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<div class="section" id="etag-and-last-modified-headers">
<span id="http-etag"></span><h1>ETag and Last-Modified Headers<a class="headerlink" href="#etag-and-last-modified-headers" title="Permalink to this headline">¶</a></h1>
<p>ETags and Last-Modified headers are two ways that feed publishers can save
bandwidth, but they only work if clients take advantage of them.
<strong class="program">Universal Feed Parser</strong> gives you the ability to take advantage of
these features, but you must use them properly.</p>
<p>The basic concept is that a feed publisher may provide a special
<abbr title="Hypertext Transfer Protocol">HTTP</abbr> header, called an ETag, when it
publishes a feed. You should send this ETag back to the server on subsequent
requests. If the feed has not changed since the last time you requested it,
the server will return a special <abbr title="Hypertext Transfer Protocol">HTTP</abbr>
status code (<tt class="docutils literal"><span class="pre">304</span></tt>) and no feed data.</p>
<div class="section" id="using-etags-to-reduce-bandwidth">
<h2>Using ETags to reduce bandwidth<a class="headerlink" href="#using-etags-to-reduce-bandwidth" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">feedparser</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="n">feedparser</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="s">'http://feedparser.org/docs/examples/atom10.xml'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">d</span><span class="o">.</span><span class="n">etag</span>
<span class="go">'"6c132-941-ad7e3080"'</span>
<span class="gp">>>> </span><span class="n">d2</span> <span class="o">=</span> <span class="n">feedparser</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="s">'http://feedparser.org/docs/examples/atom10.xml'</span><span class="p">,</span> <span class="n">etag</span><span class="o">=</span><span class="n">d</span><span class="o">.</span><span class="n">etag</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">status</span>
<span class="go">304</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">feed</span>
<span class="go">{}</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">entries</span>
<span class="go">[]</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">debug_message</span>
<span class="go">'The feed has not changed since you last checked, so</span>
<span class="go">the server sent no data. This is a feature, not a bug!'</span>
</pre></div>
</div>
<p>There is a related concept which accomplishes the same thing, but slightly
differently. In this case, the server publishes the last-modified date of the
feed in the <abbr title="Hypertext Transfer Protocol">HTTP</abbr> header. You can send
this back to the server on subsequent requests, and if the feed has not
changed, the server will return <abbr title="Hypertext Transfer Protocol">HTTP</abbr>
status code <tt class="docutils literal"><span class="pre">304</span></tt> and no feed data.</p>
</div>
<div class="section" id="using-last-modified-headers-to-reduce-bandwidth">
<h2>Using Last-Modified headers to reduce bandwidth<a class="headerlink" href="#using-last-modified-headers-to-reduce-bandwidth" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">feedparser</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="n">feedparser</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="s">'http://feedparser.org/docs/examples/atom10.xml'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">d</span><span class="o">.</span><span class="n">modified</span>
<span class="go">Fri, 11 Jun 2012 23:00:34 GMT</span>
<span class="gp">>>> </span><span class="n">d</span><span class="o">.</span><span class="n">modified_parsed</span>
<span class="go">(2004, 6, 11, 23, 0, 34, 4, 163, 0)</span>
<span class="gp">>>> </span><span class="n">d2</span> <span class="o">=</span> <span class="n">feedparser</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="s">'http://feedparser.org/docs/examples/atom10.xml'</span><span class="p">,</span> <span class="n">modified</span><span class="o">=</span><span class="n">d</span><span class="o">.</span><span class="n">modified</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">status</span>
<span class="go">304</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">feed</span>
<span class="go">{}</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">entries</span>
<span class="go">[]</span>
<span class="gp">>>> </span><span class="n">d2</span><span class="o">.</span><span class="n">debug_message</span>
<span class="go">'The feed has not changed since you last checked, so</span>
<span class="go">the server sent no data. This is a feature, not a bug!'</span>
</pre></div>
</div>
<p>Clients should support both ETag and Last-Modified headers, as some servers support one but not the other.</p>
<div class="admonition important">
<p class="first admonition-title">Important</p>
<p class="last">If you do not support ETag and Last-Modified headers, you will repeatedly
download feeds that have not changed. This wastes your bandwidth and the
publisher’s bandwidth, and the publisher may ban you from accessing their
server.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">You can control the behaviour of <abbr title="Hypertext Transfer Protocol">HTTP</abbr>
caches between your application and the origin server by using the
<tt class="docutils literal"><span class="pre">extra_headers</span></tt> parameter. For example, you may want to send
<tt class="docutils literal"><span class="pre">Cache-control:</span> <span class="pre">max-age=60</span></tt> to make the caches revalidate against the
origin server unless their cached copy is less than a minute old. Again,
this should be used with consideration.</p>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<ul class="last simple">
<li><a class="reference external" href="http://fishbowl.pastiche.org/2002/10/21/http_conditional_get_for_rss_hackers">HTTP Conditional Get For RSS Hackers</a></li>
<li><a class="reference external" href="http://diveintopython.org/http_web_services/">HTTP Web Services</a></li>
</ul>
</div>
</div>
</div>
</div>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">ETag and Last-Modified Headers</a><ul>
<li><a class="reference internal" href="#using-etags-to-reduce-bandwidth">Using ETags to reduce bandwidth</a></li>
<li><a class="reference internal" href="#using-last-modified-headers-to-reduce-bandwidth">Using Last-Modified headers to reduce bandwidth</a></li>
</ul>
</li>
</ul>
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