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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 Cookbook — Python 2.7.16 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/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.16 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="Regular Expression HOWTO" href="regex.html" /> <link rel="prev" title="Logging HOWTO" href="logging.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/2/howto/logging-cookbook.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <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="regex.html" title="Regular Expression HOWTO" accesskey="N">next</a> |</li> <li class="right" > <a href="logging.html" title="Logging 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 2.7.16 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-cookbook"> <span id="id1"></span><h1>Logging Cookbook<a class="headerlink" href="#logging-cookbook" 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> <p>This page contains a number of recipes related to logging, which have been found useful in the past.</p> <div class="section" id="using-logging-in-multiple-modules"> <h2>Using logging in multiple modules<a class="headerlink" href="#using-logging-in-multiple-modules" title="Permalink to this headline">¶</a></h2> <p>Multiple calls to <code class="docutils literal notranslate"><span class="pre">logging.getLogger('someLogger')</span></code> return a reference to the same logger object. This is true not only within the same module, but also across modules as long as it is in the same Python interpreter process. It is true for references to the same object; additionally, application code can define and configure a parent logger in one module and create (but not configure) a child logger in a separate module, and all logger calls to the child will pass up to the parent. Here is a main module:</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">auxiliary_module</span> <span class="c1"># create logger with 'spam_application'</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">'spam_application'</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 file handler which logs even debug messages</span> <span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'spam.log'</span><span class="p">)</span> <span class="n">fh</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 with a higher log level</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">ERROR</span><span class="p">)</span> <span class="c1"># create formatter and add it to the handlers</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="n">fh</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="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 the handlers to the logger</span> <span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">ch</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">'creating an instance of auxiliary_module.Auxiliary'</span><span class="p">)</span> <span class="n">a</span> <span class="o">=</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</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">'created an instance of auxiliary_module.Auxiliary'</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">'calling auxiliary_module.Auxiliary.do_something'</span><span class="p">)</span> <span class="n">a</span><span class="o">.</span><span class="n">do_something</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">'finished auxiliary_module.Auxiliary.do_something'</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">'calling auxiliary_module.some_function()'</span><span class="p">)</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</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">'done with auxiliary_module.some_function()'</span><span class="p">)</span> </pre></div> </div> <p>Here is the auxiliary module:</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">module_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">'spam_application.auxiliary'</span><span class="p">)</span> <span class="k">class</span> <span class="nc">Auxiliary</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</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">'spam_application.auxiliary.Auxiliary'</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'creating an instance of Auxiliary'</span><span class="p">)</span> <span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'doing something'</span><span class="p">)</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">+</span> <span class="mi">1</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'done doing something'</span><span class="p">)</span> <span class="k">def</span> <span class="nf">some_function</span><span class="p">():</span> <span class="n">module_logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'received a call to "some_function"'</span><span class="p">)</span> </pre></div> </div> <p>The output looks like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">663</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">creating</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">665</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">creating</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">Auxiliary</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">665</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">created</span> <span class="n">an</span> <span class="n">instance</span> <span class="n">of</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">668</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">calling</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span><span class="o">.</span><span class="n">do_something</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">668</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">doing</span> <span class="n">something</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">669</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span><span class="o">.</span><span class="n">Auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">done</span> <span class="n">doing</span> <span class="n">something</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">670</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">finished</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">Auxiliary</span><span class="o">.</span><span class="n">do_something</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">671</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">calling</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</span><span class="p">()</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">672</span> <span class="o">-</span> <span class="n">spam_application</span><span class="o">.</span><span class="n">auxiliary</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">received</span> <span class="n">a</span> <span class="n">call</span> <span class="n">to</span> <span class="s1">'some_function'</span> <span class="mi">2005</span><span class="o">-</span><span class="mi">03</span><span class="o">-</span><span class="mi">23</span> <span class="mi">23</span><span class="p">:</span><span class="mi">47</span><span class="p">:</span><span class="mi">11</span><span class="p">,</span><span class="mi">673</span> <span class="o">-</span> <span class="n">spam_application</span> <span class="o">-</span> <span class="n">INFO</span> <span class="o">-</span> <span class="n">done</span> <span class="k">with</span> <span class="n">auxiliary_module</span><span class="o">.</span><span class="n">some_function</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="logging-from-multiple-threads"> <h2>Logging from multiple threads<a class="headerlink" href="#logging-from-multiple-threads" title="Permalink to this headline">¶</a></h2> <p>Logging from multiple threads requires no special effort. The following example shows logging from the main (initIal) thread and another thread:</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">threading</span> <span class="kn">import</span> <span class="nn">time</span> <span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span> <span class="k">while</span> <span class="ow">not</span> <span class="n">arg</span><span class="p">[</span><span class="s1">'stop'</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">'Hi from myfunc'</span><span class="p">)</span> <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</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">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="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(relativeCreated)6d</span><span class="s1"> </span><span class="si">%(threadName)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="n">info</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'stop'</span><span class="p">:</span> <span class="kc">False</span><span class="p">}</span> <span class="n">thread</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">info</span><span class="p">,))</span> <span class="n">thread</span><span class="o">.</span><span class="n">start</span><span class="p">()</span> <span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="k">try</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">'Hello from main'</span><span class="p">)</span> <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.75</span><span class="p">)</span> <span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span> <span class="n">info</span><span class="p">[</span><span class="s1">'stop'</span><span class="p">]</span> <span class="o">=</span> <span class="kc">True</span> <span class="k">break</span> <span class="n">thread</span><span class="o">.</span><span class="n">join</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> <p>When run, the script should print something like the following:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="mi">0</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">3</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">505</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">755</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">1007</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">1507</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">1508</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">2010</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">2258</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">2512</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">3009</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">3013</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">3515</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">3761</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">4017</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> <span class="mi">4513</span> <span class="n">MainThread</span> <span class="n">Hello</span> <span class="kn">from</span> <span class="nn">main</span> <span class="mi">4518</span> <span class="n">Thread</span><span class="o">-</span><span class="mi">1</span> <span class="n">Hi</span> <span class="kn">from</span> <span class="nn">myfunc</span> </pre></div> </div> <p>This shows the logging output interspersed as one might expect. This approach works for more threads than shown here, of course.</p> </div> <div class="section" id="multiple-handlers-and-formatters"> <h2>Multiple handlers and formatters<a class="headerlink" href="#multiple-handlers-and-formatters" title="Permalink to this headline">¶</a></h2> <p>Loggers are plain Python objects. 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 has no minimum or maximum quota for the number of handlers you may add. Sometimes it will be beneficial for an application to log all messages of all severities to a text file while simultaneously logging errors or above to the console. To set this up, simply configure the appropriate handlers. The logging calls in the application code will remain unchanged. Here is a slight modification to the previous simple module-based configuration 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">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 file handler which logs even debug messages</span> <span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'spam.log'</span><span class="p">)</span> <span class="n">fh</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 with a higher log level</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">ERROR</span><span class="p">)</span> <span class="c1"># create formatter and add it to the handlers</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="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="n">fh</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 the handlers 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="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</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>Notice that the ‘application’ code does not care about multiple handlers. All that changed was the addition and configuration of a new handler named <em>fh</em>.</p> <p>The ability to create new handlers with higher- or lower-severity filters can be very helpful when writing and testing an application. Instead of using many <code class="docutils literal notranslate"><span class="pre">print</span></code> statements for debugging, use <code class="docutils literal notranslate"><span class="pre">logger.debug</span></code>: Unlike the print statements, which you will have to delete or comment out later, the logger.debug statements can remain intact in the source code and remain dormant until you need them again. At that time, the only change that needs to happen is to modify the severity level of the logger and/or handler to debug.</p> </div> <div class="section" id="logging-to-multiple-destinations"> <span id="multiple-destinations"></span><h2>Logging to multiple destinations<a class="headerlink" href="#logging-to-multiple-destinations" title="Permalink to this headline">¶</a></h2> <p>Let’s say you want to log to console and file with different message formats and in differing circumstances. Say you want to log messages with levels of DEBUG and higher to file, and those messages at level INFO and higher to the console. Let’s also assume that the file should contain timestamps, but the console messages should not. Here’s how you can achieve this:</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"># set up logging to file - see previous section for more details</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">logging</span><span class="o">.</span><span class="n">DEBUG</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">%(name)-12s</span><span class="s1"> </span><span class="si">%(levelname)-8s</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"> %H:%M'</span><span class="p">,</span> <span class="n">filename</span><span class="o">=</span><span class="s1">'/temp/myapp.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="c1"># define a Handler which writes INFO messages or higher to the sys.stderr</span> <span class="n">console</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">console</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">INFO</span><span class="p">)</span> <span class="c1"># set a format which is simpler for console use</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">%(name)-12s</span><span class="s1">: </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="c1"># tell the handler to use this format</span> <span class="n">console</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 the handler to the root logger</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s1">''</span><span class="p">)</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">console</span><span class="p">)</span> <span class="c1"># Now, we can log to the root logger, or any other logger. First the root...</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Jackdaws love my big sphinx of quartz.'</span><span class="p">)</span> <span class="c1"># Now, define a couple of other loggers which might represent areas in your</span> <span class="c1"># application:</span> <span class="n">logger1</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">'myapp.area1'</span><span class="p">)</span> <span class="n">logger2</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">'myapp.area2'</span><span class="p">)</span> <span class="n">logger1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Quick zephyrs blow, vexing daft Jim.'</span><span class="p">)</span> <span class="n">logger1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'How quickly daft jumping zebras vex.'</span><span class="p">)</span> <span class="n">logger2</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Jail zesty vixen who grabbed pay from quack.'</span><span class="p">)</span> <span class="n">logger2</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'The five boxing wizards jump quickly.'</span><span class="p">)</span> </pre></div> </div> <p>When you run this, on the console you will see</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">root</span> <span class="p">:</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="p">:</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="p">:</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="p">:</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span> </pre></div> </div> <p>and in the file you will see something like</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">root</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span> <span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">DEBUG</span> <span class="n">Quick</span> <span class="n">zephyrs</span> <span class="n">blow</span><span class="p">,</span> <span class="n">vexing</span> <span class="n">daft</span> <span class="n">Jim</span><span class="o">.</span> <span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span> <span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span> <span class="mi">10</span><span class="o">-</span><span class="mi">22</span> <span class="mi">22</span><span class="p">:</span><span class="mi">19</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span> </pre></div> </div> <p>As you can see, the DEBUG message only shows up in the file. The other messages are sent to both destinations.</p> <p>This example uses console and file handlers, but you can use any number and combination of handlers you choose.</p> </div> <div class="section" id="configuration-server-example"> <h2>Configuration server example<a class="headerlink" href="#configuration-server-example" title="Permalink to this headline">¶</a></h2> <p>Here is an example of a module using the logging configuration server:</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="kn">import</span> <span class="nn">time</span> <span class="kn">import</span> <span class="nn">os</span> <span class="c1"># read initial config file</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 and start listener on port 9999</span> <span class="n">t</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">9999</span><span class="p">)</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</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="k">try</span><span class="p">:</span> <span class="c1"># loop through logging calls to see the difference</span> <span class="c1"># new configurations make, until Ctrl+C is pressed</span> <span class="k">while</span> <span class="kc">True</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">'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> <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span> <span class="c1"># cleanup</span> <span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">stopListening</span><span class="p">()</span> <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span> </pre></div> </div> <p>And here is a script that takes a filename and sends that file to the server, properly preceded with the binary-encoded length, as the new logging configuration:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span> <span class="kn">import</span> <span class="nn">socket</span><span class="o">,</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">struct</span> <span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span> <span class="n">data_to_send</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span> <span class="n">HOST</span> <span class="o">=</span> <span class="s1">'localhost'</span> <span class="n">PORT</span> <span class="o">=</span> <span class="mi">9999</span> <span class="n">s</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'connecting...'</span><span class="p">)</span> <span class="n">s</span><span class="o">.</span><span class="n">connect</span><span class="p">((</span><span class="n">HOST</span><span class="p">,</span> <span class="n">PORT</span><span class="p">))</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'sending config...'</span><span class="p">)</span> <span class="n">s</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s1">'>L'</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">data_to_send</span><span class="p">)))</span> <span class="n">s</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">data_to_send</span><span class="p">)</span> <span class="n">s</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'complete'</span><span class="p">)</span> </pre></div> </div> </div> <div class="section" id="sending-and-receiving-logging-events-across-a-network"> <span id="network-logging"></span><h2>Sending and receiving logging events across a network<a class="headerlink" href="#sending-and-receiving-logging-events-across-a-network" title="Permalink to this headline">¶</a></h2> <p>Let’s say you want to send logging events across a network, and handle them at the receiving end. A simple way of doing this is attaching a <code class="xref py py-class docutils literal notranslate"><span class="pre">SocketHandler</span></code> instance to the root logger at the sending end:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">logging</span><span class="o">,</span> <span class="nn">logging.handlers</span> <span class="n">rootLogger</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">''</span><span class="p">)</span> <span class="n">rootLogger</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="n">socketHandler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">SocketHandler</span><span class="p">(</span><span class="s1">'localhost'</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">DEFAULT_TCP_LOGGING_PORT</span><span class="p">)</span> <span class="c1"># don't bother with a formatter, since a socket handler sends the event as</span> <span class="c1"># an unformatted pickle</span> <span class="n">rootLogger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">socketHandler</span><span class="p">)</span> <span class="c1"># Now, we can log to the root logger, or any other logger. First the root...</span> <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'Jackdaws love my big sphinx of quartz.'</span><span class="p">)</span> <span class="c1"># Now, define a couple of other loggers which might represent areas in your</span> <span class="c1"># application:</span> <span class="n">logger1</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">'myapp.area1'</span><span class="p">)</span> <span class="n">logger2</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">'myapp.area2'</span><span class="p">)</span> <span class="n">logger1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'Quick zephyrs blow, vexing daft Jim.'</span><span class="p">)</span> <span class="n">logger1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'How quickly daft jumping zebras vex.'</span><span class="p">)</span> <span class="n">logger2</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Jail zesty vixen who grabbed pay from quack.'</span><span class="p">)</span> <span class="n">logger2</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s1">'The five boxing wizards jump quickly.'</span><span class="p">)</span> </pre></div> </div> <p>At the receiving end, you can set up a receiver using the <a class="reference internal" href="../library/socketserver.html#module-SocketServer" title="SocketServer: A framework for network servers."><code class="xref py py-mod docutils literal notranslate"><span class="pre">SocketServer</span></code></a> module. Here is a basic working example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pickle</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="nn">logging.handlers</span> <span class="kn">import</span> <span class="nn">SocketServer</span> <span class="kn">import</span> <span class="nn">struct</span> <span class="k">class</span> <span class="nc">LogRecordStreamHandler</span><span class="p">(</span><span class="n">SocketServer</span><span class="o">.</span><span class="n">StreamRequestHandler</span><span class="p">):</span> <span class="sd">"""Handler for a streaming logging request.</span> <span class="sd"> This basically logs the record using whatever logging policy is</span> <span class="sd"> configured locally.</span> <span class="sd"> """</span> <span class="k">def</span> <span class="nf">handle</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> Handle multiple requests - each expected to be a 4-byte length,</span> <span class="sd"> followed by the LogRecord in pickle format. Logs the record</span> <span class="sd"> according to whatever policy is configured locally.</span> <span class="sd"> """</span> <span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="n">chunk</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span> <span class="o"><</span> <span class="mi">4</span><span class="p">:</span> <span class="k">break</span> <span class="n">slen</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="s1">'>L'</span><span class="p">,</span> <span class="n">chunk</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span> <span class="n">chunk</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">slen</span><span class="p">)</span> <span class="k">while</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span> <span class="o"><</span> <span class="n">slen</span><span class="p">:</span> <span class="n">chunk</span> <span class="o">=</span> <span class="n">chunk</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">slen</span> <span class="o">-</span> <span class="nb">len</span><span class="p">(</span><span class="n">chunk</span><span class="p">))</span> <span class="n">obj</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">unPickle</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span> <span class="n">record</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">makeLogRecord</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">handleLogRecord</span><span class="p">(</span><span class="n">record</span><span class="p">)</span> <span class="k">def</span> <span class="nf">unPickle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">):</span> <span class="k">return</span> <span class="n">pickle</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">data</span><span class="p">)</span> <span class="k">def</span> <span class="nf">handleLogRecord</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span> <span class="c1"># if a name is specified, we use the named logger rather than the one</span> <span class="c1"># implied by the record.</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">server</span><span class="o">.</span><span class="n">logname</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span> <span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">server</span><span class="o">.</span><span class="n">logname</span> <span class="k">else</span><span class="p">:</span> <span class="n">name</span> <span class="o">=</span> <span class="n">record</span><span class="o">.</span><span class="n">name</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="n">name</span><span class="p">)</span> <span class="c1"># N.B. EVERY record gets logged. This is because Logger.handle</span> <span class="c1"># is normally called AFTER logger-level filtering. If you want</span> <span class="c1"># to do filtering, do it at the client end to save wasting</span> <span class="c1"># cycles and network bandwidth!</span> <span class="n">logger</span><span class="o">.</span><span class="n">handle</span><span class="p">(</span><span class="n">record</span><span class="p">)</span> <span class="k">class</span> <span class="nc">LogRecordSocketReceiver</span><span class="p">(</span><span class="n">SocketServer</span><span class="o">.</span><span class="n">ThreadingTCPServer</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> Simple TCP socket-based logging receiver suitable for testing.</span> <span class="sd"> """</span> <span class="n">allow_reuse_address</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">host</span><span class="o">=</span><span class="s1">'localhost'</span><span class="p">,</span> <span class="n">port</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">DEFAULT_TCP_LOGGING_PORT</span><span class="p">,</span> <span class="n">handler</span><span class="o">=</span><span class="n">LogRecordStreamHandler</span><span class="p">):</span> <span class="n">SocketServer</span><span class="o">.</span><span class="n">ThreadingTCPServer</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="p">(</span><span class="n">host</span><span class="p">,</span> <span class="n">port</span><span class="p">),</span> <span class="n">handler</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">abort</span> <span class="o">=</span> <span class="mi">0</span> <span class="bp">self</span><span class="o">.</span><span class="n">timeout</span> <span class="o">=</span> <span class="mi">1</span> <span class="bp">self</span><span class="o">.</span><span class="n">logname</span> <span class="o">=</span> <span class="kc">None</span> <span class="k">def</span> <span class="nf">serve_until_stopped</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="kn">import</span> <span class="nn">select</span> <span class="n">abort</span> <span class="o">=</span> <span class="mi">0</span> <span class="k">while</span> <span class="ow">not</span> <span class="n">abort</span><span class="p">:</span> <span class="n">rd</span><span class="p">,</span> <span class="n">wr</span><span class="p">,</span> <span class="n">ex</span> <span class="o">=</span> <span class="n">select</span><span class="o">.</span><span class="n">select</span><span class="p">([</span><span class="bp">self</span><span class="o">.</span><span class="n">socket</span><span class="o">.</span><span class="n">fileno</span><span class="p">()],</span> <span class="p">[],</span> <span class="p">[],</span> <span class="bp">self</span><span class="o">.</span><span class="n">timeout</span><span class="p">)</span> <span class="k">if</span> <span class="n">rd</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">handle_request</span><span class="p">()</span> <span class="n">abort</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">abort</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="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(relativeCreated)5d</span><span class="s1"> </span><span class="si">%(name)-15s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="n">tcpserver</span> <span class="o">=</span> <span class="n">LogRecordSocketReceiver</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'About to start TCP server...'</span><span class="p">)</span> <span class="n">tcpserver</span><span class="o">.</span><span class="n">serve_until_stopped</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> <p>First run the server, and then the client. On the client side, nothing is printed on the console; on the server side, you should see something like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">About</span> <span class="n">to</span> <span class="n">start</span> <span class="n">TCP</span> <span class="n">server</span><span class="o">...</span> <span class="mi">59</span> <span class="n">root</span> <span class="n">INFO</span> <span class="n">Jackdaws</span> <span class="n">love</span> <span class="n">my</span> <span class="n">big</span> <span class="n">sphinx</span> <span class="n">of</span> <span class="n">quartz</span><span class="o">.</span> <span class="mi">59</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">DEBUG</span> <span class="n">Quick</span> <span class="n">zephyrs</span> <span class="n">blow</span><span class="p">,</span> <span class="n">vexing</span> <span class="n">daft</span> <span class="n">Jim</span><span class="o">.</span> <span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area1</span> <span class="n">INFO</span> <span class="n">How</span> <span class="n">quickly</span> <span class="n">daft</span> <span class="n">jumping</span> <span class="n">zebras</span> <span class="n">vex</span><span class="o">.</span> <span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">WARNING</span> <span class="n">Jail</span> <span class="n">zesty</span> <span class="n">vixen</span> <span class="n">who</span> <span class="n">grabbed</span> <span class="n">pay</span> <span class="kn">from</span> <span class="nn">quack.</span> <span class="mi">69</span> <span class="n">myapp</span><span class="o">.</span><span class="n">area2</span> <span class="n">ERROR</span> <span class="n">The</span> <span class="n">five</span> <span class="n">boxing</span> <span class="n">wizards</span> <span class="n">jump</span> <span class="n">quickly</span><span class="o">.</span> </pre></div> </div> <p>Note that there are some security issues with pickle in some scenarios. If these affect you, you can use an alternative serialization scheme by overriding the <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler.makePickle" title="logging.handlers.SocketHandler.makePickle"><code class="xref py py-meth docutils literal notranslate"><span class="pre">makePickle()</span></code></a> method and implementing your alternative there, as well as adapting the above script to use your alternative serialization.</p> </div> <div class="section" id="adding-contextual-information-to-your-logging-output"> <span id="context-info"></span><h2>Adding contextual information to your logging output<a class="headerlink" href="#adding-contextual-information-to-your-logging-output" title="Permalink to this headline">¶</a></h2> <p>Sometimes you want logging output to contain contextual information in addition to the parameters passed to the logging call. For example, in a networked application, it may be desirable to log client-specific information in the log (e.g. remote client’s username, or IP address). Although you could use the <em>extra</em> parameter to achieve this, it’s not always convenient to pass the information in this way. While it might be tempting to create <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> instances on a per-connection basis, this is not a good idea because these instances are not garbage collected. While this is not a problem in practice, when the number 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> instances is dependent on the level of granularity you want to use in logging an application, it could be hard to manage if the number 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> instances becomes effectively unbounded.</p> <div class="section" id="using-loggeradapters-to-impart-contextual-information"> <h3>Using LoggerAdapters to impart contextual information<a class="headerlink" href="#using-loggeradapters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3> <p>An easy way in which you can pass contextual information to be output along with logging event information is to use the <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a> class. This class is designed to look like a <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>, so that you can call <a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-meth 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-meth 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-meth 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-meth docutils literal notranslate"><span class="pre">error()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.exception" title="logging.exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">exception()</span></code></a>, <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-meth docutils literal notranslate"><span class="pre">critical()</span></code></a> and <a class="reference internal" href="../library/logging.html#logging.log" title="logging.log"><code class="xref py py-meth docutils literal notranslate"><span class="pre">log()</span></code></a>. These methods have the same signatures as their counterparts in <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>, so you can use the two types of instances interchangeably.</p> <p>When you create an instance of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a>, you pass it a <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> instance and a dict-like object which contains your contextual information. When you call one of the logging methods on an instance of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a>, it delegates the call to the underlying instance 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> passed to its constructor, and arranges to pass the contextual information in the delegated call. Here’s a snippet from the code of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">debug</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> Delegate a debug call to the underlying logger, after adding</span> <span class="sd"> contextual information from this adapter instance.</span> <span class="sd"> """</span> <span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> </pre></div> </div> <p>The <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter.process" title="logging.LoggerAdapter.process"><code class="xref py py-meth docutils literal notranslate"><span class="pre">process()</span></code></a> method of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a> is where the contextual information is added to the logging output. It’s passed the message and keyword arguments of the logging call, and it passes back (potentially) modified versions of these to use in the call to the underlying logger. The default implementation of this method leaves the message alone, but inserts an ‘extra’ key in the keyword argument whose value is the dict-like object passed to the constructor. Of course, if you had passed an ‘extra’ keyword argument in the call to the adapter, it will be silently overwritten.</p> <p>The advantage of using ‘extra’ is that the values in the dict-like object are merged into 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> instance’s __dict__, allowing you to use customized strings with your <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> instances which know about the keys of the dict-like object. If you need a different method, e.g. if you want to prepend or append the contextual information to the message string, you just need to subclass <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a> and override <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter.process" title="logging.LoggerAdapter.process"><code class="xref py py-meth docutils literal notranslate"><span class="pre">process()</span></code></a> to do what you need. Here is a simple example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CustomAdapter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">LoggerAdapter</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> This example adapter expects the passed in dict-like object to have a</span> <span class="sd"> 'connid' key, whose value in brackets is prepended to the log message.</span> <span class="sd"> """</span> <span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">msg</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">):</span> <span class="k">return</span> <span class="s1">'[</span><span class="si">%s</span><span class="s1">] </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">extra</span><span class="p">[</span><span class="s1">'connid'</span><span class="p">],</span> <span class="n">msg</span><span class="p">),</span> <span class="n">kwargs</span> </pre></div> </div> <p>which you can use like this:</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> <span class="n">adapter</span> <span class="o">=</span> <span class="n">CustomAdapter</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="p">{</span><span class="s1">'connid'</span><span class="p">:</span> <span class="n">some_conn_id</span><span class="p">})</span> </pre></div> </div> <p>Then any events that you log to the adapter will have the value of <code class="docutils literal notranslate"><span class="pre">some_conn_id</span></code> prepended to the log messages.</p> <div class="section" id="using-objects-other-than-dicts-to-pass-contextual-information"> <h4>Using objects other than dicts to pass contextual information<a class="headerlink" href="#using-objects-other-than-dicts-to-pass-contextual-information" title="Permalink to this headline">¶</a></h4> <p>You don’t need to pass an actual dict to a <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate"><span class="pre">LoggerAdapter</span></code></a> - you could pass an instance of a class which implements <code class="docutils literal notranslate"><span class="pre">__getitem__</span></code> and <code class="docutils literal notranslate"><span class="pre">__iter__</span></code> so that it looks like a dict to logging. This would be useful if you want to generate values dynamically (whereas the values in a dict would be constant).</p> </div> </div> <div class="section" id="using-filters-to-impart-contextual-information"> <span id="filters-contextual"></span><h3>Using Filters to impart contextual information<a class="headerlink" href="#using-filters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3> <p>You can also add contextual information to log output using a user-defined <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>. <code class="docutils literal notranslate"><span class="pre">Filter</span></code> instances are allowed to modify the <code class="docutils literal notranslate"><span class="pre">LogRecords</span></code> passed to them, including adding additional attributes which can then be output using a suitable format string, or if needed a custom <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>.</p> <p>For example in a web application, the request being processed (or at least, the interesting parts of it) can be stored in a threadlocal (<a class="reference internal" href="../library/threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.local</span></code></a>) variable, and then accessed from a <code class="docutils literal notranslate"><span class="pre">Filter</span></code> to add, say, information from the request - say, the remote IP address and remote user’s username - to the <code class="docutils literal notranslate"><span class="pre">LogRecord</span></code>, using the attribute names ‘ip’ and ‘user’ as in the <code class="docutils literal notranslate"><span class="pre">LoggerAdapter</span></code> example above. In that case, the same format string can be used to get similar output to that shown above. Here’s an example script:</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">from</span> <span class="nn">random</span> <span class="k">import</span> <span class="n">choice</span> <span class="k">class</span> <span class="nc">ContextFilter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Filter</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> This is a filter which injects contextual information into the log.</span> <span class="sd"> Rather than use actual contextual information, we just use random</span> <span class="sd"> data in this demo.</span> <span class="sd"> """</span> <span class="n">USERS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'jim'</span><span class="p">,</span> <span class="s1">'fred'</span><span class="p">,</span> <span class="s1">'sheila'</span><span class="p">]</span> <span class="n">IPS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'123.231.231.123'</span><span class="p">,</span> <span class="s1">'127.0.0.1'</span><span class="p">,</span> <span class="s1">'192.168.0.1'</span><span class="p">]</span> <span class="k">def</span> <span class="nf">filter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span> <span class="n">record</span><span class="o">.</span><span class="n">ip</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">ContextFilter</span><span class="o">.</span><span class="n">IPS</span><span class="p">)</span> <span class="n">record</span><span class="o">.</span><span class="n">user</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">ContextFilter</span><span class="o">.</span><span class="n">USERS</span><span class="p">)</span> <span class="k">return</span> <span class="kc">True</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">levels</span> <span class="o">=</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">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">WARNING</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span><span class="p">,</span> <span class="n">logging</span><span class="o">.</span><span class="n">CRITICAL</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">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(asctime)-15s</span><span class="s1"> </span><span class="si">%(name)-5s</span><span class="s1"> </span><span class="si">%(levelname)-8s</span><span class="s1"> IP: </span><span class="si">%(ip)-15s</span><span class="s1"> User: </span><span class="si">%(user)-8s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="n">a1</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">'a.b.c'</span><span class="p">)</span> <span class="n">a2</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">'d.e.f'</span><span class="p">)</span> <span class="n">f</span> <span class="o">=</span> <span class="n">ContextFilter</span><span class="p">()</span> <span class="n">a1</span><span class="o">.</span><span class="n">addFilter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="n">a2</span><span class="o">.</span><span class="n">addFilter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="n">a1</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'A debug message'</span><span class="p">)</span> <span class="n">a1</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'An info message with </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'some parameters'</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span> <span class="n">lvl</span> <span class="o">=</span> <span class="n">choice</span><span class="p">(</span><span class="n">levels</span><span class="p">)</span> <span class="n">lvlname</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">getLevelName</span><span class="p">(</span><span class="n">lvl</span><span class="p">)</span> <span class="n">a2</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">lvl</span><span class="p">,</span> <span class="s1">'A message at </span><span class="si">%s</span><span class="s1"> level with </span><span class="si">%d</span><span class="s1"> </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">lvlname</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="s1">'parameters'</span><span class="p">)</span> </pre></div> </div> <p>which, when run, produces something like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">292</span> <span class="n">a</span><span class="o">.</span><span class="n">b</span><span class="o">.</span><span class="n">c</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">debug</span> <span class="n">message</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">a</span><span class="o">.</span><span class="n">b</span><span class="o">.</span><span class="n">c</span> <span class="n">INFO</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">An</span> <span class="n">info</span> <span class="n">message</span> <span class="k">with</span> <span class="n">some</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">CRITICAL</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">300</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">192.168</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">jim</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">ERROR</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">127.0</span><span class="o">.</span><span class="mf">0.1</span> <span class="n">User</span><span class="p">:</span> <span class="n">sheila</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">DEBUG</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> <span class="mi">2010</span><span class="o">-</span><span class="mi">09</span><span class="o">-</span><span class="mi">06</span> <span class="mi">22</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">15</span><span class="p">,</span><span class="mi">301</span> <span class="n">d</span><span class="o">.</span><span class="n">e</span><span class="o">.</span><span class="n">f</span> <span class="n">INFO</span> <span class="n">IP</span><span class="p">:</span> <span class="mf">123.231</span><span class="o">.</span><span class="mf">231.123</span> <span class="n">User</span><span class="p">:</span> <span class="n">fred</span> <span class="n">A</span> <span class="n">message</span> <span class="n">at</span> <span class="n">INFO</span> <span class="n">level</span> <span class="k">with</span> <span class="mi">2</span> <span class="n">parameters</span> </pre></div> </div> </div> </div> <div class="section" id="logging-to-a-single-file-from-multiple-processes"> <span id="multiple-processes"></span><h2>Logging to a single file from multiple processes<a class="headerlink" href="#logging-to-a-single-file-from-multiple-processes" title="Permalink to this headline">¶</a></h2> <p>Although logging is thread-safe, and logging to a single file from multiple threads in a single process <em>is</em> supported, logging to a single file from <em>multiple processes</em> is <em>not</em> supported, because there is no standard way to serialize access to a single file across multiple processes in Python. If you need to log to a single file from multiple processes, one way of doing this is to have all the processes log to a <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>, and have a separate process which implements a socket server which reads from the socket and logs to file. (If you prefer, you can dedicate one thread in one of the existing processes to perform this function.) <a class="reference internal" href="#network-logging"><span class="std std-ref">This section</span></a> documents this approach in more detail and includes a working socket receiver which can be used as a starting point for you to adapt in your own applications.</p> <p>If you are using a recent version of Python which includes the <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based "threading" interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module, you could write your own handler which uses the <a class="reference internal" href="../library/multiprocessing.html#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> class from this module to serialize access to the file from your processes. The existing <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> and subclasses do not make use of <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based "threading" interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> at present, though they may do so in the future. Note that at present, the <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based "threading" interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module does not provide working lock functionality on all platforms (see <a class="reference external" href="https://bugs.python.org/issue3770">https://bugs.python.org/issue3770</a>).</p> </div> <div class="section" id="using-file-rotation"> <h2>Using file rotation<a class="headerlink" href="#using-file-rotation" title="Permalink to this headline">¶</a></h2> <p>Sometimes you want to let a log file grow to a certain size, then open a new file and log to that. You may want to keep a certain number of these files, and when that many files have been created, rotate the files so that the number of files and the size of the files both remain bounded. For this usage pattern, the logging package provides a <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>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">glob</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="kn">import</span> <span class="nn">logging.handlers</span> <span class="n">LOG_FILENAME</span> <span class="o">=</span> <span class="s1">'logging_rotatingfile_example.out'</span> <span class="c1"># Set up a specific logger with our desired output level</span> <span class="n">my_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">'MyLogger'</span><span class="p">)</span> <span class="n">my_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"># Add the log message handler to the logger</span> <span class="n">handler</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">handlers</span><span class="o">.</span><span class="n">RotatingFileHandler</span><span class="p">(</span> <span class="n">LOG_FILENAME</span><span class="p">,</span> <span class="n">maxBytes</span><span class="o">=</span><span class="mi">20</span><span class="p">,</span> <span class="n">backupCount</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> <span class="n">my_logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span> <span class="c1"># Log some messages</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span> <span class="n">my_logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'i = </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="n">i</span><span class="p">)</span> <span class="c1"># See what files are created</span> <span class="n">logfiles</span> <span class="o">=</span> <span class="n">glob</span><span class="o">.</span><span class="n">glob</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="s1">*'</span> <span class="o">%</span> <span class="n">LOG_FILENAME</span><span class="p">)</span> <span class="k">for</span> <span class="n">filename</span> <span class="ow">in</span> <span class="n">logfiles</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span> </pre></div> </div> <p>The result should be 6 separate files, each with part of the log history for the application:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span> <span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">1</span> <span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">2</span> <span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">3</span> <span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">4</span> <span class="n">logging_rotatingfile_example</span><span class="o">.</span><span class="n">out</span><span class="o">.</span><span class="mi">5</span> </pre></div> </div> <p>The most current file is always <code class="file docutils literal notranslate"><span class="pre">logging_rotatingfile_example.out</span></code>, and each time it reaches the size limit it is renamed with the suffix <code class="docutils literal notranslate"><span class="pre">.1</span></code>. Each of the existing backup files is renamed to increment the suffix (<code class="docutils literal notranslate"><span class="pre">.1</span></code> becomes <code class="docutils literal notranslate"><span class="pre">.2</span></code>, etc.) and the <code class="docutils literal notranslate"><span class="pre">.6</span></code> file is erased.</p> <p>Obviously this example sets the log length much too small as an extreme example. You would want to set <em>maxBytes</em> to an appropriate value.</p> </div> <div class="section" id="an-example-dictionary-based-configuration"> <h2>An example dictionary-based configuration<a class="headerlink" href="#an-example-dictionary-based-configuration" title="Permalink to this headline">¶</a></h2> <p>Below is an example of a logging configuration dictionary - it’s taken from the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">documentation on the Django project</a>. This dictionary is 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> to put the configuration into effect:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span> <span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span> <span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'verbose'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'format'</span><span class="p">:</span> <span class="s1">'</span><span class="si">%(levelname)s</span><span class="s1"> </span><span class="si">%(asctime)s</span><span class="s1"> </span><span class="si">%(module)s</span><span class="s1"> </span><span class="si">%(process)d</span><span class="s1"> </span><span class="si">%(thread)d</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span> <span class="p">},</span> <span class="s1">'simple'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'format'</span><span class="p">:</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="p">},</span> <span class="s1">'filters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'special'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'()'</span><span class="p">:</span> <span class="s1">'project.logging.SpecialFilter'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">:</span> <span class="s1">'bar'</span><span class="p">,</span> <span class="p">}</span> <span class="p">},</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'null'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span> <span class="s1">'class'</span><span class="p">:</span><span class="s1">'django.utils.log.NullHandler'</span><span class="p">,</span> <span class="p">},</span> <span class="s1">'console'</span><span class="p">:{</span> <span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span> <span class="s1">'class'</span><span class="p">:</span><span class="s1">'logging.StreamHandler'</span><span class="p">,</span> <span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'simple'</span> <span class="p">},</span> <span class="s1">'mail_admins'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span> <span class="s1">'class'</span><span class="p">:</span> <span class="s1">'django.utils.log.AdminEmailHandler'</span><span class="p">,</span> <span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'special'</span><span class="p">]</span> <span class="p">}</span> <span class="p">},</span> <span class="s1">'loggers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'django'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:[</span><span class="s1">'null'</span><span class="p">],</span> <span class="s1">'propagate'</span><span class="p">:</span> <span class="kc">True</span><span class="p">,</span> <span class="s1">'level'</span><span class="p">:</span><span class="s1">'INFO'</span><span class="p">,</span> <span class="p">},</span> <span class="s1">'django.request'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'mail_admins'</span><span class="p">],</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'ERROR'</span><span class="p">,</span> <span class="s1">'propagate'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span> <span class="p">},</span> <span class="s1">'myproject.custom'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">,</span> <span class="s1">'mail_admins'</span><span class="p">],</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'INFO'</span><span class="p">,</span> <span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'special'</span><span class="p">]</span> <span class="p">}</span> <span class="p">}</span> <span class="p">}</span> </pre></div> </div> <p>For more information about this configuration, you can see the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">relevant section</a> of the Django documentation.</p> </div> <div class="section" id="inserting-a-bom-into-messages-sent-to-a-sysloghandler"> <h2>Inserting a BOM into messages sent to a SysLogHandler<a class="headerlink" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler" title="Permalink to this headline">¶</a></h2> <p><a class="reference external" href="https://tools.ietf.org/html/rfc5424">RFC 5424</a> requires that a Unicode message be sent to a syslog daemon as a set of bytes which have the following structure: an optional pure-ASCII component, followed by a UTF-8 Byte Order Mark (BOM), followed by Unicode encoded using UTF-8. (See the <a class="reference external" href="https://tools.ietf.org/html/rfc5424#section-6">relevant section of the specification</a>.)</p> <p>In Python 2.6 and 2.7, code was added to <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> to insert a BOM into the message, but unfortunately, it was implemented incorrectly, with the BOM appearing at the beginning of the message and hence not allowing any pure-ASCII component to appear before it.</p> <p>As this behaviour is broken, the incorrect BOM insertion code is being removed from Python 2.7.4 and later. However, it is not being replaced, and if you want to produce RFC 5424-compliant messages which include a BOM, an optional pure-ASCII sequence before it and arbitrary Unicode after it, encoded using UTF-8, then you need to do the following:</p> <ol class="arabic"> <li><p class="first">Attach a <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> instance to your <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> instance, with a format string such as:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="sa">u</span><span class="s1">'ASCII section</span><span class="se">\ufeff</span><span class="s1">Unicode section'</span> </pre></div> </div> <p>The Unicode code point <code class="docutils literal notranslate"><span class="pre">u'\ufeff'</span></code>, when encoded using UTF-8, will be encoded as a UTF-8 BOM – the byte-string <code class="docutils literal notranslate"><span class="pre">'\xef\xbb\xbf'</span></code>.</p> </li> <li><p class="first">Replace the ASCII section with whatever placeholders you like, but make sure that the data that appears in there after substitution is always ASCII (that way, it will remain unchanged after UTF-8 encoding).</p> </li> <li><p class="first">Replace the Unicode section with whatever placeholders you like; if the data which appears there after substitution contains characters outside the ASCII range, that’s fine – it will be encoded using UTF-8.</p> </li> </ol> <p>If the formatted message is Unicode, it <em>will</em> be encoded using UTF-8 encoding by <code class="docutils literal notranslate"><span class="pre">SysLogHandler</span></code>. If you follow the above rules, you should be able to produce RFC 5424-compliant messages. If you don’t, logging may not complain, but your messages will not be RFC 5424-compliant, and your syslog daemon may complain.</p> </div> <div class="section" id="implementing-structured-logging"> <h2>Implementing structured logging<a class="headerlink" href="#implementing-structured-logging" title="Permalink to this headline">¶</a></h2> <p>Although most logging messages are intended for reading by humans, and thus not readily machine-parseable, there might be circumstances where you want to output messages in a structured format which <em>is</em> capable of being parsed by a program (without needing complex regular expressions to parse the log message). This is straightforward to achieve using the logging package. There are a number of ways in which this could be achieved, but the following is a simple approach which uses JSON to serialise the event in a machine-parseable manner:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="k">class</span> <span class="nc">StructuredMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">message</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span> <span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span> <span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> >>> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">message</span><span class="p">,</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">))</span> <span class="n">_</span> <span class="o">=</span> <span class="n">StructuredMessage</span> <span class="c1"># optional, to improve readability</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">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</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">info</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'message 1'</span><span class="p">,</span> <span class="n">foo</span><span class="o">=</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">bar</span><span class="o">=</span><span class="s1">'baz'</span><span class="p">,</span> <span class="n">num</span><span class="o">=</span><span class="mi">123</span><span class="p">,</span> <span class="n">fnum</span><span class="o">=</span><span class="mf">123.456</span><span class="p">))</span> </pre></div> </div> <p>If the above script is run, it prints:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">message</span> <span class="mi">1</span> <span class="o">>>></span> <span class="p">{</span><span class="s2">"fnum"</span><span class="p">:</span> <span class="mf">123.456</span><span class="p">,</span> <span class="s2">"num"</span><span class="p">:</span> <span class="mi">123</span><span class="p">,</span> <span class="s2">"bar"</span><span class="p">:</span> <span class="s2">"baz"</span><span class="p">,</span> <span class="s2">"foo"</span><span class="p">:</span> <span class="s2">"bar"</span><span class="p">}</span> </pre></div> </div> <p>Note that the order of items might be different according to the version of Python used.</p> <p>If you need more specialised processing, you can use a custom JSON encoder, as in the following complete example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">unicode_literals</span> <span class="kn">import</span> <span class="nn">json</span> <span class="kn">import</span> <span class="nn">logging</span> <span class="c1"># This next bit is to ensure the script runs unchanged on 2.x and 3.x</span> <span class="k">try</span><span class="p">:</span> <span class="n">unicode</span> <span class="k">except</span> <span class="ne">NameError</span><span class="p">:</span> <span class="n">unicode</span> <span class="o">=</span> <span class="nb">str</span> <span class="k">class</span> <span class="nc">Encoder</span><span class="p">(</span><span class="n">json</span><span class="o">.</span><span class="n">JSONEncoder</span><span class="p">):</span> <span class="k">def</span> <span class="nf">default</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">o</span><span class="p">):</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="nb">set</span><span class="p">):</span> <span class="k">return</span> <span class="nb">tuple</span><span class="p">(</span><span class="n">o</span><span class="p">)</span> <span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="n">unicode</span><span class="p">):</span> <span class="k">return</span> <span class="n">o</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">'unicode_escape'</span><span class="p">)</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s1">'ascii'</span><span class="p">)</span> <span class="k">return</span> <span class="nb">super</span><span class="p">(</span><span class="n">Encoder</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">default</span><span class="p">(</span><span class="n">o</span><span class="p">)</span> <span class="k">class</span> <span class="nc">StructuredMessage</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">message</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span> <span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span> <span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="n">s</span> <span class="o">=</span> <span class="n">Encoder</span><span class="p">()</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span> <span class="k">return</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> >>> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">message</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span> <span class="n">_</span> <span class="o">=</span> <span class="n">StructuredMessage</span> <span class="c1"># optional, to improve readability</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">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="nb">format</span><span class="o">=</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">info</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'message 1'</span><span class="p">,</span> <span class="n">set_value</span><span class="o">=</span><span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]),</span> <span class="n">snowman</span><span class="o">=</span><span class="s1">'</span><span class="se">\u2603</span><span class="s1">'</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> <p>When the above script is run, it prints:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">message</span> <span class="mi">1</span> <span class="o">>>></span> <span class="p">{</span><span class="s2">"snowman"</span><span class="p">:</span> <span class="s2">"</span><span class="se">\u2603</span><span class="s2">"</span><span class="p">,</span> <span class="s2">"set_value"</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]}</span> </pre></div> </div> <p>Note that the order of items might be different according to the version of Python used.</p> </div> <div class="section" id="customizing-handlers-with-dictconfig"> <span id="custom-handlers"></span><h2>Customizing handlers with <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><a class="headerlink" href="#customizing-handlers-with-dictconfig" title="Permalink to this headline">¶</a></h2> <p>There are times when you want to customize logging handlers in particular ways, and if you use <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> you may be able to do this without subclassing. As an example, consider that you may want to set the ownership of a log file. On POSIX, this is easily done using <a class="reference internal" href="../library/os.html#os.chown" title="os.chown"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.chown()</span></code></a>, but the file handlers in the stdlib don’t offer built-in support. You can customize handler creation using a plain function such as:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">owned_file_handler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'a'</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">owner</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">owner</span><span class="p">:</span> <span class="kn">import</span> <span class="nn">os</span><span class="o">,</span> <span class="nn">pwd</span><span class="o">,</span> <span class="nn">grp</span> <span class="c1"># convert user and group names to uid and gid</span> <span class="n">uid</span> <span class="o">=</span> <span class="n">pwd</span><span class="o">.</span><span class="n">getpwnam</span><span class="p">(</span><span class="n">owner</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span><span class="o">.</span><span class="n">pw_uid</span> <span class="n">gid</span> <span class="o">=</span> <span class="n">grp</span><span class="o">.</span><span class="n">getgrnam</span><span class="p">(</span><span class="n">owner</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span><span class="o">.</span><span class="n">gr_gid</span> <span class="n">owner</span> <span class="o">=</span> <span class="p">(</span><span class="n">uid</span><span class="p">,</span> <span class="n">gid</span><span class="p">)</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">)</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="n">os</span><span class="o">.</span><span class="n">chown</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="o">*</span><span class="n">owner</span><span class="p">)</span> <span class="k">return</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">,</span> <span class="n">encoding</span><span class="p">)</span> </pre></div> </div> <p>You can then specify, in a logging configuration 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>, that a logging handler be created by calling this function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span> <span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span> <span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'default'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'format'</span><span class="p">:</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">%(name)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span> <span class="p">},</span> <span class="p">},</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'file'</span><span class="p">:{</span> <span class="c1"># The values below are popped from this dictionary and</span> <span class="c1"># used to create the handler, set the handler's level and</span> <span class="c1"># its formatter.</span> <span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span> <span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span> <span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'default'</span><span class="p">,</span> <span class="c1"># The values below are passed to the handler creator callable</span> <span class="c1"># as keyword arguments.</span> <span class="s1">'owner'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'pulse'</span><span class="p">,</span> <span class="s1">'pulse'</span><span class="p">],</span> <span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'chowntest.log'</span><span class="p">,</span> <span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span> <span class="s1">'encoding'</span><span class="p">:</span> <span class="s1">'utf-8'</span><span class="p">,</span> <span class="p">},</span> <span class="p">},</span> <span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'file'</span><span class="p">],</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span> <span class="p">},</span> <span class="p">}</span> </pre></div> </div> <p>In this example I am setting the ownership using the <code class="docutils literal notranslate"><span class="pre">pulse</span></code> user and group, just for the purposes of illustration. Putting it together into a working script, <code class="docutils literal notranslate"><span class="pre">chowntest.py</span></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="o">,</span> <span class="nn">logging.config</span><span class="o">,</span> <span class="nn">os</span><span class="o">,</span> <span class="nn">shutil</span> <span class="k">def</span> <span class="nf">owned_file_handler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'a'</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">owner</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">owner</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">)</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="n">shutil</span><span class="o">.</span><span class="n">chown</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="o">*</span><span class="n">owner</span><span class="p">)</span> <span class="k">return</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">,</span> <span class="n">encoding</span><span class="p">)</span> <span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span> <span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span> <span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'default'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'format'</span><span class="p">:</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">%(name)s</span><span class="s1"> </span><span class="si">%(message)s</span><span class="s1">'</span> <span class="p">},</span> <span class="p">},</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'file'</span><span class="p">:{</span> <span class="c1"># The values below are popped from this dictionary and</span> <span class="c1"># used to create the handler, set the handler's level and</span> <span class="c1"># its formatter.</span> <span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span> <span class="s1">'level'</span><span class="p">:</span><span class="s1">'DEBUG'</span><span class="p">,</span> <span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'default'</span><span class="p">,</span> <span class="c1"># The values below are passed to the handler creator callable</span> <span class="c1"># as keyword arguments.</span> <span class="s1">'owner'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'pulse'</span><span class="p">,</span> <span class="s1">'pulse'</span><span class="p">],</span> <span class="s1">'filename'</span><span class="p">:</span> <span class="s1">'chowntest.log'</span><span class="p">,</span> <span class="s1">'mode'</span><span class="p">:</span> <span class="s1">'w'</span><span class="p">,</span> <span class="s1">'encoding'</span><span class="p">:</span> <span class="s1">'utf-8'</span><span class="p">,</span> <span class="p">},</span> <span class="p">},</span> <span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'file'</span><span class="p">],</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span> <span class="p">},</span> <span class="p">}</span> <span class="n">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</span><span class="p">)</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">'mylogger'</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">'A debug message'</span><span class="p">)</span> </pre></div> </div> <p>To run this, you will probably need to run as <code class="docutils literal notranslate"><span class="pre">root</span></code>:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> sudo python3.3 chowntest.py <span class="gp">$</span> cat chowntest.log <span class="go">2013-11-05 09:34:51,128 DEBUG mylogger A debug message</span> <span class="gp">$</span> ls -l chowntest.log <span class="go">-rw-r--r-- 1 pulse pulse 55 2013-11-05 09:34 chowntest.log</span> </pre></div> </div> <p>Note that this example uses Python 3.3 because that’s where <code class="xref py py-func docutils literal notranslate"><span class="pre">shutil.chown()</span></code> makes an appearance. This approach should work with any Python version that supports <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> - namely, Python 2.7, 3.2 or later. With pre-3.3 versions, you would need to implement the actual ownership change using e.g. <a class="reference internal" href="../library/os.html#os.chown" title="os.chown"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.chown()</span></code></a>.</p> <p>In practice, the handler-creating function may be in a utility module somewhere in your project. Instead of the line in the configuration:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="s1">'()'</span><span class="p">:</span> <span class="n">owned_file_handler</span><span class="p">,</span> </pre></div> </div> <p>you could use e.g.:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="s1">'()'</span><span class="p">:</span> <span class="s1">'ext://project.util.owned_file_handler'</span><span class="p">,</span> </pre></div> </div> <p>where <code class="docutils literal notranslate"><span class="pre">project.util</span></code> can be replaced with the actual name of the package where the function resides. In the above working script, using <code class="docutils literal notranslate"><span class="pre">'ext://__main__.owned_file_handler'</span></code> should work. Here, the actual callable is resolved by <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> from the <code class="docutils literal notranslate"><span class="pre">ext://</span></code> specification.</p> <p>This example hopefully also points the way to how you could implement other types of file change - e.g. setting specific POSIX permission bits - in the same way, using <a class="reference internal" href="../library/os.html#os.chmod" title="os.chmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.chmod()</span></code></a>.</p> <p>Of course, the approach could also be extended to types of handler other than a <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> - for example, one of the rotating file handlers, or a different type of handler altogether.</p> </div> <div class="section" id="configuring-filters-with-dictconfig"> <span id="filters-dictconfig"></span><h2>Configuring filters with <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><a class="headerlink" href="#configuring-filters-with-dictconfig" title="Permalink to this headline">¶</a></h2> <p>You <em>can</em> configure filters using <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>, though it might not be obvious at first glance how to do it (hence this recipe). Since <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> is the only filter class included in the standard library, and it is unlikely to cater to many requirements (it’s only there as a base class), you will typically need to define your own <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> subclass with an overridden <a class="reference internal" href="../library/logging.html#logging.Filter.filter" title="logging.Filter.filter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">filter()</span></code></a> method. To do this, specify the <code class="docutils literal notranslate"><span class="pre">()</span></code> key in the configuration dictionary for the filter, specifying a callable which will be used to create the filter (a class is the most obvious, but you can provide any callable which returns a <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> instance). Here is a complete 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="kn">import</span> <span class="nn">logging.config</span> <span class="kn">import</span> <span class="nn">sys</span> <span class="k">class</span> <span class="nc">MyFilter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Filter</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">param</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="o">=</span> <span class="n">param</span> <span class="k">def</span> <span class="nf">filter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">allow</span> <span class="o">=</span> <span class="kc">True</span> <span class="k">else</span><span class="p">:</span> <span class="n">allow</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">param</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">record</span><span class="o">.</span><span class="n">msg</span> <span class="k">if</span> <span class="n">allow</span><span class="p">:</span> <span class="n">record</span><span class="o">.</span><span class="n">msg</span> <span class="o">=</span> <span class="s1">'changed: '</span> <span class="o">+</span> <span class="n">record</span><span class="o">.</span><span class="n">msg</span> <span class="k">return</span> <span class="n">allow</span> <span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span> <span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'filters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'myfilter'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'()'</span><span class="p">:</span> <span class="n">MyFilter</span><span class="p">,</span> <span class="s1">'param'</span><span class="p">:</span> <span class="s1">'noshow'</span><span class="p">,</span> <span class="p">}</span> <span class="p">},</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'console'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span> <span class="s1">'filters'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'myfilter'</span><span class="p">]</span> <span class="p">}</span> <span class="p">},</span> <span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'level'</span><span class="p">:</span> <span class="s1">'DEBUG'</span><span class="p">,</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console'</span><span class="p">]</span> <span class="p">},</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">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</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">'hello'</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">'hello - noshow'</span><span class="p">)</span> </pre></div> </div> <p>This example shows how you can pass configuration data to the callable which constructs the instance, in the form of keyword parameters. When run, the above script will print:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">changed</span><span class="p">:</span> <span class="n">hello</span> </pre></div> </div> <p>which shows that the filter is working as configured.</p> <p>A couple of extra points to note:</p> <ul class="simple"> <li>If you can’t refer to the callable directly in the configuration (e.g. if it lives in a different module, and you can’t import it directly where the configuration dictionary is), you can use the form <code class="docutils literal notranslate"><span class="pre">ext://...</span></code> as described in <a class="reference internal" href="../library/logging.config.html#logging-config-dict-externalobj"><span class="std std-ref">Access to external objects</span></a>. For example, you could have used the text <code class="docutils literal notranslate"><span class="pre">'ext://__main__.MyFilter'</span></code> instead of <code class="docutils literal notranslate"><span class="pre">MyFilter</span></code> in the above example.</li> <li>As well as for filters, this technique can also be used to configure custom handlers and formatters. See <a class="reference internal" href="../library/logging.config.html#logging-config-dict-userdef"><span class="std std-ref">User-defined objects</span></a> for more information on how logging supports using user-defined objects in its configuration, and see the other cookbook recipe <a class="reference internal" href="#custom-handlers"><span class="std std-ref">Customizing handlers with dictConfig()</span></a> above.</li> </ul> </div> <div class="section" id="customized-exception-formatting"> <span id="custom-format-exception"></span><h2>Customized exception formatting<a class="headerlink" href="#customized-exception-formatting" title="Permalink to this headline">¶</a></h2> <p>There might be times when you want to do customized exception formatting - for argument’s sake, let’s say you want exactly one line per logged event, even when exception information is present. You can do this with a custom formatter class, as shown in the following 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="k">class</span> <span class="nc">OneLineExceptionFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span> <span class="k">def</span> <span class="nf">formatException</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_info</span><span class="p">):</span> <span class="sd">"""</span> <span class="sd"> Format an exception so that it prints on a single line.</span> <span class="sd"> """</span> <span class="n">result</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">OneLineExceptionFormatter</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">formatException</span><span class="p">(</span><span class="n">exc_info</span><span class="p">)</span> <span class="k">return</span> <span class="nb">repr</span><span class="p">(</span><span class="n">result</span><span class="p">)</span> <span class="c1"># or format into one line however you want to</span> <span class="k">def</span> <span class="nf">format</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span> <span class="n">s</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">OneLineExceptionFormatter</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">record</span><span class="p">)</span> <span class="k">if</span> <span class="n">record</span><span class="o">.</span><span class="n">exc_text</span><span class="p">:</span> <span class="n">s</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">,</span> <span class="s1">''</span><span class="p">)</span> <span class="o">+</span> <span class="s1">'|'</span> <span class="k">return</span> <span class="n">s</span> <span class="k">def</span> <span class="nf">configure_logging</span><span class="p">():</span> <span class="n">fh</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">FileHandler</span><span class="p">(</span><span class="s1">'output.txt'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span> <span class="n">f</span> <span class="o">=</span> <span class="n">OneLineExceptionFormatter</span><span class="p">(</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><span class="p">,</span> <span class="s1">'</span><span class="si">%d</span><span class="s1">/%m/%Y %H:%M:%S'</span><span class="p">)</span> <span class="n">fh</span><span class="o">.</span><span class="n">setFormatter</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="n">root</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="n">root</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="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">fh</span><span class="p">)</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span> <span class="n">configure_logging</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">'Sample message'</span><span class="p">)</span> <span class="k">try</span><span class="p">:</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span> <span class="k">except</span> <span class="ne">ZeroDivisionError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span> <span class="n">logging</span><span class="o">.</span><span class="n">exception</span><span class="p">(</span><span class="s1">'ZeroDivisionError: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">e</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> <p>When run, this produces a file with exactly two lines:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">28</span><span class="o">/</span><span class="mi">01</span><span class="o">/</span><span class="mi">2015</span> <span class="mi">07</span><span class="p">:</span><span class="mi">21</span><span class="p">:</span><span class="mi">23</span><span class="o">|</span><span class="n">INFO</span><span class="o">|</span><span class="n">Sample</span> <span class="n">message</span><span class="o">|</span> <span class="mi">28</span><span class="o">/</span><span class="mi">01</span><span class="o">/</span><span class="mi">2015</span> <span class="mi">07</span><span class="p">:</span><span class="mi">21</span><span class="p">:</span><span class="mi">23</span><span class="o">|</span><span class="n">ERROR</span><span class="o">|</span><span class="ne">ZeroDivisionError</span><span class="p">:</span> <span class="n">integer</span> <span class="n">division</span> <span class="ow">or</span> <span class="n">modulo</span> <span class="n">by</span> <span class="n">zero</span><span class="o">|</span><span class="s1">'Traceback (most recent call last):</span><span class="se">\n</span><span class="s1"> File "logtest7.py", line 30, in main</span><span class="se">\n</span><span class="s1"> x = 1 / 0</span><span class="se">\n</span><span class="s1">ZeroDivisionError: integer division or modulo by zero'</span><span class="o">|</span> </pre></div> </div> <p>While the above treatment is simplistic, it points the way to how exception information can be formatted to your liking. The <a class="reference internal" href="../library/traceback.html#module-traceback" title="traceback: Print or retrieve a stack traceback."><code class="xref py py-mod docutils literal notranslate"><span class="pre">traceback</span></code></a> module may be helpful for more specialized needs.</p> </div> <div class="section" id="speaking-logging-messages"> <span id="spoken-messages"></span><h2>Speaking logging messages<a class="headerlink" href="#speaking-logging-messages" title="Permalink to this headline">¶</a></h2> <p>There might be situations when it is desirable to have logging messages rendered in an audible rather than a visible format. This is easy to do if you have text-to-speech (TTS) functionality available in your system, even if it doesn’t have a Python binding. Most TTS systems have a command line program you can run, and this can be invoked from a handler using <a class="reference internal" href="../library/subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a>. It’s assumed here that TTS command line programs won’t expect to interact with users or take a long time to complete, and that the frequency of logged messages will be not so high as to swamp the user with messages, and that it’s acceptable to have the messages spoken one at a time rather than concurrently, The example implementation below waits for one message to be spoken before the next is processed, and this might cause other handlers to be kept waiting. Here is a short example showing the approach, which assumes that the <code class="docutils literal notranslate"><span class="pre">espeak</span></code> TTS package is available:</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">subprocess</span> <span class="kn">import</span> <span class="nn">sys</span> <span class="k">class</span> <span class="nc">TTSHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Handler</span><span class="p">):</span> <span class="k">def</span> <span class="nf">emit</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">record</span><span class="p">):</span> <span class="n">msg</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">record</span><span class="p">)</span> <span class="c1"># Speak slowly in a female English voice</span> <span class="n">cmd</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'espeak'</span><span class="p">,</span> <span class="s1">'-s150'</span><span class="p">,</span> <span class="s1">'-ven+f3'</span><span class="p">,</span> <span class="n">msg</span><span class="p">]</span> <span class="n">p</span> <span class="o">=</span> <span class="n">subprocess</span><span class="o">.</span><span class="n">Popen</span><span class="p">(</span><span class="n">cmd</span><span class="p">,</span> <span class="n">stdout</span><span class="o">=</span><span class="n">subprocess</span><span class="o">.</span><span class="n">PIPE</span><span class="p">,</span> <span class="n">stderr</span><span class="o">=</span><span class="n">subprocess</span><span class="o">.</span><span class="n">STDOUT</span><span class="p">)</span> <span class="c1"># wait for the program to finish</span> <span class="n">p</span><span class="o">.</span><span class="n">communicate</span><span class="p">()</span> <span class="k">def</span> <span class="nf">configure_logging</span><span class="p">():</span> <span class="n">h</span> <span class="o">=</span> <span class="n">TTSHandler</span><span class="p">()</span> <span class="n">root</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="n">root</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">h</span><span class="p">)</span> <span class="c1"># the default formatter just returns the message</span> <span class="n">root</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="k">def</span> <span class="nf">main</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">'Hello'</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">'Goodbye'</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">configure_logging</span><span class="p">()</span> <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="n">main</span><span class="p">())</span> </pre></div> </div> <p>When run, this script should say “Hello” and then “Goodbye” in a female voice.</p> <p>The above approach can, of course, be adapted to other TTS systems and even other systems altogether which can process messages via external programs run from a command line.</p> </div> <div class="section" id="buffering-logging-messages-and-outputting-them-conditionally"> <span id="buffered-logging"></span><h2>Buffering logging messages and outputting them conditionally<a class="headerlink" href="#buffering-logging-messages-and-outputting-them-conditionally" title="Permalink to this headline">¶</a></h2> <p>There might be situations where you want to log messages in a temporary area and only output them if a certain condition occurs. For example, you may want to start logging debug events in a function, and if the function completes without errors, you don’t want to clutter the log with the collected debug information, but if there is an error, you want all the debug information to be output as well as the error.</p> <p>Here is an example which shows how you could do this using a decorator for your functions where you want logging to behave this way. It makes use of the <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">logging.handlers.MemoryHandler</span></code></a>, which allows buffering of logged events until some condition occurs, at which point the buffered events are <code class="docutils literal notranslate"><span class="pre">flushed</span></code> - passed to another handler (the <code class="docutils literal notranslate"><span class="pre">target</span></code> handler) for processing. By default, the <code class="docutils literal notranslate"><span class="pre">MemoryHandler</span></code> flushed when its buffer gets filled up or an event whose level is greater than or equal to a specified threshold is seen. You can use this recipe with a more specialised subclass of <code class="docutils literal notranslate"><span class="pre">MemoryHandler</span></code> if you want custom flushing behavior.</p> <p>The example script has a simple function, <code class="docutils literal notranslate"><span class="pre">foo</span></code>, which just cycles through all the logging levels, writing to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> to say what level it’s about to log at, and then actually logging a message at that level. You can pass a parameter to <code class="docutils literal notranslate"><span class="pre">foo</span></code> which, if true, will log at ERROR and CRITICAL levels - otherwise, it only logs at DEBUG, INFO and WARNING levels.</p> <p>The script just arranges to decorate <code class="docutils literal notranslate"><span class="pre">foo</span></code> with a decorator which will do the conditional logging that’s required. The decorator takes a logger as a parameter and attaches a memory handler for the duration of the call to the decorated function. The decorator can be additionally parameterised using a target handler, a level at which flushing should occur, and a capacity for the buffer. These default to 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> which writes to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code>, <code class="docutils literal notranslate"><span class="pre">logging.ERROR</span></code> and <code class="docutils literal notranslate"><span class="pre">100</span></code> respectively.</p> <p>Here’s the script:</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">from</span> <span class="nn">logging.handlers</span> <span class="k">import</span> <span class="n">MemoryHandler</span> <span class="kn">import</span> <span class="nn">sys</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> <span class="n">logger</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> <span class="k">def</span> <span class="nf">log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">,</span> <span class="n">target_handler</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">flush_level</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">capacity</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">target_handler</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">target_handler</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="k">if</span> <span class="n">flush_level</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">flush_level</span> <span class="o">=</span> <span class="n">logging</span><span class="o">.</span><span class="n">ERROR</span> <span class="k">if</span> <span class="n">capacity</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">capacity</span> <span class="o">=</span> <span class="mi">100</span> <span class="n">handler</span> <span class="o">=</span> <span class="n">MemoryHandler</span><span class="p">(</span><span class="n">capacity</span><span class="p">,</span> <span class="n">flushLevel</span><span class="o">=</span><span class="n">flush_level</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">target_handler</span><span class="p">)</span> <span class="k">def</span> <span class="nf">decorator</span><span class="p">(</span><span class="n">fn</span><span class="p">):</span> <span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span> <span class="k">try</span><span class="p">:</span> <span class="k">return</span> <span class="n">fn</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span> <span class="n">logger</span><span class="o">.</span><span class="n">exception</span><span class="p">(</span><span class="s1">'call failed'</span><span class="p">)</span> <span class="k">raise</span> <span class="k">finally</span><span class="p">:</span> <span class="nb">super</span><span class="p">(</span><span class="n">MemoryHandler</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span> <span class="n">logger</span><span class="o">.</span><span class="n">removeHandler</span><span class="p">(</span><span class="n">handler</span><span class="p">)</span> <span class="k">return</span> <span class="n">wrapper</span> <span class="k">return</span> <span class="n">decorator</span> <span class="k">def</span> <span class="nf">write_line</span><span class="p">(</span><span class="n">s</span><span class="p">):</span> <span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="se">\n</span><span class="s1">'</span> <span class="o">%</span> <span class="n">s</span><span class="p">)</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">fail</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at 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">'Actually logged at DEBUG'</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at INFO ...'</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">'Actually logged at INFO'</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at WARNING ...'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">'Actually logged at WARNING'</span><span class="p">)</span> <span class="k">if</span> <span class="n">fail</span><span class="p">:</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at ERROR ...'</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">'Actually logged at ERROR'</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'about to log at CRITICAL ...'</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">'Actually logged at CRITICAL'</span><span class="p">)</span> <span class="k">return</span> <span class="n">fail</span> <span class="n">decorated_foo</span> <span class="o">=</span> <span class="n">log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">)(</span><span class="n">foo</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">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="n">write_line</span><span class="p">(</span><span class="s1">'Calling undecorated foo with False'</span><span class="p">)</span> <span class="k">assert</span> <span class="ow">not</span> <span class="n">foo</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling undecorated foo with True'</span><span class="p">)</span> <span class="k">assert</span> <span class="n">foo</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling decorated foo with False'</span><span class="p">)</span> <span class="k">assert</span> <span class="ow">not</span> <span class="n">decorated_foo</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span> <span class="n">write_line</span><span class="p">(</span><span class="s1">'Calling decorated foo with True'</span><span class="p">)</span> <span class="k">assert</span> <span class="n">decorated_foo</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span> </pre></div> </div> <p>When this script is run, the following output should be observed:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Calling</span> <span class="n">undecorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">False</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span> <span class="n">Calling</span> <span class="n">undecorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">True</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="o">...</span> <span class="n">Calling</span> <span class="n">decorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">False</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span> <span class="n">Calling</span> <span class="n">decorated</span> <span class="n">foo</span> <span class="k">with</span> <span class="kc">True</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">INFO</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="o">...</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="o">...</span> <span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">DEBUG</span> <span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">INFO</span> <span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">WARNING</span> <span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">ERROR</span> <span class="n">about</span> <span class="n">to</span> <span class="n">log</span> <span class="n">at</span> <span class="n">CRITICAL</span> <span class="o">...</span> <span class="n">Actually</span> <span class="n">logged</span> <span class="n">at</span> <span class="n">CRITICAL</span> </pre></div> </div> <p>As you can see, actual logging output only occurs when an event is logged whose severity is ERROR or greater, but in that case, any previous events at lower severities are also logged.</p> <p>You can of course use the conventional means of decoration:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@log_if_errors</span><span class="p">(</span><span class="n">logger</span><span class="p">)</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">fail</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> </div> <div class="section" id="formatting-times-using-utc-gmt-via-configuration"> <span id="utc-formatting"></span><h2>Formatting times using UTC (GMT) via configuration<a class="headerlink" href="#formatting-times-using-utc-gmt-via-configuration" title="Permalink to this headline">¶</a></h2> <p>Sometimes you want to format times using UTC, which can be done using a class such as <cite>UTCFormatter</cite>, shown below:</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">time</span> <span class="k">class</span> <span class="nc">UTCFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span> <span class="n">converter</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">gmtime</span> </pre></div> </div> <p>and you can then use the <code class="docutils literal notranslate"><span class="pre">UTCFormatter</span></code> in your code instead of <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>. If you want to do that via configuration, you can use 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> API with an approach illustrated by the following complete 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="kn">import</span> <span class="nn">logging.config</span> <span class="kn">import</span> <span class="nn">time</span> <span class="k">class</span> <span class="nc">UTCFormatter</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">Formatter</span><span class="p">):</span> <span class="n">converter</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">gmtime</span> <span class="n">LOGGING</span> <span class="o">=</span> <span class="p">{</span> <span class="s1">'version'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'disable_existing_loggers'</span><span class="p">:</span> <span class="kc">False</span><span class="p">,</span> <span class="s1">'formatters'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'utc'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'()'</span><span class="p">:</span> <span class="n">UTCFormatter</span><span class="p">,</span> <span class="s1">'format'</span><span class="p">:</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="p">},</span> <span class="s1">'local'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'format'</span><span class="p">:</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="p">}</span> <span class="p">},</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'console1'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span> <span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'utc'</span><span class="p">,</span> <span class="p">},</span> <span class="s1">'console2'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'class'</span><span class="p">:</span> <span class="s1">'logging.StreamHandler'</span><span class="p">,</span> <span class="s1">'formatter'</span><span class="p">:</span> <span class="s1">'local'</span><span class="p">,</span> <span class="p">},</span> <span class="p">},</span> <span class="s1">'root'</span><span class="p">:</span> <span class="p">{</span> <span class="s1">'handlers'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'console1'</span><span class="p">,</span> <span class="s1">'console2'</span><span class="p">],</span> <span class="p">}</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">logging</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">dictConfig</span><span class="p">(</span><span class="n">LOGGING</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">'The local time is </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="n">time</span><span class="o">.</span><span class="n">asctime</span><span class="p">())</span> </pre></div> </div> <p>When this script is run, it should print something like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">2015</span><span class="o">-</span><span class="mi">10</span><span class="o">-</span><span class="mi">17</span> <span class="mi">12</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span><span class="p">,</span><span class="mi">501</span> <span class="n">The</span> <span class="n">local</span> <span class="n">time</span> <span class="ow">is</span> <span class="n">Sat</span> <span class="n">Oct</span> <span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span> <span class="mi">2015</span> <span class="mi">2015</span><span class="o">-</span><span class="mi">10</span><span class="o">-</span><span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span><span class="p">,</span><span class="mi">501</span> <span class="n">The</span> <span class="n">local</span> <span class="n">time</span> <span class="ow">is</span> <span class="n">Sat</span> <span class="n">Oct</span> <span class="mi">17</span> <span class="mi">13</span><span class="p">:</span><span class="mi">53</span><span class="p">:</span><span class="mi">29</span> <span class="mi">2015</span> </pre></div> </div> <p>showing how the time is formatted both as local time and UTC, one for each handler.</p> </div> <div class="section" id="using-a-context-manager-for-selective-logging"> <span id="context-manager"></span><h2>Using a context manager for selective logging<a class="headerlink" href="#using-a-context-manager-for-selective-logging" title="Permalink to this headline">¶</a></h2> <p>There are times when it would be useful to temporarily change the logging configuration and revert it back after doing something. For this, a context manager is the most obvious way of saving and restoring the logging context. Here is a simple example of such a context manager, which allows you to optionally change the logging level and add a logging handler purely in the scope of the context manager:</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">sys</span> <span class="k">class</span> <span class="nc">LoggingContext</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">logger</span><span class="p">,</span> <span class="n">level</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">handler</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">close</span><span class="o">=</span><span class="kc">True</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span> <span class="o">=</span> <span class="n">logger</span> <span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="o">=</span> <span class="n">level</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span> <span class="o">=</span> <span class="n">handler</span> <span class="bp">self</span><span class="o">.</span><span class="n">close</span> <span class="o">=</span> <span class="n">close</span> <span class="k">def</span> <span class="nf">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">old_level</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">level</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">level</span><span class="p">)</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">)</span> <span class="k">def</span> <span class="nf">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">et</span><span class="p">,</span> <span class="n">ev</span><span class="p">,</span> <span class="n">tb</span><span class="p">):</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">level</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">old_level</span><span class="p">)</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">removeHandler</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="p">)</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">close</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">handler</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="c1"># implicit return of None => don't swallow exceptions</span> </pre></div> </div> <p>If you specify a level value, the logger’s level is set to that value in the scope of the with block covered by the context manager. If you specify a handler, it is added to the logger on entry to the block and removed on exit from the block. You can also ask the manager to close the handler for you on block exit - you could do this if you don’t need the handler any more.</p> <p>To illustrate how it works, we can add the following block of code to the above:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></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">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">'foo'</span><span class="p">)</span> <span class="n">logger</span><span class="o">.</span><span class="n">addHandler</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">StreamHandler</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">INFO</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">'1. This should appear just once on stderr.'</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">'2. This should not appear.'</span><span class="p">)</span> <span class="k">with</span> <span class="n">LoggingContext</span><span class="p">(</span><span class="n">logger</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">logger</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s1">'3. This should appear once on stderr.'</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">'4. This should not appear.'</span><span class="p">)</span> <span class="n">h</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">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span> <span class="k">with</span> <span class="n">LoggingContext</span><span class="p">(</span><span class="n">logger</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">handler</span><span class="o">=</span><span class="n">h</span><span class="p">,</span> <span class="n">close</span><span class="o">=</span><span class="kc">True</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">'5. This should appear twice - once on stderr and once on stdout.'</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">'6. This should appear just once on stderr.'</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">'7. This should not appear.'</span><span class="p">)</span> </pre></div> </div> <p>We initially set the logger’s level to <code class="docutils literal notranslate"><span class="pre">INFO</span></code>, so message #1 appears and message #2 doesn’t. We then change the level to <code class="docutils literal notranslate"><span class="pre">DEBUG</span></code> temporarily in the following <code class="docutils literal notranslate"><span class="pre">with</span></code> block, and so message #3 appears. After the block exits, the logger’s level is restored to <code class="docutils literal notranslate"><span class="pre">INFO</span></code> and so message #4 doesn’t appear. In the next <code class="docutils literal notranslate"><span class="pre">with</span></code> block, we set the level to <code class="docutils literal notranslate"><span class="pre">DEBUG</span></code> again but also add a handler writing to <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code>. Thus, message #5 appears twice on the console (once via <code class="docutils literal notranslate"><span class="pre">stderr</span></code> and once via <code class="docutils literal notranslate"><span class="pre">stdout</span></code>). After the <code class="docutils literal notranslate"><span class="pre">with</span></code> statement’s completion, the status is as it was before so message #6 appears (like message #1) whereas message #7 doesn’t (just like message #2).</p> <p>If we run the resulting script, the result is as follows:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py <span class="go">1. This should appear just once on stderr.</span> <span class="go">3. This should appear once on stderr.</span> <span class="go">5. This should appear twice - once on stderr and once on stdout.</span> <span class="go">5. This should appear twice - once on stderr and once on stdout.</span> <span class="go">6. This should appear just once on stderr.</span> </pre></div> </div> <p>If we run it again, but pipe <code class="docutils literal notranslate"><span class="pre">stderr</span></code> to <code class="docutils literal notranslate"><span class="pre">/dev/null</span></code>, we see the following, which is the only message written to <code class="docutils literal notranslate"><span class="pre">stdout</span></code>:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py <span class="m">2</span>>/dev/null <span class="go">5. This should appear twice - once on stderr and once on stdout.</span> </pre></div> </div> <p>Once again, but piping <code class="docutils literal notranslate"><span class="pre">stdout</span></code> to <code class="docutils literal notranslate"><span class="pre">/dev/null</span></code>, we get:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python logctx.py >/dev/null <span class="go">1. This should appear just once on stderr.</span> <span class="go">3. This should appear once on stderr.</span> <span class="go">5. This should appear twice - once on stderr and once on stdout.</span> <span class="go">6. This should appear just once on stderr.</span> </pre></div> </div> <p>In this case, the message #5 printed to <code class="docutils literal notranslate"><span class="pre">stdout</span></code> doesn’t appear, as expected.</p> <p>Of course, the approach described here can be generalised, for example to attach logging filters temporarily. Note that the above code works in Python 2 as well as Python 3.</p> </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 Cookbook</a><ul> <li><a class="reference internal" href="#using-logging-in-multiple-modules">Using logging in multiple modules</a></li> <li><a class="reference internal" href="#logging-from-multiple-threads">Logging from multiple threads</a></li> <li><a class="reference internal" href="#multiple-handlers-and-formatters">Multiple handlers and formatters</a></li> <li><a class="reference internal" href="#logging-to-multiple-destinations">Logging to multiple destinations</a></li> <li><a class="reference internal" href="#configuration-server-example">Configuration server example</a></li> <li><a class="reference internal" href="#sending-and-receiving-logging-events-across-a-network">Sending and receiving logging events across a network</a></li> <li><a class="reference internal" href="#adding-contextual-information-to-your-logging-output">Adding contextual information to your logging output</a><ul> <li><a class="reference internal" href="#using-loggeradapters-to-impart-contextual-information">Using LoggerAdapters to impart contextual information</a><ul> <li><a class="reference internal" href="#using-objects-other-than-dicts-to-pass-contextual-information">Using objects other than dicts to pass contextual information</a></li> </ul> </li> <li><a class="reference internal" href="#using-filters-to-impart-contextual-information">Using Filters to impart contextual information</a></li> </ul> </li> <li><a class="reference internal" href="#logging-to-a-single-file-from-multiple-processes">Logging to a single file from multiple processes</a></li> <li><a class="reference internal" href="#using-file-rotation">Using file rotation</a></li> <li><a class="reference internal" href="#an-example-dictionary-based-configuration">An example dictionary-based configuration</a></li> <li><a class="reference internal" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler">Inserting a BOM into messages sent to a SysLogHandler</a></li> <li><a class="reference internal" href="#implementing-structured-logging">Implementing structured logging</a></li> <li><a class="reference internal" href="#customizing-handlers-with-dictconfig">Customizing handlers with <code class="docutils literal notranslate"><span class="pre">dictConfig()</span></code></a></li> <li><a class="reference internal" href="#configuring-filters-with-dictconfig">Configuring filters with <code class="docutils literal notranslate"><span class="pre">dictConfig()</span></code></a></li> <li><a class="reference internal" href="#customized-exception-formatting">Customized exception formatting</a></li> <li><a class="reference internal" href="#speaking-logging-messages">Speaking logging messages</a></li> <li><a class="reference internal" href="#buffering-logging-messages-and-outputting-them-conditionally">Buffering logging messages and outputting them conditionally</a></li> <li><a class="reference internal" href="#formatting-times-using-utc-gmt-via-configuration">Formatting times using UTC (GMT) via configuration</a></li> <li><a class="reference internal" href="#using-a-context-manager-for-selective-logging">Using a context manager for selective logging</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="logging.html" title="previous chapter">Logging HOWTO</a></p> <h4>Next topic</h4> <p class="topless"><a href="regex.html" title="next chapter">Regular Expression HOWTO</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/howto/logging-cookbook.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"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </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" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="regex.html" title="Regular Expression HOWTO" >next</a> |</li> <li class="right" > <a href="logging.html" title="Logging HOWTO" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; 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