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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&#8217;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&#8217;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">&#39;yourname@example.com&#39;</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">&#39;127.0.0.1&#39;</span><span class="p">,</span>
- <span class="s">&#39;server-error@example.com&#39;</span><span class="p">,</span>
- <span class="n">ADMINS</span><span class="p">,</span> <span class="s">&#39;YourApplication Failed&#39;</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&#64;example.com</em> with the subject &#8220;YourApplication Failed&#8221;. 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&#8217;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&#8217;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&#8217;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&#8217;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">&#39;&#39;&#39;</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">&#39;&#39;&#39;</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">&#39;</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"> &#39;</span>
- <span class="s">&#39;[in </span><span class="si">%(pathname)s</span><span class="s">:</span><span class="si">%(lineno)d</span><span class="s">]&#39;</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">&quot;2003-07-08</span> <span class="pre">16:49:45,896&quot;</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&#8217;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">&#39;sqlalchemy&#39;</span><span class="p">),</span>
- <span class="n">getLogger</span><span class="p">(</span><span class="s">&#39;otherlibrary&#39;</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>
- </div>
- </div>
- <div class="sphinxsidebar">
- <div class="sphinxsidebarwrapper"><p class="logo"><a href="index.html">
- <img class="logo" src="_static/flask.png" alt="Logo"/>
-</a></p>
- <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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