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<div class="section" id="custom-error-pages">
<h1>Custom Error Pages<a class="headerlink" href="#custom-error-pages" title="Permalink to this headline">¶</a></h1>
<p>Flask comes with a handy <a class="reference internal" href="../api.html#flask.abort" title="flask.abort"><tt class="xref py py-func docutils literal"><span class="pre">abort()</span></tt></a> function that aborts a
request with an HTTP error code early. It will also provide a plain black
and white error page for you with a basic description, but nothing fancy.</p>
<p>Depending on the error code it is less or more likely for the user to
actually see such an error.</p>
<div class="section" id="common-error-codes">
<h2>Common Error Codes<a class="headerlink" href="#common-error-codes" title="Permalink to this headline">¶</a></h2>
<p>The following error codes are some that are often displayed to the user,
even if the application behaves correctly:</p>
<dl class="docutils">
<dt><em>404 Not Found</em></dt>
<dd>The good old “chap, you made a mistake typing that URL” message. So
common that even novices to the internet know that 404 means: damn,
the thing I was looking for is not there. It’s a very good idea to
make sure there is actually something useful on a 404 page, at least a
link back to the index.</dd>
<dt><em>403 Forbidden</em></dt>
<dd>If you have some kind of access control on your website, you will have
to send a 403 code for disallowed resources. So make sure the user
is not lost when they try to access a forbidden resource.</dd>
<dt><em>410 Gone</em></dt>
<dd>Did you know that there the “404 Not Found” has a brother named “410
Gone”? Few people actually implement that, but the idea is that
resources that previously existed and got deleted answer with 410
instead of 404. If you are not deleting documents permanently from
the database but just mark them as deleted, do the user a favour and
use the 410 code instead and display a message that what they were
looking for was deleted for all eternity.</dd>
<dt><em>500 Internal Server Error</em></dt>
<dd>Usually happens on programming errors or if the server is overloaded.
A terrible good idea to have a nice page there, because your
application <em>will</em> fail sooner or later (see also:
<a class="reference internal" href="../errorhandling.html#application-errors"><em>Handling Application Errors</em></a>).</dd>
</dl>
</div>
<div class="section" id="error-handlers">
<h2>Error Handlers<a class="headerlink" href="#error-handlers" title="Permalink to this headline">¶</a></h2>
<p>An error handler is a function, just like a view function, but it is
called when an error happens and is passed that error. The error is most
likely a <a class="reference external" href="http://werkzeug.pocoo.org/docs/exceptions/#werkzeug.exceptions.HTTPException" title="(in Werkzeug v0.7)"><tt class="xref py py-exc docutils literal"><span class="pre">HTTPException</span></tt></a>, but in one case it
can be a different error: a handler for internal server errors will be
passed other exception instances as well if they are uncaught.</p>
<p>An error handler is registered with the <a class="reference internal" href="../api.html#flask.Flask.errorhandler" title="flask.Flask.errorhandler"><tt class="xref py py-meth docutils literal"><span class="pre">errorhandler()</span></tt></a>
decorator and the error code of the exception. Keep in mind that Flask
will <em>not</em> set the error code for you, so make sure to also provide the
HTTP status code when returning a response.</p>
<p>Here an example implementation for a “404 Page Not Found” exception:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">render_template</span>
<span class="nd">@app.errorhandler</span><span class="p">(</span><span class="mi">404</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">page_not_found</span><span class="p">(</span><span class="n">e</span><span class="p">):</span>
<span class="k">return</span> <span class="n">render_template</span><span class="p">(</span><span class="s">'404.html'</span><span class="p">),</span> <span class="mi">404</span>
</pre></div>
</div>
<p>An example template might be this:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="cp">{%</span> <span class="k">extends</span> <span class="s2">"layout.html"</span> <span class="cp">%}</span>
<span class="cp">{%</span> <span class="k">block</span> <span class="nv">title</span> <span class="cp">%}</span>Page Not Found<span class="cp">{%</span> <span class="k">endblock</span> <span class="cp">%}</span>
<span class="cp">{%</span> <span class="k">block</span> <span class="nv">body</span> <span class="cp">%}</span>
<span class="nt"><h1></span>Page Not Found<span class="nt"></h1></span>
<span class="nt"><p></span>What you were looking for is just not there.
<span class="nt"><p><a</span> <span class="na">href=</span><span class="s">"</span><span class="cp">{{</span> <span class="nv">url_for</span><span class="o">(</span><span class="s1">'index'</span><span class="o">)</span> <span class="cp">}}</span><span class="s">"</span><span class="nt">></span>go somewhere nice<span class="nt"></a></span>
<span class="cp">{%</span> <span class="k">endblock</span> <span class="cp">%}</span>
</pre></div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Custom Error Pages</a><ul>
<li><a class="reference internal" href="#common-error-codes">Common Error Codes</a></li>
<li><a class="reference internal" href="#error-handlers">Error Handlers</a></li>
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