doc/web/howto/web-in-60/logging-errors.html

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   3 <title>Twisted Documentation: Logging Errors</title>
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   8     <h1 class="title">Logging Errors</h1>
   9     <div class="toc"><ol/></div>
  10     <div class="content">
  11 <span/>
  12
  13 <p>The <a href="interrupted.html" shape="rect">previous example</a> created a server that
  14 dealt with response errors by aborting response generation, potentially avoiding
  15 pointless work. However, it did this silently for any error. In this example,
  16 we'll modify the previous example so that it logs each failed response.</p>
  17
  18 <p>This example will use the Twisted API for logging errors. As was
  19 mentioned in the <a href="asynchronous-deferred.html" shape="rect">first example
  20 covering Deferreds</a>, errbacks are passed an error. In the previous
  21 example, the <code>_responseFailed</code> errback accepted this error
  22 as a parameter but ignored it. The only way this example will differ
  23 is that this <code>_responseFailed</code> will use that error
  24 parameter to log a message.</p>
  25
  26 <p>This example will require all of the imports required by the previous example
  27 plus one new import:</p>
  28
  29 <pre class="python"><p class="py-linenumber">1
  30 </p><span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">python</span>.<span class="py-src-variable">log</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">err</span>
  31 </pre>
  32
  33 <p>The only other part of the previous example which changes is
  34 the <code>_responseFailed</code> callback, which will now log the
  35 error passed to it:</p>
  36
  37 <pre class="python"><p class="py-linenumber">1
  38 2
  39 3
  40 4
  41 </p>...
  42     <span class="py-src-keyword">def</span> <span class="py-src-identifier">_responseFailed</span>(<span class="py-src-parameter">self</span>, <span class="py-src-parameter">failure</span>, <span class="py-src-parameter">call</span>):
  43         <span class="py-src-variable">call</span>.<span class="py-src-variable">cancel</span>()
  44         <span class="py-src-variable">err</span>(<span class="py-src-variable">failure</span>, <span class="py-src-string">&quot;Async response demo interrupted response&quot;</span>)
  45 </pre>
  46
  47 <p>We're passing two arguments to <code class="API"><a href="http://twistedmatrix.com/documents/12.1.0/api/twisted.python.log.err.html" title="twisted.python.log.err">err</a></code> here. The first is the error which is being
  48 passed in to the callback. This is always an object of type <code class="API"><a href="http://twistedmatrix.com/documents/12.1.0/api/twisted.python.failure.Failure.html" title="twisted.python.failure.Failure">Failure</a></code>, a class which represents an
  49 exception and (sometimes, but not always) a traceback. <code>err</code> will
  50 format this nicely for the log. The second argument is a descriptive string that
  51 tells someone reading the log what the source of the error was.</p>
  52
  53 <p>Here's the full example with the two above modifications:</p>
  54
  55 <pre class="python"><p class="py-linenumber"> 1
  56  2
  57  3
  58  4
  59  5
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  74 20
  75 </p><span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">web</span>.<span class="py-src-variable">resource</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">Resource</span>
  76 <span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">web</span>.<span class="py-src-variable">server</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">NOT_DONE_YET</span>
  77 <span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">internet</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">reactor</span>
  78 <span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">python</span>.<span class="py-src-variable">log</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">err</span>
  79
  80 <span class="py-src-keyword">class</span> <span class="py-src-identifier">DelayedResource</span>(<span class="py-src-parameter">Resource</span>):
  81     <span class="py-src-keyword">def</span> <span class="py-src-identifier">_delayedRender</span>(<span class="py-src-parameter">self</span>, <span class="py-src-parameter">request</span>):
  82         <span class="py-src-variable">request</span>.<span class="py-src-variable">write</span>(<span class="py-src-string">&quot;&lt;html&gt;&lt;body&gt;Sorry to keep you waiting.&lt;/body&gt;&lt;/html&gt;&quot;</span>)
  83         <span class="py-src-variable">request</span>.<span class="py-src-variable">finish</span>()
  84
  85     <span class="py-src-keyword">def</span> <span class="py-src-identifier">_responseFailed</span>(<span class="py-src-parameter">self</span>, <span class="py-src-parameter">failure</span>, <span class="py-src-parameter">call</span>):
  86         <span class="py-src-variable">call</span>.<span class="py-src-variable">cancel</span>()
  87         <span class="py-src-variable">err</span>(<span class="py-src-variable">failure</span>, <span class="py-src-string">&quot;Async response demo interrupted response&quot;</span>)
  88
  89     <span class="py-src-keyword">def</span> <span class="py-src-identifier">render_GET</span>(<span class="py-src-parameter">self</span>, <span class="py-src-parameter">request</span>):
  90         <span class="py-src-variable">call</span> = <span class="py-src-variable">reactor</span>.<span class="py-src-variable">callLater</span>(<span class="py-src-number">5</span>, <span class="py-src-variable">self</span>.<span class="py-src-variable">_delayedRender</span>, <span class="py-src-variable">request</span>)
  91         <span class="py-src-variable">request</span>.<span class="py-src-variable">notifyFinish</span>().<span class="py-src-variable">addErrback</span>(<span class="py-src-variable">self</span>.<span class="py-src-variable">_responseFailed</span>, <span class="py-src-variable">call</span>)
  92         <span class="py-src-keyword">return</span> <span class="py-src-variable">NOT_DONE_YET</span>
  93
  94 <span class="py-src-variable">resource</span> = <span class="py-src-variable">DelayedResource</span>()
  95 </pre>
  96
  97 <p>Run this server as in the <a href="interrupted.html" shape="rect">previous example</a>
  98 and interrupt a request. Unlike the previous example, where the server gave no
  99 indication that this had happened, you'll see a message in the log output with
 100 this version.</p>
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