span>
<span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Exception: </span><span class="si">%s</span><span class="s1"> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">e</span><span class="p">,</span> <span class="n">url</span><span class="p">))</span>
<span class="k">raise</span> <span class="n">gen</span><span class="o">.</span><span class="n">Return</span><span class="p">([])</span>
<span class="k">raise</span> <span class="n">gen</span><span class="o">.</span><span class="n">Return</span><span class="p">(</span><span class="n">urls</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">remove_fragment</span><span class="p">(</span><span class="n">url</span><span class="p">):</span>
<span class="n">pure_url</span><span class="p">,</span> <span class="n">frag</span> <span class="o">=</span> <span class="n">urldefrag</span><span class="p">(</span><span class="n">url</span><span class="p">)</span>
<span class="k">return</span> <span class="n">pure_url</span>
<span class="k">def</span> <span class="nf">get_links</span><span class="p">(</span><span class="n">html</span><span class="p">):</span>
<span class="k">class</span> <span class="nc">URLSeeker</span><span class="p">(</span><span class="n">HTMLParser</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">HTMLParser</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">urls</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">def</span> <span class="nf">handle_starttag</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">tag</span><span class="p">,</span> <span class="n">attrs</span><span class="p">):</span>
<span class="n">href</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="n">attrs</span><span class="p">)</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'href'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">href</span> <span class="ow">and</span> <span class="n">tag</span> <span class="o">==</span> <span class="s1">'a'</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">urls</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">href</span><span class="p">)</span>
<span class="n">url_seeker</span> <span class="o">=</span> <span class="n">URLSeeker</span><span class="p">()</span>
<span class="n">url_seeker</span><span class="o">.</span><span class="n">feed</span><span class="p">(</span><span class="n">html</span><span class="p">)</span>
<span class="k">return</span> <span class="n">url_seeker</span><span class="o">.</span><span class="n">urls</span>
<span class="nd">@gen</span><span class="o">.</span><span class="n">coroutine</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">q</span> <span class="o">=</span> <span class="n">queues</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>
<span class="n">start</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span>
<span class="n">fetching</span><span class="p">,</span> <span class="n">fetched</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(),</span> <span class="nb">set</span><span class="p">()</span>
<span class="nd">@gen</span><span class="o">.</span><span class="n">coroutine</span>
<span class="k">def</span> <span class="nf">fetch_url</span><span class="p">():</span>
<span class="n">current_url</span> <span class="o">=</span> <span class="k">yield</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">if</span> <span class="n">current_url</span> <span class="ow">in</span> <span class="n">fetching</span><span class="p">:</span>
<span class="k">return</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'fetching </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">current_url</span><span class="p">)</span>
<span class="n">fetching</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">current_url</span><span class="p">)</span>
<span class="n">urls</span> <span class="o">=</span> <span class="k">yield</span> <span class="n">get_links_from_url</span><span class="p">(</span><span class="n">current_url</span><span class="p">)</span>
<span class="n">fetched</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">current_url</span><span class="p">)</span>
<span class="k">for</span> <span class="n">new_url</span> <span class="ow">in</span> <span class="n">urls</span><span class="p">:</span>
<span class="c1"># Only follow links beneath the base URL</span>
<span class="k">if</span> <span class="n">new_url</span><span class="o">.</span><span class="n">startswith</span><span class="p">(</span><span class="n">base_url</span><span class="p">):</span>
<span class="k">yield</span> <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">new_url</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">q</span><span class="o">.</span><span class="n">task_done</span><span class="p">()</span>
<span class="nd">@gen</span><span class="o">.</span><span class="n">coroutine</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">fetch_url</span><span class="p">()</span>
<span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">base_url</span><span class="p">)</span>
<span class="c1"># Start workers, then wait for the work queue to be empty.</span>
<span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">concurrency</span><span class="p">):</span>
<span class="n">worker</span><span class="p">()</span>
<span class="k">yield</span> <span class="n">q</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="n">timedelta</span><span class="p">(</span><span class="n">seconds</span><span class="o">=</span><span class="mi">300</span><span class="p">))</span>
<span class="k">assert</span> <span class="n">fetching</span> <span class="o">==</span> <span class="n">fetched</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'Done in </span><span class="si">%d</span><span class="s1"> seconds, fetched </span><span class="si">%s</span><span class="s1"> URLs.'</span> <span class="o">%</span> <span class="p">(</span>
<span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="o">-</span> <span class="n">start</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">fetched</span><span class="p">)))</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">()</span>
<span class="n">io_loop</span> <span class="o">=</span> <span class="n">ioloop</span><span class="o">.</span><span class="n">IOLoop</span><span class="o">.</span><span class="n">current</span><span class="p">()</span>
<span class="n">io_loop</span><span class="o">.</span><span class="n">run_sync</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
</pre></div>
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