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<div class="section" id="handling-application-errors">
<span id="application-errors"></span><h1>Handling Application Errors<a class="headerlink" href="#handling-application-errors" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 0.3.</span></p>
<p>Applications fail, servers fail. Sooner or later you will see an exception
in production. Even if your code is 100% correct, you will still see
exceptions from time to time. Why? Because everything else involved will
fail. Here some situations where perfectly fine code can lead to server
errors:</p>
<ul class="simple">
<li>the client terminated the request early and the application was still
reading from the incoming data.</li>
<li>the database server was overloaded and could not handle the query.</li>
<li>a filesystem is full</li>
<li>a harddrive crashed</li>
<li>a backend server overloaded</li>
<li>a programming error in a library you are using</li>
<li>network connection of the server to another system failed.</li>
</ul>
<p>And that’s just a small sample of issues you could be facing. So how do we
deal with that sort of problem? By default if your application runs in
production mode, Flask will display a very simple page for you and log the
exception to the <a class="reference internal" href="api.html#flask.Flask.logger" title="flask.Flask.logger"><tt class="xref py py-attr docutils literal"><span class="pre">logger</span></tt></a>.</p>
<p>But there is more you can do, and we will cover some better setups to deal
with errors.</p>
<div class="section" id="error-mails">
<h2>Error Mails<a class="headerlink" href="#error-mails" title="Permalink to this headline">¶</a></h2>
<p>If the application runs in production mode (which it will do on your
server) you won’t see any log messages by default. Why is that? Flask
tries to be a zero-configuration framework. Where should it drop the logs
for you if there is no configuration? Guessing is not a good idea because
chances are, the place it guessed is not the place where the user has
permission to create a logfile. Also, for most small applications nobody
will look at the logs anyways.</p>
<p>In fact, I promise you right now that if you configure a logfile for the
application errors you will never look at it except for debugging an issue
when a user reported it for you. What you want instead is a mail the
second the exception happened. Then you get an alert and you can do
something about it.</p>
<p>Flask uses the Python builtin logging system, and it can actually send
you mails for errors which is probably what you want. Here is how you can
configure the Flask logger to send you mails for exceptions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">ADMINS</span> <span class="o">=</span> <span class="p">[</span><span class="s">'yourname@example.com'</span><span class="p">]</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">app</span><span class="o">.</span><span class="n">debug</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">from</span> <span class="nn">logging.handlers</span> <span class="kn">import</span> <span class="n">SMTPHandler</span>
<span class="n">mail_handler</span> <span class="o">=</span> <span class="n">SMTPHandler</span><span class="p">(</span><span class="s">'127.0.0.1'</span><span class="p">,</span>
<span class="s">'server-error@example.com'</span><span class="p">,</span>
<span class="n">ADMINS</span><span class="p">,</span> <span class="s">'YourApplication Failed'</span><span class="p">)</span>
<span class="n">mail_handler</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">)</span>
<span class="n">app</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">mail_handler</span><span class="p">)</span>
</pre></div>
</div>
<p>So what just happened? We created a new
<a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.handlers.SMTPHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">SMTPHandler</span></tt></a> that will send mails with the mail
server listening on <tt class="docutils literal"><span class="pre">127.0.0.1</span></tt> to all the <cite>ADMINS</cite> from the address
<em>server-error@example.com</em> with the subject “YourApplication Failed”. If
your mail server requires credentials, these can also be provided. For
that check out the documentation for the
<a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.handlers.SMTPHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">SMTPHandler</span></tt></a>.</p>
<p>We also tell the handler to only send errors and more critical messages.
Because we certainly don’t want to get a mail for warnings or other
useless logs that might happen during request handling.</p>
<p>Before you run that in production, please also look at <a class="reference internal" href="#logformat"><em>Controlling the Log Format</em></a> to
put more information into that error mail. That will save you from a lot
of frustration.</p>
</div>
<div class="section" id="logging-to-a-file">
<h2>Logging to a File<a class="headerlink" href="#logging-to-a-file" title="Permalink to this headline">¶</a></h2>
<p>Even if you get mails, you probably also want to log warnings. It’s a
good idea to keep as much information around that might be required to
debug a problem. Please note that Flask itself will not issue any
warnings in the core system, so it’s your responsibility to warn in the
code if something seems odd.</p>
<p>There are a couple of handlers provided by the logging system out of the
box but not all of them are useful for basic error logging. The most
interesting are probably the following:</p>
<ul class="simple">
<li><a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.FileHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">FileHandler</span></tt></a> - logs messages to a file on the
filesystem.</li>
<li><a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.handlers.RotatingFileHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">RotatingFileHandler</span></tt></a> - logs messages to a file
on the filesystem and will rotate after a certain number of messages.</li>
<li><a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.handlers.NTEventLogHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">NTEventLogHandler</span></tt></a> - will log to the system
event log of a Windows system. If you are deploying on a Windows box,
this is what you want to use.</li>
<li><a class="reference external" href="http://docs.python.org/dev/library/logging.handlers.html#logging.handlers.SysLogHandler" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">SysLogHandler</span></tt></a> - sends logs to a UNIX
syslog.</li>
</ul>
<p>Once you picked your log handler, do like you did with the SMTP handler
above, just make sure to use a lower setting (I would recommend
<cite>WARNING</cite>):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="ow">not</span> <span class="n">app</span><span class="o">.</span><span class="n">debug</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">from</span> <span class="nn">themodule</span> <span class="kn">import</span> <span class="n">TheHandlerYouWant</span>
<span class="n">file_handler</span> <span class="o">=</span> <span class="n">TheHandlerYouWant</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
<span class="n">file_handler</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">)</span>
<span class="n">app</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">file_handler</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="controlling-the-log-format">
<span id="logformat"></span><h2>Controlling the Log Format<a class="headerlink" href="#controlling-the-log-format" title="Permalink to this headline">¶</a></h2>
<p>By default a handler will only write the message string into a file or
send you that message as mail. A log record stores more information,
and it makes a lot of sense to configure your logger to also contain that
information so that you have a better idea of why that error happened, and
more importantly, where it did.</p>
<p>A formatter can be instantiated with a format string. Note that
tracebacks are appended to the log entry automatically. You don’t have to
do that in the log formatter format string.</p>
<p>Here some example setups:</p>
<div class="section" id="email">
<h3>Email<a class="headerlink" href="#email" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="n">Formatter</span>
<span class="n">mail_handler</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">Formatter</span><span class="p">(</span><span class="s">'''</span>
<span class="s">Message type: </span><span class="si">%(levelname)s</span><span class="s"></span>
<span class="s">Location: </span><span class="si">%(pathname)s</span><span class="s">:</span><span class="si">%(lineno)d</span><span class="s"></span>
<span class="s">Module: </span><span class="si">%(module)s</span><span class="s"></span>
<span class="s">Function: </span><span class="si">%(funcName)s</span><span class="s"></span>
<span class="s">Time: </span><span class="si">%(asctime)s</span><span class="s"></span>
<span class="s">Message:</span>
<span class="si">%(message)s</span><span class="s"></span>
<span class="s">'''</span><span class="p">))</span>
</pre></div>
</div>
</div>
<div class="section" id="file-logging">
<h3>File logging<a class="headerlink" href="#file-logging" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="n">Formatter</span>
<span class="n">file_handler</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">Formatter</span><span class="p">(</span>
<span class="s">'</span><span class="si">%(asctime)s</span><span class="s"> </span><span class="si">%(levelname)s</span><span class="s">: </span><span class="si">%(message)s</span><span class="s"> '</span>
<span class="s">'[in </span><span class="si">%(pathname)s</span><span class="s">:</span><span class="si">%(lineno)d</span><span class="s">]'</span>
<span class="p">))</span>
</pre></div>
</div>
</div>
<div class="section" id="complex-log-formatting">
<h3>Complex Log Formatting<a class="headerlink" href="#complex-log-formatting" title="Permalink to this headline">¶</a></h3>
<p>Here is a list of useful formatting variables for the format string. Note
that this list is not complete, consult the official documentation of the
<a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging" title="(in Python v3.3)"><tt class="xref py py-mod docutils literal"><span class="pre">logging</span></tt></a> package for a full list.</p>
<table border="1" class="docutils">
<colgroup>
<col width="26%" />
<col width="74%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Format</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">%(levelname)s</span></tt></td>
<td>Text logging level for the message
(<tt class="docutils literal"><span class="pre">'DEBUG'</span></tt>, <tt class="docutils literal"><span class="pre">'INFO'</span></tt>, <tt class="docutils literal"><span class="pre">'WARNING'</span></tt>,
<tt class="docutils literal"><span class="pre">'ERROR'</span></tt>, <tt class="docutils literal"><span class="pre">'CRITICAL'</span></tt>).</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">%(pathname)s</span></tt></td>
<td>Full pathname of the source file where the
logging call was issued (if available).</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">%(filename)s</span></tt></td>
<td>Filename portion of pathname.</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">%(module)s</span></tt></td>
<td>Module (name portion of filename).</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">%(funcName)s</span></tt></td>
<td>Name of function containing the logging call.</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">%(lineno)d</span></tt></td>
<td>Source line number where the logging call was
issued (if available).</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">%(asctime)s</span></tt></td>
<td>Human-readable time when the LogRecord` was
created. By default this is of the form
<tt class="docutils literal"><span class="pre">"2003-07-08</span> <span class="pre">16:49:45,896"</span></tt> (the numbers after
the comma are millisecond portion of the time).
This can be changed by subclassing the formatter
and overriding the
<a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.Formatter.formatTime" title="(in Python v3.3)"><tt class="xref py py-meth docutils literal"><span class="pre">formatTime()</span></tt></a> method.</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">%(message)s</span></tt></td>
<td>The logged message, computed as <tt class="docutils literal"><span class="pre">msg</span> <span class="pre">%</span> <span class="pre">args</span></tt></td>
</tr>
</tbody>
</table>
<p>If you want to further customize the formatting, you can subclass the
formatter. The formatter has three interesting methods:</p>
<dl class="docutils">
<dt><a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.Formatter.format" title="(in Python v3.3)"><tt class="xref py py-meth docutils literal"><span class="pre">format()</span></tt></a>:</dt>
<dd>handles the actual formatting. It is passed a
<a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.LogRecord" title="(in Python v3.3)"><tt class="xref py py-class docutils literal"><span class="pre">LogRecord</span></tt></a> object and has to return the formatted
string.</dd>
<dt><a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.Formatter.formatTime" title="(in Python v3.3)"><tt class="xref py py-meth docutils literal"><span class="pre">formatTime()</span></tt></a>:</dt>
<dd>called for <cite>asctime</cite> formatting. If you want a different time format
you can override this method.</dd>
<dt><a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.Formatter.formatException" title="(in Python v3.3)"><tt class="xref py py-meth docutils literal"><span class="pre">formatException()</span></tt></a></dt>
<dd>called for exception formatting. It is passed an <tt class="xref py py-attr docutils literal"><span class="pre">exc_info</span></tt>
tuple and has to return a string. The default is usually fine, you
don’t have to override it.</dd>
</dl>
<p>For more information, head over to the official documentation.</p>
</div>
</div>
<div class="section" id="other-libraries">
<h2>Other Libraries<a class="headerlink" href="#other-libraries" title="Permalink to this headline">¶</a></h2>
<p>So far we only configured the logger your application created itself.
Other libraries might log themselves as well. For example, SQLAlchemy uses
logging heavily in its core. While there is a method to configure all
loggers at once in the <a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging" title="(in Python v3.3)"><tt class="xref py py-mod docutils literal"><span class="pre">logging</span></tt></a> package, I would not recommend using
it. There might be a situation in which you want to have multiple
separate applications running side by side in the same Python interpreter
and then it becomes impossible to have different logging setups for those.</p>
<p>Instead, I would recommend figuring out which loggers you are interested
in, getting the loggers with the <a class="reference external" href="http://docs.python.org/dev/library/logging.html#logging.getLogger" title="(in Python v3.3)"><tt class="xref py py-func docutils literal"><span class="pre">getLogger()</span></tt></a> function and
iterating over them to attach handlers:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="n">getLogger</span>
<span class="n">loggers</span> <span class="o">=</span> <span class="p">[</span><span class="n">app</span><span class="o">.</span><span class="n">logger</span><span class="p">,</span> <span class="n">getLogger</span><span class="p">(</span><span class="s">'sqlalchemy'</span><span class="p">),</span>
<span class="n">getLogger</span><span class="p">(</span><span class="s">'otherlibrary'</span><span class="p">)]</span>
<span class="k">for</span> <span class="n">logger</span> <span class="ow">in</span> <span class="n">loggers</span><span class="p">:</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">mail_handler</span><span class="p">)</span>
<span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">file_handler</span><span class="p">)</span>
</pre></div>
</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="#">Handling Application Errors</a><ul>
<li><a class="reference internal" href="#error-mails">Error Mails</a></li>
<li><a class="reference internal" href="#logging-to-a-file">Logging to a File</a></li>
<li><a class="reference internal" href="#controlling-the-log-format">Controlling the Log Format</a><ul>
<li><a class="reference internal" href="#email">Email</a></li>
<li><a class="reference internal" href="#file-logging">File logging</a></li>
<li><a class="reference internal" href="#complex-log-formatting">Complex Log Formatting</a></li>
</ul>
</li>
<li><a class="reference internal" href="#other-libraries">Other Libraries</a></li>
</ul>
</li>
</ul>
<h3>Related Topics</h3>
<ul>
<li><a href="index.html">Documentation overview</a><ul>
<li>Previous: <a href="testing.html" title="previous chapter">Testing Flask Applications</a></li>
<li>Next: <a href="config.html" title="next chapter">Configuration Handling</a></li>
</ul></li>
</ul>
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