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<h1 class="heading"><a href="index.html"
title="back to the documentation overview"><span>Werkzeug</span></a></h1>
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<li class="indexlink"><a href="index.html">Overview</a></li>
<li><a href="test.html">« Test Utilities</a></li>
<li class="active"><a href="#">Debugging Applications</a></li>
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<h3>Table Of Contents</h3>
<div class="inner"><ul>
<li><a class="reference external" href="#">Debugging Applications</a><ul>
<li><a class="reference external" href="#enabling-the-debugger">Enabling the Debugger</a></li>
<li><a class="reference external" href="#using-the-debugger">Using the Debugger</a></li>
<li><a class="reference external" href="#pasting-errors">Pasting Errors</a></li>
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<div class="section" id="module-werkzeug">
<span id="debugging-applications"></span><h1>Debugging Applications<a class="headerlink" href="#module-werkzeug" title="Permalink to this headline">¶</a></h1>
<p>Depending on the WSGI gateway/server, exceptions are handled differently.
But most of the time, exceptions go to stderr or the error log.</p>
<p>Since this is not the best debugging environment, Werkzeug provides a
WSGI middleware that renders nice debugging tracebacks, optionally with an
AJAX based debugger (which allows to execute code in the context of the
traceback’s frames).</p>
<p>The interactive debugger however does not work in forking environments
which makes it nearly impossible to use on production servers. Also the
debugger allows the execution of arbitrary code which makes it a major
security risk and <strong>must never be used on production machines</strong> because of
that.</p>
<div class="section" id="enabling-the-debugger">
<h2>Enabling the Debugger<a class="headerlink" href="#enabling-the-debugger" title="Permalink to this headline">¶</a></h2>
<p>You can enable the debugger by wrapping the application in a
<a title="werkzeug.DebuggedApplication" class="reference internal" href="#werkzeug.DebuggedApplication"><tt class="xref py py-class docutils literal"><span class="pre">DebuggedApplication</span></tt></a> middleware. Additionally there are
parameters to the <a title="werkzeug.run_simple" class="reference external" href="serving.html#werkzeug.run_simple"><tt class="xref py py-func docutils literal"><span class="pre">run_simple()</span></tt></a> function to enable it because this
is a common task during development.</p>
<dl class="class">
<dt id="werkzeug.DebuggedApplication">
<em class="property">class </em><tt class="descclassname">werkzeug.</tt><tt class="descname">DebuggedApplication</tt><big>(</big><em>app</em>, <em>evalex=False</em>, <em>request_key='werkzeug.request'</em>, <em>console_path='/console'</em>, <em>console_init_func=None</em>, <em>show_hidden_frames=False</em><big>)</big><a class="headerlink" href="#werkzeug.DebuggedApplication" title="Permalink to this definition">¶</a></dt>
<dd><p>Enables debugging support for a given application:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug.debug</span> <span class="kn">import</span> <span class="n">DebuggedApplication</span>
<span class="kn">from</span> <span class="nn">myapp</span> <span class="kn">import</span> <span class="n">app</span>
<span class="n">app</span> <span class="o">=</span> <span class="n">DebuggedApplication</span><span class="p">(</span><span class="n">app</span><span class="p">,</span> <span class="n">evalex</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
</pre></div>
</div>
<p>The <cite>evalex</cite> keyword argument allows evaluating expressions in a
traceback’s frame context.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>app</strong> – the WSGI application to run debugged.</li>
<li><strong>evalex</strong> – enable exception evaluation feature (interactive
debugging). This requires a non-forking server.</li>
<li><strong>request_key</strong> – The key that points to the request object in ths
environment. This parameter is ignored in current
versions.</li>
<li><strong>console_path</strong> – the URL for a general purpose console.</li>
<li><strong>console_init_func</strong> – the function that is executed before starting
the general purpose console. The return value
is used as initial namespace.</li>
<li><strong>show_hidden_frames</strong> – by default hidden traceback frames are skipped.
You can show them by setting this parameter
to <cite>True</cite>.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
</div>
<div class="section" id="using-the-debugger">
<h2>Using the Debugger<a class="headerlink" href="#using-the-debugger" title="Permalink to this headline">¶</a></h2>
<p>Once enabled and an error happens during a request you will see a detailed
traceback instead of a general “internal server error”. If you have the
<cite>evalex</cite> feature enabled you can also get a traceback for every frame in
the traceback by clicking on the console icon.</p>
<p>Once clicked a console opens where you can execute Python code in:</p>
<div align="center" class="align-center"><img alt="a screenshot of the interactive debugger" class="align-center" src="_images/debug-screenshot.png" /></div>
<p>Inside the interactive consoles you can execute any kind of Python code.
Unlike regular Python consoles the output of the object reprs is colored
and stripped to a reasonable size by default. If the output is longer
than what the console decides to display a small plus sign is added to
the repr and a click will expand the repr.</p>
<p>To display all variables that are defined in the current frame you can
use the <cite>dump()</cite> function. You can call it without arguments to get a
detailed list of all variables and their values, or with an object as
argument to get a detailed list of all the attributes it has.</p>
</div>
<div class="section" id="pasting-errors">
<h2>Pasting Errors<a class="headerlink" href="#pasting-errors" title="Permalink to this headline">¶</a></h2>
<p>If you click on the <cite>Traceback</cite> title the traceback switches over to a text
based one. The text based one can be pasted to <a class="reference external" href="http://paste.pocoo.org/">paste.pocoo.org</a> with one
click.</p>
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