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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Logging HOWTO — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="Logging Cookbook" href="logging-cookbook.html" /> <link rel="prev" title="Functional Programming HOWTO" href="functional.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/howto/logging.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/howto/logging.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="logging-cookbook.html" title="Logging Cookbook" accesskey="N">next</a> |</li> <li class="right" > <a href="functional.html" title="Functional Programming HOWTO" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python HOWTOs</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="logging-howto"> <h1>Logging HOWTO<a class="headerlink" href="#logging-howto" title="Permalink to this headline">¶</a></h1> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">Vinay Sajip <vinay_sajip at red-dove dot com></td> </tr> </tbody> </table> <div class="section" id="basic-logging-tutorial"> <span id="logging-basic-tutorial"></span><h2>Basic Logging Tutorial<a class="headerlink" href="#basic-logging-tutorial" title="Permalink to this headline">¶</a></h2> <p>Logging is a means of tracking events that happen when some software runs. The software’s developer adds logging calls to their code to indicate that certain events have occurred. An event is described by a descriptive message which can optionally contain variable data (i.e. data that is potentially different for each occurrence of the event). Events also have an importance which the developer ascribes to the event; the importance can also be called the <em>level</em> or <em>severity</em>.</p> <div class="section" id="when-to-use-logging"> <h3>When to use logging<a class="headerlink" href="#when-to-use-logging" title="Permalink to this headline">¶</a></h3> <p>Logging provides a set of convenience functions for simple logging usage. These are <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-func docutils literal notranslate"><span class="pre">info()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.warning" title="logging.warning"><code class="xref py py-func docutils literal notranslate"><span class="pre">warning()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.error" title="logging.error"><code class="xref py py-func docutils literal notranslate"><span class="pre">error()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-func docutils literal notranslate"><span class="pre">critical()</span></code></a>. To determine when to use logging, see the table below, which states, for each of a set of common tasks, the best tool to use for it.</p> <table border="1" class="docutils"> <colgroup> <col width="49%" /> <col width="51%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Task you want to perform</th> <th class="head">The best tool for the task</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td>Display console output for ordinary usage of a command line script or program</td> <td><a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a></td> </tr> <tr class="row-odd"><td>Report events that occur during normal operation of a program (e.g. for status monitoring or fault investigation)</td> <td><a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.info()</span></code></a> (or <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.debug()</span></code></a> for very detailed output for diagnostic purposes)</td> </tr> <tr class="row-even"><td>Issue a warning regarding a particular runtime event</td> <td><p class="first"><a class="reference internal" href="../library/warnings.html#warnings.warn" title="warnings.warn"><code class="xref py py-func docutils literal notranslate"><span class="pre">warnings.warn()</span></code></a> in library code if the issue is avoidable and the client application should be modified to eliminate the warning</p> <p class="last"><a class="reference internal" href="../library/logging.html#logging.warning" title="logging.warning"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.warning()</span></code></a> if there is nothing the client application can do about the situation, but the event should still be noted</p> </td> </tr> <tr class="row-odd"><td>Report an error regarding a particular runtime event</td> <td>Raise an exception</td> </tr> <tr class="row-even"><td>Report suppression of an error without raising an exception (e.g. error handler in a long-running server process)</td> <td><a class="reference internal" href="../library/logging.html#logging.error" title="logging.error"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.error()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.exception" title="logging.exception"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.exception()</span></code></a> or <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-func docutils literal notranslate"><span class="pre">logging.critical()</span></code></a> as appropriate for the specific error and application domain</td> </tr> </tbody> </table> <p>The logging functions are named after the level or severity of the events they are used to track. The standard levels and their applicability are described below (in increasing order of severity):</p> <table border="1" class="docutils"> <colgroup> <col width="24%" /> <col width="76%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Level</th> <th class="head">When it’s used</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">DEBUG</span></code></td> <td>Detailed information, typically of interest only when diagnosing problems.</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">INFO</span></code></td> <td>Confirmation that things are working as expected.</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">WARNING</span></code></td> <td>An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected.</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">ERROR</span></code></td> <td>Due to a more serious problem, the software has not been able to perform some function.</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">CRITICAL</span></code></td> <td>A serious error, indicating that the program itself may be unable to continue running.</td> </tr> </tbody> </table> <p>The default level is <code class="docutils literal notranslate"><span class="pre">WARNING</span></code>, which means that only events of this level and above will be tracked, unless the logging package is configured to do otherwise.</p> <p>Events that are tracked can be handled in different ways. The simplest way of handling tracked events is to print them to the console. Another common way is to write them to a disk file.</p> </div> <div class="section" id="a-simple-example"> <span id="howto-minimal-example"></span><h3>A simple example<a class="headerlink" href="#a-simple-example" title="Permalink to this headline">¶</a></h3> <p>A very simple example is:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Watch out!'</span><span class="p">)</span> <span class="c1"># will print a message to the console</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'I told you so'</span><span class="p">)</span> <span class="c1"># will not print anything</span> </pre></div> </div> <p>If you type these lines into a script and run it, you’ll see:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>WARNING:root:Watch out! </pre></div> </div> <p>printed out on the console. The <code class="docutils literal notranslate"><span class="pre">INFO</span></code> message doesn’t appear because the default level is <code class="docutils literal notranslate"><span class="pre">WARNING</span></code>. The printed message includes the indication of the level and the description of the event provided in the logging call, i.e. ‘Watch out!’. Don’t worry about the ‘root’ part for now: it will be explained later. The actual output can be formatted quite flexibly if you need that; formatting options will also be explained later.</p> </div> <div class="section" id="logging-to-a-file"> <h3>Logging to a file<a class="headerlink" href="#logging-to-a-file" title="Permalink to this headline">¶</a></h3> <p>A very common situation is that of recording logging events in a file, so let’s look at that next. Be sure to try the following in a newly-started Python interpreter, and don’t just continue from the session described above:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">filename</span><span class="o">=</span><span class="s1">'example.log'</span><span class="p">,</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'This message should go to the log file'</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'So should this'</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="s1">'And this, too'</span><span class="p">)</span> </pre></div> </div> <p>And now if we open the file and look at what we have, we should find the log messages:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">DEBUG</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">This</span> <span class="n">message</span> <span class="n">should</span> <span class="n">go</span> <span class="n">to</span> <span class="n">the</span> <span class="n">log</span> <span class="n">file</span> <span class="n">INFO</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">So</span> <span class="n">should</span> <span class="n">this</span> <span class="n">WARNING</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">And</span> <span class="n">this</span><span class="p">,</span> <span class="n">too</span> </pre></div> </div> <p>This example also shows how you can set the logging level which acts as the threshold for tracking. In this case, because we set the threshold to <code class="docutils literal notranslate"><span class="pre">DEBUG</span></code>, all of the messages were printed.</p> <p>If you want to set the logging level from a command-line option such as:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">--</span><span class="n">log</span><span class="o">=</span><span class="n">INFO</span> </pre></div> </div> <p>and you have the value of the parameter passed for <code class="docutils literal notranslate"><span class="pre">--log</span></code> in some variable <em>loglevel</em>, you can use:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">getattr</span><span class="p">(</span><span class="n">logging</span><span class="p">,</span> <span class="n">loglevel</span><span class="o">.</span><span class="n">upper</span><span class="p">())</span> </pre></div> </div> <p>to get the value which you’ll pass to <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">basicConfig()</span></code></a> via the <em>level</em> argument. You may want to error check any user input value, perhaps as in the following example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># assuming loglevel is bound to the string value obtained from the</span> <span class="c1"># command line argument. Convert to upper case to allow the user to</span> <span class="c1"># specify --log=DEBUG or --log=debug</span> <span class="n">numeric_level</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">logging</span><span class="p">,</span> <span class="n">loglevel</span><span class="o">.</span><span class="n">upper</span><span class="p">(),</span> <span class="kc">None</span><span class="p">)</span> <span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">numeric_level</span><span class="p">,</span> <span class="nb">int</span><span class="p">):</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'Invalid log level: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">loglevel</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">numeric_level</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span> </pre></div> </div> <p>The call to <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">basicConfig()</span></code></a> should come <em>before</em> any calls to <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-func docutils literal notranslate"><span class="pre">info()</span></code></a> etc. As it’s intended as a one-off simple configuration facility, only the first call will actually do anything: subsequent calls are effectively no-ops.</p> <p>If you run the above script several times, the messages from successive runs are appended to the file <em>example.log</em>. If you want each run to start afresh, not remembering the messages from earlier runs, you can specify the <em>filemode</em> argument, by changing the call in the above example to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">filename</span><span class="o">=</span><span class="s1">'example.log'</span><span class="p">,</span> <span class="n">filemode</span><span class="o">=</span><span class="s1">'w'</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span> </pre></div> </div> <p>The output will be the same as before, but the log file is no longer appended to, so the messages from earlier runs are lost.</p> </div> <div class="section" id="logging-from-multiple-modules"> <h3>Logging from multiple modules<a class="headerlink" href="#logging-from-multiple-modules" title="Permalink to this headline">¶</a></h3> <p>If your program consists of multiple modules, here’s an example of how you could organize logging in it:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># myapp.py</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="nn">mylib</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">filename</span><span class="o">=</span><span class="s1">'myapp.log'</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Started'</span><span class="p">)</span> <span class="n">mylib</span><span class="o">.</span><span class="n">do_something</span><span class="p">()</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Finished'</span><span class="p">)</span> <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span> <span class="n">main</span><span class="p">()</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># mylib.py</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="k">def</span> <span class="nf">do_something</span><span class="p">():</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Doing something'</span><span class="p">)</span> </pre></div> </div> <p>If you run <em>myapp.py</em>, you should see this in <em>myapp.log</em>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">INFO</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">Started</span> <span class="n">INFO</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">Doing</span> <span class="n">something</span> <span class="n">INFO</span><span class="p">:</span><span class="n">root</span><span class="p">:</span><span class="n">Finished</span> </pre></div> </div> <p>which is hopefully what you were expecting to see. You can generalize this to multiple modules, using the pattern in <em>mylib.py</em>. Note that for this simple usage pattern, you won’t know, by looking in the log file, <em>where</em> in your application your messages came from, apart from looking at the event description. If you want to track the location of your messages, you’ll need to refer to the documentation beyond the tutorial level – see <a class="reference internal" href="#logging-advanced-tutorial"><span class="std std-ref">Advanced Logging Tutorial</span></a>.</p> </div> <div class="section" id="logging-variable-data"> <h3>Logging variable data<a class="headerlink" href="#logging-variable-data" title="Permalink to this headline">¶</a></h3> <p>To log variable data, use a format string for the event description message and append the variable data as arguments. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="s1"> before you </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'Look'</span><span class="p">,</span> <span class="s1">'leap!'</span><span class="p">)</span> </pre></div> </div> <p>will display:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>WARNING:root:Look before you leap! </pre></div> </div> <p>As you can see, merging of variable data into the event description message uses the old, %-style of string formatting. This is for backwards compatibility: the logging package pre-dates newer formatting options such as <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.format()</span></code></a> and <a class="reference internal" href="../library/string.html#string.Template" title="string.Template"><code class="xref py py-class docutils literal notranslate"><span class="pre">string.Template</span></code></a>. These newer formatting options <em>are</em> supported, but exploring them is outside the scope of this tutorial.</p> </div> <div class="section" id="changing-the-format-of-displayed-messages"> <h3>Changing the format of displayed messages<a class="headerlink" href="#changing-the-format-of-displayed-messages" title="Permalink to this headline">¶</a></h3> <p>To change the format which is used to display messages, you need to specify the format you want to use:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(levelname)s</span><span class="s1">:</span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'This message should appear on the console'</span><span class="p">)</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'So should this'</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="s1">'And this, too'</span><span class="p">)</span> </pre></div> </div> <p>which would print:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">DEBUG</span><span class="p">:</span><span class="n">This</span> <span class="n">message</span> <span class="n">should</span> <span class="n">appear</span> <span class="n">on</span> <span class="n">the</span> <span class="n">console</span> <span class="n">INFO</span><span class="p">:</span><span class="n">So</span> <span class="n">should</span> <span class="n">this</span> <span class="n">WARNING</span><span class="p">:</span><span class="n">And</span> <span class="n">this</span><span class="p">,</span> <span class="n">too</span> </pre></div> </div> <p>Notice that the ‘root’ which appeared in earlier examples has disappeared. For a full set of things that can appear in format strings, you can refer to the documentation for <a class="reference internal" href="../library/logging.html#logrecord-attributes"><span class="std std-ref">LogRecord attributes</span></a>, but for simple usage, you just need the <em>levelname</em> (severity), <em>message</em> (event description, including variable data) and perhaps to display when the event occurred. This is described in the next section.</p> </div> <div class="section" id="displaying-the-date-time-in-messages"> <h3>Displaying the date/time in messages<a class="headerlink" href="#displaying-the-date-time-in-messages" title="Permalink to this headline">¶</a></h3> <p>To display the date and time of an event, you would place ‘%(asctime)s’ in your format string:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</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="s1">'is when this event was logged.'</span><span class="p">)</span> </pre></div> </div> <p>which should print something like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">2010</span><span class="o">-</span><span class="mi">12</span><span class="o">-</span><span class="mi">12</span> <span class="mi">11</span><span class="p">:</span><span class="mi">41</span><span class="p">:</span><span class="mi">42</span><span class="p">,</span><span class="mi">612</span> <span class="ow">is</span> <span class="n">when</span> <span class="n">this</span> <span class="n">event</span> <span class="n">was</span> <span class="n">logged</span><span class="o">.</span> </pre></div> </div> <p>The default format for date/time display (shown above) is ISO8601. If you need more control over the formatting of the date/time, provide a <em>datefmt</em> argument to <code class="docutils literal notranslate"><span class="pre">basicConfig</span></code>, as in this example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">,</span> <span class="n">datefmt</span><span class="o">=</span><span class="s1">'%m/</span><span class="si">%d</span><span class="s1">/%Y %I:%M:%S %p'</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="s1">'is when this event was logged.'</span><span class="p">)</span> </pre></div> </div> <p>which would display something like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">12</span><span class="o">/</span><span class="mi">12</span><span class="o">/</span><span class="mi">2010</span> <span class="mi">11</span><span class="p">:</span><span class="mi">46</span><span class="p">:</span><span class="mi">36</span> <span class="n">AM</span> <span class="ow">is</span> <span class="n">when</span> <span class="n">this</span> <span class="n">event</span> <span class="n">was</span> <span class="n">logged</span><span class="o">.</span> </pre></div> </div> <p>The format of the <em>datefmt</em> argument is the same as supported by <a class="reference internal" href="../library/time.html#time.strftime" title="time.strftime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.strftime()</span></code></a>.</p> </div> <div class="section" id="next-steps"> <h3>Next Steps<a class="headerlink" href="#next-steps" title="Permalink to this headline">¶</a></h3> <p>That concludes the basic tutorial. It should be enough to get you up and running with logging. There’s a lot more that the logging package offers, but to get the best out of it, you’ll need to invest a little more of your time in reading the following sections. If you’re ready for that, grab some of your favourite beverage and carry on.</p> <p>If your logging needs are simple, then use the above examples to incorporate logging into your own scripts, and if you run into problems or don’t understand something, please post a question on the comp.lang.python Usenet group (available at <a class="reference external" href="https://groups.google.com/group/comp.lang.python">https://groups.google.com/group/comp.lang.python</a>) and you should receive help before too long.</p> <p>Still here? You can carry on reading the next few sections, which provide a slightly more advanced/in-depth tutorial than the basic one above. After that, you can take a look at the <a class="reference internal" href="logging-cookbook.html#logging-cookbook"><span class="std std-ref">Logging Cookbook</span></a>.</p> </div> </div> <div class="section" id="advanced-logging-tutorial"> <span id="logging-advanced-tutorial"></span><h2>Advanced Logging Tutorial<a class="headerlink" href="#advanced-logging-tutorial" title="Permalink to this headline">¶</a></h2> <p>The logging library takes a modular approach and offers several categories of components: loggers, handlers, filters, and formatters.</p> <ul class="simple"> <li>Loggers expose the interface that application code directly uses.</li> <li>Handlers send the log records (created by loggers) to the appropriate destination.</li> <li>Filters provide a finer grained facility for determining which log records to output.</li> <li>Formatters specify the layout of log records in the final output.</li> </ul> <p>Log event information is passed between loggers, handlers, filters and formatters in a <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate"><span class="pre">LogRecord</span></code></a> instance.</p> <p>Logging is performed by calling methods on instances of the <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate"><span class="pre">Logger</span></code></a> class (hereafter called <em class="dfn">loggers</em>). Each instance has a name, and they are conceptually arranged in a namespace hierarchy using dots (periods) as separators. For example, a logger named ‘scan’ is the parent of loggers ‘scan.text’, ‘scan.html’ and ‘scan.pdf’. Logger names can be anything you want, and indicate the area of an application in which a logged message originates.</p> <p>A good convention to use when naming loggers is to use a module-level logger, in each module which uses logging, named as follows:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="vm">__name__</span><span class="p">)</span> </pre></div> </div> <p>This means that logger names track the package/module hierarchy, and it’s intuitively obvious where events are logged just from the logger name.</p> <p>The root of the hierarchy of loggers is called the root logger. That’s the logger used by the functions <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-func docutils literal notranslate"><span class="pre">info()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.warning" title="logging.warning"><code class="xref py py-func docutils literal notranslate"><span class="pre">warning()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.error" title="logging.error"><code class="xref py py-func docutils literal notranslate"><span class="pre">error()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-func docutils literal notranslate"><span class="pre">critical()</span></code></a>, which just call the same-named method of the root logger. The functions and the methods have the same signatures. The root logger’s name is printed as ‘root’ in the logged output.</p> <p>It is, of course, possible to log messages to different destinations. Support is included in the package for writing log messages to files, HTTP GET/POST locations, email via SMTP, generic sockets, or OS-specific logging mechanisms such as syslog or the Windows NT event log. Destinations are served by <em class="dfn">handler</em> classes. You can create your own log destination class if you have special requirements not met by any of the built-in handler classes.</p> <p>By default, no destination is set for any logging messages. You can specify a destination (such as console or file) by using <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">basicConfig()</span></code></a> as in the tutorial examples. If you call the functions <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-func docutils literal notranslate"><span class="pre">info()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.warning" title="logging.warning"><code class="xref py py-func docutils literal notranslate"><span class="pre">warning()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.error" title="logging.error"><code class="xref py py-func docutils literal notranslate"><span class="pre">error()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-func docutils literal notranslate"><span class="pre">critical()</span></code></a>, they will check to see if no destination is set; and if one is not set, they will set a destination of the console (<code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code>) and a default format for the displayed message before delegating to the root logger to do the actual message output.</p> <p>The default format set by <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">basicConfig()</span></code></a> for messages is:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">severity</span><span class="p">:</span><span class="n">logger</span> <span class="n">name</span><span class="p">:</span><span class="n">message</span> </pre></div> </div> <p>You can change this by passing a format string to <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">basicConfig()</span></code></a> with the <em>format</em> keyword argument. For all options regarding how a format string is constructed, see <a class="reference internal" href="../library/logging.html#formatter-objects"><span class="std std-ref">Formatter Objects</span></a>.</p> <div class="section" id="logging-flow"> <h3>Logging Flow<a class="headerlink" href="#logging-flow" title="Permalink to this headline">¶</a></h3> <p>The flow of log event information in loggers and handlers is illustrated in the following diagram.</p> <img alt="../_images/logging_flow.png" src="../_images/logging_flow.png" /> </div> <div class="section" id="loggers"> <h3>Loggers<a class="headerlink" href="#loggers" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate"><span class="pre">Logger</span></code></a> objects have a threefold job. First, they expose several methods to application code so that applications can log messages at runtime. Second, logger objects determine which log messages to act upon based upon severity (the default filtering facility) or filter objects. Third, logger objects pass along relevant log messages to all interested log handlers.</p> <p>The most widely used methods on logger objects fall into two categories: configuration and message sending.</p> <p>These are the most common configuration methods:</p> <ul class="simple"> <li><a class="reference internal" href="../library/logging.html#logging.Logger.setLevel" title="logging.Logger.setLevel"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.setLevel()</span></code></a> specifies the lowest-severity log message a logger will handle, where debug is the lowest built-in severity level and critical is the highest built-in severity. For example, if the severity level is INFO, the logger will handle only INFO, WARNING, ERROR, and CRITICAL messages and will ignore DEBUG messages.</li> <li><a class="reference internal" href="../library/logging.html#logging.Logger.addHandler" title="logging.Logger.addHandler"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.addHandler()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.Logger.removeHandler" title="logging.Logger.removeHandler"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.removeHandler()</span></code></a> add and remove handler objects from the logger object. Handlers are covered in more detail in <a class="reference internal" href="#handler-basic"><span class="std std-ref">Handlers</span></a>.</li> <li><a class="reference internal" href="../library/logging.html#logging.Logger.addFilter" title="logging.Logger.addFilter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.addFilter()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.Logger.removeFilter" title="logging.Logger.removeFilter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.removeFilter()</span></code></a> add and remove filter objects from the logger object. Filters are covered in more detail in <a class="reference internal" href="../library/logging.html#filter"><span class="std std-ref">Filter Objects</span></a>.</li> </ul> <p>You don’t need to always call these methods on every logger you create. See the last two paragraphs in this section.</p> <p>With the logger object configured, the following methods create log messages:</p> <ul class="simple"> <li><a class="reference internal" href="../library/logging.html#logging.Logger.debug" title="logging.Logger.debug"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.debug()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.Logger.info" title="logging.Logger.info"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.info()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.Logger.warning" title="logging.Logger.warning"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.warning()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.Logger.error" title="logging.Logger.error"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.error()</span></code></a>, and <a class="reference internal" href="../library/logging.html#logging.Logger.critical" title="logging.Logger.critical"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.critical()</span></code></a> all create log records with a message and a level that corresponds to their respective method names. The message is actually a format string, which may contain the standard string substitution syntax of <code class="xref py py-const docutils literal notranslate"><span class="pre">%s</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">%d</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">%f</span></code>, and so on. The rest of their arguments is a list of objects that correspond with the substitution fields in the message. With regard to <code class="xref py py-const docutils literal notranslate"><span class="pre">**kwargs</span></code>, the logging methods care only about a keyword of <code class="xref py py-const docutils literal notranslate"><span class="pre">exc_info</span></code> and use it to determine whether to log exception information.</li> <li><a class="reference internal" href="../library/logging.html#logging.Logger.exception" title="logging.Logger.exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.exception()</span></code></a> creates a log message similar to <a class="reference internal" href="../library/logging.html#logging.Logger.error" title="logging.Logger.error"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.error()</span></code></a>. The difference is that <a class="reference internal" href="../library/logging.html#logging.Logger.exception" title="logging.Logger.exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.exception()</span></code></a> dumps a stack trace along with it. Call this method only from an exception handler.</li> <li><a class="reference internal" href="../library/logging.html#logging.Logger.log" title="logging.Logger.log"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Logger.log()</span></code></a> takes a log level as an explicit argument. This is a little more verbose for logging messages than using the log level convenience methods listed above, but this is how to log at custom log levels.</li> </ul> <p><a class="reference internal" href="../library/logging.html#logging.getLogger" title="logging.getLogger"><code class="xref py py-func docutils literal notranslate"><span class="pre">getLogger()</span></code></a> returns a reference to a logger instance with the specified name if it is provided, or <code class="docutils literal notranslate"><span class="pre">root</span></code> if not. The names are period-separated hierarchical structures. Multiple calls to <a class="reference internal" href="../library/logging.html#logging.getLogger" title="logging.getLogger"><code class="xref py py-func docutils literal notranslate"><span class="pre">getLogger()</span></code></a> with the same name will return a reference to the same logger object. Loggers that are further down in the hierarchical list are children of loggers higher up in the list. For example, given a logger with a name of <code class="docutils literal notranslate"><span class="pre">foo</span></code>, loggers with names of <code class="docutils literal notranslate"><span class="pre">foo.bar</span></code>, <code class="docutils literal notranslate"><span class="pre">foo.bar.baz</span></code>, and <code class="docutils literal notranslate"><span class="pre">foo.bam</span></code> are all descendants of <code class="docutils literal notranslate"><span class="pre">foo</span></code>.</p> <p>Loggers have a concept of <em>effective level</em>. If a level is not explicitly set on a logger, the level of its parent is used instead as its effective level. If the parent has no explicit level set, <em>its</em> parent is examined, and so on - all ancestors are searched until an explicitly set level is found. The root logger always has an explicit level set (<code class="docutils literal notranslate"><span class="pre">WARNING</span></code> by default). When deciding whether to process an event, the effective level of the logger is used to determine whether the event is passed to the logger’s handlers.</p> <p>Child loggers propagate messages up to the handlers associated with their ancestor loggers. Because of this, it is unnecessary to define and configure handlers for all the loggers an application uses. It is sufficient to configure handlers for a top-level logger and create child loggers as needed. (You can, however, turn off propagation by setting the <em>propagate</em> attribute of a logger to <code class="docutils literal notranslate"><span class="pre">False</span></code>.)</p> </div> <div class="section" id="handlers"> <span id="handler-basic"></span><h3>Handlers<a class="headerlink" href="#handlers" title="Permalink to this headline">¶</a></h3> <p><code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> objects are responsible for dispatching the appropriate log messages (based on the log messages’ severity) to the handler’s specified destination. <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate"><span class="pre">Logger</span></code></a> objects can add zero or more handler objects to themselves with an <a class="reference internal" href="../library/logging.html#logging.Logger.addHandler" title="logging.Logger.addHandler"><code class="xref py py-meth docutils literal notranslate"><span class="pre">addHandler()</span></code></a> method. As an example scenario, an application may want to send all log messages to a log file, all log messages of error or higher to stdout, and all messages of critical to an email address. This scenario requires three individual handlers where each handler is responsible for sending messages of a specific severity to a specific location.</p> <p>The standard library includes quite a few handler types (see <a class="reference internal" href="#useful-handlers"><span class="std std-ref">Useful Handlers</span></a>); the tutorials use mainly <a class="reference internal" href="../library/logging.handlers.html#logging.StreamHandler" title="logging.StreamHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">StreamHandler</span></code></a> and <a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileHandler</span></code></a> in its examples.</p> <p>There are very few methods in a handler for application developers to concern themselves with. The only handler methods that seem relevant for application developers who are using the built-in handler objects (that is, not creating custom handlers) are the following configuration methods:</p> <ul class="simple"> <li>The <a class="reference internal" href="../library/logging.html#logging.Handler.setLevel" title="logging.Handler.setLevel"><code class="xref py py-meth docutils literal notranslate"><span class="pre">setLevel()</span></code></a> method, just as in logger objects, specifies the lowest severity that will be dispatched to the appropriate destination. Why are there two <code class="xref py py-func docutils literal notranslate"><span class="pre">setLevel()</span></code> methods? The level set in the logger determines which severity of messages it will pass to its handlers. The level set in each handler determines which messages that handler will send on.</li> <li><a class="reference internal" href="../library/logging.html#logging.Handler.setFormatter" title="logging.Handler.setFormatter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">setFormatter()</span></code></a> selects a Formatter object for this handler to use.</li> <li><a class="reference internal" href="../library/logging.html#logging.Handler.addFilter" title="logging.Handler.addFilter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">addFilter()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.Handler.removeFilter" title="logging.Handler.removeFilter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">removeFilter()</span></code></a> respectively configure and deconfigure filter objects on handlers.</li> </ul> <p>Application code should not directly instantiate and use instances of <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code>. Instead, the <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> class is a base class that defines the interface that all handlers should have and establishes some default behavior that child classes can use (or override).</p> </div> <div class="section" id="formatters"> <h3>Formatters<a class="headerlink" href="#formatters" title="Permalink to this headline">¶</a></h3> <p>Formatter objects configure the final order, structure, and contents of the log message. Unlike the base <code class="xref py py-class docutils literal notranslate"><span class="pre">logging.Handler</span></code> class, application code may instantiate formatter classes, although you could likely subclass the formatter if your application needs special behavior. The constructor takes two optional arguments – a message format string and a date format string.</p> <dl class="method"> <dt id="logging.logging.Formatter.__init__"> <code class="descclassname">logging.Formatter.</code><code class="descname">__init__</code><span class="sig-paren">(</span><em>fmt=None</em>, <em>datefmt=None</em><span class="sig-paren">)</span><a class="headerlink" href="#logging.logging.Formatter.__init__" title="Permalink to this definition">¶</a></dt> <dd></dd></dl> <p>If there is no message format string, the default is to use the raw message. If there is no date format string, the default date format is:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="n">Y</span><span class="o">-%</span><span class="n">m</span><span class="o">-%</span><span class="n">d</span> <span class="o">%</span><span class="n">H</span><span class="p">:</span><span class="o">%</span><span class="n">M</span><span class="p">:</span><span class="o">%</span><span class="n">S</span> </pre></div> </div> <p>with the milliseconds tacked on at the end.</p> <p>The message format string uses <code class="docutils literal notranslate"><span class="pre">%(<dictionary</span> <span class="pre">key>)s</span></code> styled string substitution; the possible keys are documented in <a class="reference internal" href="../library/logging.html#logrecord-attributes"><span class="std std-ref">LogRecord attributes</span></a>.</p> <p>The following message format string will log the time in a human-readable format, the severity of the message, and the contents of the message, in that order:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> - </span><span class="si">%(levelname)s</span><span class="s1"> - </span><span class="si">%(message)s</span><span class="s1">'</span> </pre></div> </div> <p>Formatters use a user-configurable function to convert the creation time of a record to a tuple. By default, <a class="reference internal" href="../library/time.html#time.localtime" title="time.localtime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.localtime()</span></code></a> is used; to change this for a particular formatter instance, set the <code class="docutils literal notranslate"><span class="pre">converter</span></code> attribute of the instance to a function with the same signature as <a class="reference internal" href="../library/time.html#time.localtime" title="time.localtime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.localtime()</span></code></a> or <a class="reference internal" href="../library/time.html#time.gmtime" title="time.gmtime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.gmtime()</span></code></a>. To change it for all formatters, for example if you want all logging times to be shown in GMT, set the <code class="docutils literal notranslate"><span class="pre">converter</span></code> attribute in the Formatter class (to <code class="docutils literal notranslate"><span class="pre">time.gmtime</span></code> for GMT display).</p> </div> <div class="section" id="configuring-logging"> <h3>Configuring Logging<a class="headerlink" href="#configuring-logging" title="Permalink to this headline">¶</a></h3> <p>Programmers can configure logging in three ways:</p> <ol class="arabic simple"> <li>Creating loggers, handlers, and formatters explicitly using Python code that calls the configuration methods listed above.</li> <li>Creating a logging config file and reading it using the <a class="reference internal" href="../library/logging.config.html#logging.config.fileConfig" title="logging.config.fileConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">fileConfig()</span></code></a> function.</li> <li>Creating a dictionary of configuration information and passing it to the <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">dictConfig()</span></code></a> function.</li> </ol> <p>For the reference documentation on the last two options, see <a class="reference internal" href="../library/logging.config.html#logging-config-api"><span class="std std-ref">Configuration functions</span></a>. The following example configures a very simple logger, a console handler, and a simple formatter using Python code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="c1"># create logger</span> <span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'simple_example'</span><span class="p">)</span> <span class="n">logger</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">DEBUG</span><span class="p">)</span> <span class="c1"># create console handler and set level to debug</span> <span class="n">ch</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span><span class="p">()</span> <span class="n">ch</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">DEBUG</span><span class="p">)</span> <span class="c1"># create formatter</span> <span class="n">formatter</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">(</span><span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> - </span><span class="si">%(name)s</span><span class="s1"> - </span><span class="si">%(levelname)s</span><span class="s1"> - </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="c1"># add formatter to ch</span> <span class="n">ch</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="c1"># add ch to logger</span> <span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">ch</span><span class="p">)</span> <span class="c1"># 'application' code</span> <span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'debug message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'info message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">'warn message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'error message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'critical message'</span><span class="p">)</span> </pre></div> </div> <p>Running this module from the command line produces the following output:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python simple_logging_module.py <span class="go">2005-03-19 15:10:26,618 - simple_example - DEBUG - debug message</span> <span class="go">2005-03-19 15:10:26,620 - simple_example - INFO - info message</span> <span class="go">2005-03-19 15:10:26,695 - simple_example - WARNING - warn message</span> <span class="go">2005-03-19 15:10:26,697 - simple_example - ERROR - error message</span> <span class="go">2005-03-19 15:10:26,773 - simple_example - CRITICAL - critical message</span> </pre></div> </div> <p>The following Python module creates a logger, handler, and formatter nearly identical to those in the example listed above, with the only difference being the names of the objects:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="nn">logging.config</span> <span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">fileConfig</span><span class="p">(</span><span class="s1">'logging.conf'</span><span class="p">)</span> <span class="c1"># create logger</span> <span class="n">logger</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'simpleExample'</span><span class="p">)</span> <span class="c1"># 'application' code</span> <span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'debug message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'info message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">'warn message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'error message'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">critical</span><span class="p">(</span><span class="s1">'critical message'</span><span class="p">)</span> </pre></div> </div> <p>Here is the logging.conf file:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">loggers</span><span class="p">]</span> <span class="n">keys</span><span class="o">=</span><span class="n">root</span><span class="p">,</span><span class="n">simpleExample</span> <span class="p">[</span><span class="n">handlers</span><span class="p">]</span> <span class="n">keys</span><span class="o">=</span><span class="n">consoleHandler</span> <span class="p">[</span><span class="n">formatters</span><span class="p">]</span> <span class="n">keys</span><span class="o">=</span><span class="n">simpleFormatter</span> <span class="p">[</span><span class="n">logger_root</span><span class="p">]</span> <span class="n">level</span><span class="o">=</span><span class="n">DEBUG</span> <span class="n">handlers</span><span class="o">=</span><span class="n">consoleHandler</span> <span class="p">[</span><span class="n">logger_simpleExample</span><span class="p">]</span> <span class="n">level</span><span class="o">=</span><span class="n">DEBUG</span> <span class="n">handlers</span><span class="o">=</span><span class="n">consoleHandler</span> <span class="n">qualname</span><span class="o">=</span><span class="n">simpleExample</span> <span class="n">propagate</span><span class="o">=</span><span class="mi">0</span> <span class="p">[</span><span class="n">handler_consoleHandler</span><span class="p">]</span> <span class="n">class</span><span class="o">=</span><span class="n">StreamHandler</span> <span class="n">level</span><span class="o">=</span><span class="n">DEBUG</span> <span class="n">formatter</span><span class="o">=</span><span class="n">simpleFormatter</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">,)</span> <span class="p">[</span><span class="n">formatter_simpleFormatter</span><span class="p">]</span> <span class="nb">format</span><span class="o">=%</span><span class="p">(</span><span class="n">asctime</span><span class="p">)</span><span class="n">s</span> <span class="o">-</span> <span class="o">%</span><span class="p">(</span><span class="n">name</span><span class="p">)</span><span class="n">s</span> <span class="o">-</span> <span class="o">%</span><span class="p">(</span><span class="n">levelname</span><span class="p">)</span><span class="n">s</span> <span class="o">-</span> <span class="o">%</span><span class="p">(</span><span class="n">message</span><span class="p">)</span><span class="n">s</span> <span class="n">datefmt</span><span class="o">=</span> </pre></div> </div> <p>The output is nearly identical to that of the non-config-file-based example:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python simple_logging_config.py <span class="go">2005-03-19 15:38:55,977 - simpleExample - DEBUG - debug message</span> <span class="go">2005-03-19 15:38:55,979 - simpleExample - INFO - info message</span> <span class="go">2005-03-19 15:38:56,054 - simpleExample - WARNING - warn message</span> <span class="go">2005-03-19 15:38:56,055 - simpleExample - ERROR - error message</span> <span class="go">2005-03-19 15:38:56,130 - simpleExample - CRITICAL - critical message</span> </pre></div> </div> <p>You can see that the config file approach has a few advantages over the Python code approach, mainly separation of configuration and code and the ability of noncoders to easily modify the logging properties.</p> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p>The <a class="reference internal" href="../library/logging.config.html#logging.config.fileConfig" title="logging.config.fileConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">fileConfig()</span></code></a> function takes a default parameter, <code class="docutils literal notranslate"><span class="pre">disable_existing_loggers</span></code>, which defaults to <code class="docutils literal notranslate"><span class="pre">True</span></code> for reasons of backward compatibility. This may or may not be what you want, since it will cause any loggers existing before the <a class="reference internal" href="../library/logging.config.html#logging.config.fileConfig" title="logging.config.fileConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">fileConfig()</span></code></a> call to be disabled unless they (or an ancestor) are explicitly named in the configuration. Please refer to the reference documentation for more information, and specify <code class="docutils literal notranslate"><span class="pre">False</span></code> for this parameter if you wish.</p> <p class="last">The dictionary passed to <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate"><span class="pre">dictConfig()</span></code></a> can also specify a Boolean value with key <code class="docutils literal notranslate"><span class="pre">disable_existing_loggers</span></code>, which if not specified explicitly in the dictionary also defaults to being interpreted as <code class="docutils literal notranslate"><span class="pre">True</span></code>. This leads to the logger-disabling behaviour described above, which may not be what you want - in which case, provide the key explicitly with a value of <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p> </div> <p>Note that the class names referenced in config files need to be either relative to the logging module, or absolute values which can be resolved using normal import mechanisms. Thus, you could use either <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.WatchedFileHandler" title="logging.handlers.WatchedFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">WatchedFileHandler</span></code></a> (relative to the logging module) or <code class="docutils literal notranslate"><span class="pre">mypackage.mymodule.MyHandler</span></code> (for a class defined in package <code class="docutils literal notranslate"><span class="pre">mypackage</span></code> and module <code class="docutils literal notranslate"><span class="pre">mymodule</span></code>, where <code class="docutils literal notranslate"><span class="pre">mypackage</span></code> is available on the Python import path).</p> <p>In Python 2.7, a new means of configuring logging has been introduced, using dictionaries to hold configuration information. This provides a superset of the functionality of the config-file-based approach outlined above, and is the recommended configuration method for new applications and deployments. Because a Python dictionary is used to hold configuration information, and since you can populate that dictionary using different means, you have more options for configuration. For example, you can use a configuration file in JSON format, or, if you have access to YAML processing functionality, a file in YAML format, to populate the configuration dictionary. Or, of course, you can construct the dictionary in Python code, receive it in pickled form over a socket, or use whatever approach makes sense for your application.</p> <p>Here’s an example of the same configuration as above, in YAML format for the new dictionary-based approach:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">version</span><span class="p">:</span> <span class="mi">1</span> <span class="n">formatters</span><span class="p">:</span> <span class="n">simple</span><span class="p">:</span> <span class="nb">format</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(asctime)s</span><span class="s1"> - </span><span class="si">%(name)s</span><span class="s1"> - </span><span class="si">%(levelname)s</span><span class="s1"> - </span><span class="si">%(message)s</span><span class="s1">'</span> <span class="n">handlers</span><span class="p">:</span> <span class="n">console</span><span class="p">:</span> <span class="n">class</span><span class="p">:</span> <span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</span> <span class="n">level</span><span class="p">:</span> <span class="n">DEBUG</span> <span class="n">formatter</span><span class="p">:</span> <span class="n">simple</span> <span class="n">stream</span><span class="p">:</span> <span class="n">ext</span><span class="p">:</span><span class="o">//</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span> <span class="n">loggers</span><span class="p">:</span> <span class="n">simpleExample</span><span class="p">:</span> <span class="n">level</span><span class="p">:</span> <span class="n">DEBUG</span> <span class="n">handlers</span><span class="p">:</span> <span class="p">[</span><span class="n">console</span><span class="p">]</span> <span class="n">propagate</span><span class="p">:</span> <span class="n">no</span> <span class="n">root</span><span class="p">:</span> <span class="n">level</span><span class="p">:</span> <span class="n">DEBUG</span> <span class="n">handlers</span><span class="p">:</span> <span class="p">[</span><span class="n">console</span><span class="p">]</span> </pre></div> </div> <p>For more information about logging using a dictionary, see <a class="reference internal" href="../library/logging.config.html#logging-config-api"><span class="std std-ref">Configuration functions</span></a>.</p> </div> <div class="section" id="what-happens-if-no-configuration-is-provided"> <h3>What happens if no configuration is provided<a class="headerlink" href="#what-happens-if-no-configuration-is-provided" title="Permalink to this headline">¶</a></h3> <p>If no logging configuration is provided, it is possible to have a situation where a logging event needs to be output, but no handlers can be found to output the event. The behaviour of the logging package in these circumstances is dependent on the Python version.</p> <p>For Python 2.x, the behaviour is as follows:</p> <ul class="simple"> <li>If <em>logging.raiseExceptions</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> (production mode), the event is silently dropped.</li> <li>If <em>logging.raiseExceptions</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (development mode), a message ‘No handlers could be found for logger X.Y.Z’ is printed once.</li> </ul> </div> <div class="section" id="configuring-logging-for-a-library"> <span id="library-config"></span><h3>Configuring Logging for a Library<a class="headerlink" href="#configuring-logging-for-a-library" title="Permalink to this headline">¶</a></h3> <p>When developing a library which uses logging, you should take care to document how the library uses logging - for example, the names of loggers used. Some consideration also needs to be given to its logging configuration. If the using application does not configure logging, and library code makes logging calls, then (as described in the previous section) an error message will be printed to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code>.</p> <p>If for some reason you <em>don’t</em> want this message printed in the absence of any logging configuration, you can attach a do-nothing handler to the top-level logger for your library. This avoids the message being printed, since a handler will be always be found for the library’s events: it just doesn’t produce any output. If the library user configures logging for application use, presumably that configuration will add some handlers, and if levels are suitably configured then logging calls made in library code will send output to those handlers, as normal.</p> <p>A do-nothing handler is included in the logging package: <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NullHandler</span></code></a> (since Python 2.7). An instance of this handler could be added to the top-level logger of the logging namespace used by the library (<em>if</em> you want to prevent an error message being output to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> in the absence of logging configuration). If all logging by a library <em>foo</em> is done using loggers with names matching ‘foo.x’, ‘foo.x.y’, etc. then the code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">NullHandler</span><span class="p">())</span> </pre></div> </div> <p>should have the desired effect. If an organisation produces a number of libraries, then the logger name specified can be ‘orgname.foo’ rather than just ‘foo’.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">It is strongly advised that you <em>do not add any handlers other than</em> <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NullHandler</span></code></a> <em>to your library’s loggers</em>. This is because the configuration of handlers is the prerogative of the application developer who uses your library. The application developer knows their target audience and what handlers are most appropriate for their application: if you add handlers ‘under the hood’, you might well interfere with their ability to carry out unit tests and deliver logs which suit their requirements.</p> </div> </div> </div> <div class="section" id="logging-levels"> <h2>Logging Levels<a class="headerlink" href="#logging-levels" title="Permalink to this headline">¶</a></h2> <p>The numeric values of logging levels are given in the following table. These are primarily of interest if you want to define your own levels, and need them to have specific values relative to the predefined levels. If you define a level with the same numeric value, it overwrites the predefined value; the predefined name is lost.</p> <table border="1" class="docutils"> <colgroup> <col width="48%" /> <col width="52%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Level</th> <th class="head">Numeric value</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">CRITICAL</span></code></td> <td>50</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">ERROR</span></code></td> <td>40</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">WARNING</span></code></td> <td>30</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">INFO</span></code></td> <td>20</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">DEBUG</span></code></td> <td>10</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">NOTSET</span></code></td> <td>0</td> </tr> </tbody> </table> <p>Levels can also be associated with loggers, being set either by the developer or through loading a saved logging configuration. When a logging method is called on a logger, the logger compares its own level with the level associated with the method call. If the logger’s level is higher than the method call’s, no logging message is actually generated. This is the basic mechanism controlling the verbosity of logging output.</p> <p>Logging messages are encoded as instances of the <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate"><span class="pre">LogRecord</span></code></a> class. When a logger decides to actually log an event, a <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate"><span class="pre">LogRecord</span></code></a> instance is created from the logging message.</p> <p>Logging messages are subjected to a dispatch mechanism through the use of <em class="dfn">handlers</em>, which are instances of subclasses of the <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> class. Handlers are responsible for ensuring that a logged message (in the form of a <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate"><span class="pre">LogRecord</span></code></a>) ends up in a particular location (or set of locations) which is useful for the target audience for that message (such as end users, support desk staff, system administrators, developers). Handlers are passed <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate"><span class="pre">LogRecord</span></code></a> instances intended for particular destinations. Each logger can have zero, one or more handlers associated with it (via the <a class="reference internal" href="../library/logging.html#logging.Logger.addHandler" title="logging.Logger.addHandler"><code class="xref py py-meth docutils literal notranslate"><span class="pre">addHandler()</span></code></a> method of <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate"><span class="pre">Logger</span></code></a>). In addition to any handlers directly associated with a logger, <em>all handlers associated with all ancestors of the logger</em> are called to dispatch the message (unless the <em>propagate</em> flag for a logger is set to a false value, at which point the passing to ancestor handlers stops).</p> <p>Just as for loggers, handlers can have levels associated with them. A handler’s level acts as a filter in the same way as a logger’s level does. If a handler decides to actually dispatch an event, the <a class="reference internal" href="../library/logging.html#logging.Handler.emit" title="logging.Handler.emit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">emit()</span></code></a> method is used to send the message to its destination. Most user-defined subclasses of <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> will need to override this <a class="reference internal" href="../library/logging.html#logging.Handler.emit" title="logging.Handler.emit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">emit()</span></code></a>.</p> <div class="section" id="custom-levels"> <span id="id1"></span><h3>Custom Levels<a class="headerlink" href="#custom-levels" title="Permalink to this headline">¶</a></h3> <p>Defining your own levels is possible, but should not be necessary, as the existing levels have been chosen on the basis of practical experience. However, if you are convinced that you need custom levels, great care should be exercised when doing this, and it is possibly <em>a very bad idea to define custom levels if you are developing a library</em>. That’s because if multiple library authors all define their own custom levels, there is a chance that the logging output from such multiple libraries used together will be difficult for the using developer to control and/or interpret, because a given numeric value might mean different things for different libraries.</p> </div> </div> <div class="section" id="useful-handlers"> <span id="id2"></span><h2>Useful Handlers<a class="headerlink" href="#useful-handlers" title="Permalink to this headline">¶</a></h2> <p>In addition to the base <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> class, many useful subclasses are provided:</p> <ol class="arabic simple"> <li><a class="reference internal" href="../library/logging.handlers.html#logging.StreamHandler" title="logging.StreamHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">StreamHandler</span></code></a> instances send messages to streams (file-like objects).</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileHandler</span></code></a> instances send messages to disk files.</li> <li><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseRotatingHandler</span></code> is the base class for handlers that rotate log files at a certain point. It is not meant to be instantiated directly. Instead, use <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.RotatingFileHandler" title="logging.handlers.RotatingFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">RotatingFileHandler</span></code></a> or <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.TimedRotatingFileHandler" title="logging.handlers.TimedRotatingFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">TimedRotatingFileHandler</span></code></a>.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.RotatingFileHandler" title="logging.handlers.RotatingFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">RotatingFileHandler</span></code></a> instances send messages to disk files, with support for maximum log file sizes and log file rotation.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.TimedRotatingFileHandler" title="logging.handlers.TimedRotatingFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">TimedRotatingFileHandler</span></code></a> instances send messages to disk files, rotating the log file at certain timed intervals.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">SocketHandler</span></code></a> instances send messages to TCP/IP sockets.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.DatagramHandler" title="logging.handlers.DatagramHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">DatagramHandler</span></code></a> instances send messages to UDP sockets.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SMTPHandler" title="logging.handlers.SMTPHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">SMTPHandler</span></code></a> instances send messages to a designated email address.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SysLogHandler" title="logging.handlers.SysLogHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">SysLogHandler</span></code></a> instances send messages to a Unix syslog daemon, possibly on a remote machine.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.NTEventLogHandler" title="logging.handlers.NTEventLogHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NTEventLogHandler</span></code></a> instances send messages to a Windows NT/2000/XP event log.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.MemoryHandler" title="logging.handlers.MemoryHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">MemoryHandler</span></code></a> instances send messages to a buffer in memory, which is flushed whenever specific criteria are met.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.HTTPHandler" title="logging.handlers.HTTPHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">HTTPHandler</span></code></a> instances send messages to an HTTP server using either <code class="docutils literal notranslate"><span class="pre">GET</span></code> or <code class="docutils literal notranslate"><span class="pre">POST</span></code> semantics.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.handlers.WatchedFileHandler" title="logging.handlers.WatchedFileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">WatchedFileHandler</span></code></a> instances watch the file they are logging to. If the file changes, it is closed and reopened using the file name. This handler is only useful on Unix-like systems; Windows does not support the underlying mechanism used.</li> <li><a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NullHandler</span></code></a> instances do nothing with error messages. They are used by library developers who want to use logging, but want to avoid the ‘No handlers could be found for logger XXX’ message which can be displayed if the library user has not configured logging. See <a class="reference internal" href="#library-config"><span class="std std-ref">Configuring Logging for a Library</span></a> for more information.</li> </ol> <div class="versionadded"> <p><span class="versionmodified">New in version 2.7: </span>The <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NullHandler</span></code></a> class.</p> </div> <p>The <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">NullHandler</span></code></a>, <a class="reference internal" href="../library/logging.handlers.html#logging.StreamHandler" title="logging.StreamHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">StreamHandler</span></code></a> and <a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">FileHandler</span></code></a> classes are defined in the core logging package. The other handlers are defined in a sub- module, <a class="reference internal" href="../library/logging.handlers.html#module-logging.handlers" title="logging.handlers: Handlers for the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.handlers</span></code></a>. (There is also another sub-module, <a class="reference internal" href="../library/logging.config.html#module-logging.config" title="logging.config: Configuration of the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.config</span></code></a>, for configuration functionality.)</p> <p>Logged messages are formatted for presentation through instances of the <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">Formatter</span></code></a> class. They are initialized with a format string suitable for use with the % operator and a dictionary.</p> <p>For formatting multiple messages in a batch, instances of <code class="xref py py-class docutils literal notranslate"><span class="pre">BufferingFormatter</span></code> can be used. In addition to the format string (which is applied to each message in the batch), there is provision for header and trailer format strings.</p> <p>When filtering based on logger level and/or handler level is not enough, instances of <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate"><span class="pre">Filter</span></code></a> can be added to both <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate"><span class="pre">Logger</span></code></a> and <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> instances (through their <a class="reference internal" href="../library/logging.html#logging.Handler.addFilter" title="logging.Handler.addFilter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">addFilter()</span></code></a> method). Before deciding to process a message further, both loggers and handlers consult all their filters for permission. If any filter returns a false value, the message is not processed further.</p> <p>The basic <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate"><span class="pre">Filter</span></code></a> functionality allows filtering by specific logger name. If this feature is used, messages sent to the named logger and its children are allowed through the filter, and all others dropped.</p> </div> <div class="section" id="exceptions-raised-during-logging"> <span id="logging-exceptions"></span><h2>Exceptions raised during logging<a class="headerlink" href="#exceptions-raised-during-logging" title="Permalink to this headline">¶</a></h2> <p>The logging package is designed to swallow exceptions which occur while logging in production. This is so that errors which occur while handling logging events - such as logging misconfiguration, network or other similar errors - do not cause the application using logging to terminate prematurely.</p> <p><code class="xref py py-class docutils literal notranslate"><span class="pre">SystemExit</span></code> and <code class="xref py py-class docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code> exceptions are never swallowed. Other exceptions which occur during the <a class="reference internal" href="../library/logging.html#logging.Handler.emit" title="logging.Handler.emit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">emit()</span></code></a> method of a <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> subclass are passed to its <a class="reference internal" href="../library/logging.html#logging.Handler.handleError" title="logging.Handler.handleError"><code class="xref py py-meth docutils literal notranslate"><span class="pre">handleError()</span></code></a> method.</p> <p>The default implementation of <a class="reference internal" href="../library/logging.html#logging.Handler.handleError" title="logging.Handler.handleError"><code class="xref py py-meth docutils literal notranslate"><span class="pre">handleError()</span></code></a> in <code class="xref py py-class docutils literal notranslate"><span class="pre">Handler</span></code> checks to see if a module-level variable, <code class="xref py py-data docutils literal notranslate"><span class="pre">raiseExceptions</span></code>, is set. If set, a traceback is printed to <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a>. If not set, the exception is swallowed.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The default value of <code class="xref py py-data docutils literal notranslate"><span class="pre">raiseExceptions</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code>. This is because during development, you typically want to be notified of any exceptions that occur. It’s advised that you set <code class="xref py py-data docutils literal notranslate"><span class="pre">raiseExceptions</span></code> to <code class="docutils literal notranslate"><span class="pre">False</span></code> for production usage.</p> </div> </div> <div class="section" id="using-arbitrary-objects-as-messages"> <span id="arbitrary-object-messages"></span><h2>Using arbitrary objects as messages<a class="headerlink" href="#using-arbitrary-objects-as-messages" title="Permalink to this headline">¶</a></h2> <p>In the preceding sections and examples, it has been assumed that the message passed when logging the event is a string. However, this is not the only possibility. You can pass an arbitrary object as a message, and its <a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a> method will be called when the logging system needs to convert it to a string representation. In fact, if you want to, you can avoid computing a string representation altogether - for example, the <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal notranslate"><span class="pre">SocketHandler</span></code></a> emits an event by pickling it and sending it over the wire.</p> </div> <div class="section" id="optimization"> <h2>Optimization<a class="headerlink" href="#optimization" title="Permalink to this headline">¶</a></h2> <p>Formatting of message arguments is deferred until it cannot be avoided. However, computing the arguments passed to the logging method can also be expensive, and you may want to avoid doing it if the logger will just throw away your event. To decide what to do, you can call the <a class="reference internal" href="../library/logging.html#logging.Logger.isEnabledFor" title="logging.Logger.isEnabledFor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">isEnabledFor()</span></code></a> method which takes a level argument and returns true if the event would be created by the Logger for that level of call. You can write code like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">logger</span><span class="o">.</span><span class="n">isEnabledFor</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">):</span> <span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Message with </span><span class="si">%s</span><span class="s1">, </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">expensive_func1</span><span class="p">(),</span> <span class="n">expensive_func2</span><span class="p">())</span> </pre></div> </div> <p>so that if the logger’s threshold is set above <code class="docutils literal notranslate"><span class="pre">DEBUG</span></code>, the calls to <code class="xref py py-func docutils literal notranslate"><span class="pre">expensive_func1()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">expensive_func2()</span></code> are never made.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">In some cases, <a class="reference internal" href="../library/logging.html#logging.Logger.isEnabledFor" title="logging.Logger.isEnabledFor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">isEnabledFor()</span></code></a> can itself be more expensive than you’d like (e.g. for deeply nested loggers where an explicit level is only set high up in the logger hierarchy). In such cases (or if you want to avoid calling a method in tight loops), you can cache the result of a call to <a class="reference internal" href="../library/logging.html#logging.Logger.isEnabledFor" title="logging.Logger.isEnabledFor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">isEnabledFor()</span></code></a> in a local or instance variable, and use that instead of calling the method each time. Such a cached value would only need to be recomputed when the logging configuration changes dynamically while the application is running (which is not all that common).</p> </div> <p>There are other optimizations which can be made for specific applications which need more precise control over what logging information is collected. Here’s a list of things you can do to avoid processing during logging which you don’t need:</p> <table border="1" class="docutils"> <colgroup> <col width="54%" /> <col width="46%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">What you don’t want to collect</th> <th class="head">How to avoid collecting it</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td>Information about where calls were made from.</td> <td>Set <code class="docutils literal notranslate"><span class="pre">logging._srcfile</span></code> to <code class="docutils literal notranslate"><span class="pre">None</span></code>. This avoids calling <a class="reference internal" href="../library/sys.html#sys._getframe" title="sys._getframe"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys._getframe()</span></code></a>, which may help to speed up your code in environments like PyPy (which can’t speed up code that uses <a class="reference internal" href="../library/sys.html#sys._getframe" title="sys._getframe"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys._getframe()</span></code></a>).</td> </tr> <tr class="row-odd"><td>Threading information.</td> <td>Set <code class="docutils literal notranslate"><span class="pre">logging.logThreads</span></code> to <code class="docutils literal notranslate"><span class="pre">0</span></code>.</td> </tr> <tr class="row-even"><td>Process information.</td> <td>Set <code class="docutils literal notranslate"><span class="pre">logging.logProcesses</span></code> to <code class="docutils literal notranslate"><span class="pre">0</span></code>.</td> </tr> </tbody> </table> <p>Also note that the core logging module only includes the basic handlers. If you don’t import <a class="reference internal" href="../library/logging.handlers.html#module-logging.handlers" title="logging.handlers: Handlers for the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.handlers</span></code></a> and <a class="reference internal" href="../library/logging.config.html#module-logging.config" title="logging.config: Configuration of the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.config</span></code></a>, they won’t take up any memory.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="docutils"> <dt>Module <a class="reference internal" href="../library/logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging</span></code></a></dt> <dd>API reference for the logging module.</dd> <dt>Module <a class="reference internal" href="../library/logging.config.html#module-logging.config" title="logging.config: Configuration of the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.config</span></code></a></dt> <dd>Configuration API for the logging module.</dd> <dt>Module <a class="reference internal" href="../library/logging.handlers.html#module-logging.handlers" title="logging.handlers: Handlers for the logging module."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging.handlers</span></code></a></dt> <dd>Useful handlers included with the logging module.</dd> </dl> <p class="last"><a class="reference internal" href="logging-cookbook.html#logging-cookbook"><span class="std std-ref">A logging cookbook</span></a></p> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Logging HOWTO</a><ul> <li><a class="reference internal" href="#basic-logging-tutorial">Basic Logging Tutorial</a><ul> <li><a class="reference internal" href="#when-to-use-logging">When to use logging</a></li> <li><a class="reference internal" href="#a-simple-example">A simple example</a></li> <li><a class="reference internal" href="#logging-to-a-file">Logging to a file</a></li> <li><a class="reference internal" href="#logging-from-multiple-modules">Logging from multiple modules</a></li> <li><a class="reference internal" href="#logging-variable-data">Logging variable data</a></li> <li><a class="reference internal" href="#changing-the-format-of-displayed-messages">Changing the format of displayed messages</a></li> <li><a class="reference internal" href="#displaying-the-date-time-in-messages">Displaying the date/time in messages</a></li> <li><a class="reference internal" href="#next-steps">Next Steps</a></li> </ul> </li> <li><a class="reference internal" href="#advanced-logging-tutorial">Advanced Logging Tutorial</a><ul> <li><a class="reference internal" href="#logging-flow">Logging Flow</a></li> <li><a class="reference internal" href="#loggers">Loggers</a></li> <li><a class="reference internal" href="#handlers">Handlers</a></li> <li><a class="reference internal" href="#formatters">Formatters</a></li> <li><a class="reference internal" href="#configuring-logging">Configuring Logging</a></li> <li><a class="reference internal" href="#what-happens-if-no-configuration-is-provided">What happens if no configuration is provided</a></li> <li><a class="reference internal" href="#configuring-logging-for-a-library">Configuring Logging for a Library</a></li> </ul> </li> <li><a class="reference internal" href="#logging-levels">Logging Levels</a><ul> <li><a class="reference internal" href="#custom-levels">Custom Levels</a></li> </ul> </li> <li><a class="reference internal" href="#useful-handlers">Useful Handlers</a></li> <li><a class="reference internal" href="#exceptions-raised-during-logging">Exceptions raised during logging</a></li> <li><a class="reference internal" href="#using-arbitrary-objects-as-messages">Using arbitrary objects as messages</a></li> <li><a class="reference internal" href="#optimization">Optimization</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="functional.html" title="previous chapter">Functional Programming HOWTO</a></p> <h4>Next topic</h4> <p class="topless"><a href="logging-cookbook.html" title="next chapter">Logging Cookbook</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/howto/logging.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> 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