<!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="Content-Type" content="text/html; charset=utf-8" /> <title>Capturing of the stdout/stderr output</title> <link rel="stylesheet" href="_static/sphinxdoc.css" type="text/css" /> <link rel="stylesheet" href="_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '', VERSION: '2.3.4.1', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </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> <link rel="top" title="None" href="index.html" /> <link rel="up" title="py.test reference documentation" href="apiref.html" /> <link rel="next" title="Monkeypatching/mocking modules and environments" href="monkeypatch.html" /> <link rel="prev" title="classic xunit-style setup" href="xunit_setup.html" /> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="monkeypatch.html" title="Monkeypatching/mocking modules and environments" accesskey="N">next</a></li> <li class="right" > <a href="xunit_setup.html" title="classic xunit-style setup" accesskey="P">previous</a> |</li> <li><a href="contents.html">pytest-2.3.4.1</a> »</li> <li><a href="apiref.html" accesskey="U">py.test reference documentation</a> »</li> <g:plusone></g:plusone> </ul> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <div id="searchbox" style="display: none"> <form class="search" action="search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Search" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('#searchbox').show(0);</script> <h3>quicklinks</h3> <div style="text-align: left; font-size: 100%; vertical-align: middle;"> <table> <tr> <td> <a href="index.html">home</a> </td><td> <a href="contents.html">TOC/contents</a> </td></tr><tr><td> <a href="getting-started.html">install</a> </td><td> <a href="changelog.html">changelog</a> </td></tr><tr><td> <a href="example/index.html">examples</a> </td><td> <a href="customize.html">customize</a> </td></tr><tr><td> <a href="https://bitbucket.org/hpk42/pytest/issues?status=new&status=open">issues[bb]</a> </td><td> <a href="contact.html">contact</a> </td></tr></table> </div> <h3><a href="contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Capturing of the stdout/stderr output</a><ul> <li><a class="reference internal" href="#default-stdout-stderr-stdin-capturing-behaviour">Default stdout/stderr/stdin capturing behaviour</a></li> <li><a class="reference internal" href="#setting-capturing-methods-or-disabling-capturing">Setting capturing methods or disabling capturing</a></li> <li><a class="reference internal" href="#using-print-statements-for-debugging">Using print statements for debugging</a></li> <li><a class="reference internal" href="#accessing-captured-output-from-a-test-function">Accessing captured output from a test function</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="xunit_setup.html" title="previous chapter">classic xunit-style setup</a></p> <h4>Next topic</h4> <p class="topless"><a href="monkeypatch.html" title="next chapter">Monkeypatching/mocking modules and environments</a></p> </div> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="capturing-of-the-stdout-stderr-output"> <span id="captures"></span><h1>Capturing of the stdout/stderr output<a class="headerlink" href="#capturing-of-the-stdout-stderr-output" title="Permalink to this headline">¶</a></h1> <div class="section" id="default-stdout-stderr-stdin-capturing-behaviour"> <h2>Default stdout/stderr/stdin capturing behaviour<a class="headerlink" href="#default-stdout-stderr-stdin-capturing-behaviour" title="Permalink to this headline">¶</a></h2> <p>During test execution any output sent to <tt class="docutils literal"><span class="pre">stdout</span></tt> and <tt class="docutils literal"><span class="pre">stderr</span></tt> is captured. If a test or a setup method fails its according captured output will usually be shown along with the failure traceback.</p> <p>In addition, <tt class="docutils literal"><span class="pre">stdin</span></tt> is set to a “null” object which will fail on attempts to read from it because it is rarely desired to wait for interactive input when running automated tests.</p> <p>By default capturing is done by intercepting writes to low level file descriptors. This allows to capture output from simple print statements as well as output from a subprocess started by a test.</p> </div> <div class="section" id="setting-capturing-methods-or-disabling-capturing"> <h2>Setting capturing methods or disabling capturing<a class="headerlink" href="#setting-capturing-methods-or-disabling-capturing" title="Permalink to this headline">¶</a></h2> <p>There are two ways in which <tt class="docutils literal"><span class="pre">py.test</span></tt> can perform capturing:</p> <ul class="simple"> <li>file descriptor (FD) level capturing (default): All writes going to the operating system file descriptors 1 and 2 will be captured.</li> <li><tt class="docutils literal"><span class="pre">sys</span></tt> level capturing: Only writes to Python files <tt class="docutils literal"><span class="pre">sys.stdout</span></tt> and <tt class="docutils literal"><span class="pre">sys.stderr</span></tt> will be captured. No capturing of writes to filedescriptors is performed.</li> </ul> <p id="disable-capturing">You can influence output capturing mechanisms from the command line:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">py</span><span class="o">.</span><span class="n">test</span> <span class="o">-</span><span class="n">s</span> <span class="c"># disable all capturing</span> <span class="n">py</span><span class="o">.</span><span class="n">test</span> <span class="o">--</span><span class="n">capture</span><span class="o">=</span><span class="n">sys</span> <span class="c"># replace sys.stdout/stderr with in-mem files</span> <span class="n">py</span><span class="o">.</span><span class="n">test</span> <span class="o">--</span><span class="n">capture</span><span class="o">=</span><span class="n">fd</span> <span class="c"># also point filedescriptors 1 and 2 to temp file</span> </pre></div> </div> </div> <div class="section" id="using-print-statements-for-debugging"> <span id="printdebugging"></span><h2>Using print statements for debugging<a class="headerlink" href="#using-print-statements-for-debugging" title="Permalink to this headline">¶</a></h2> <p>One primary benefit of the default capturing of stdout/stderr output is that you can use print statements for debugging:</p> <div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_module.py</span> <span class="k">def</span> <span class="nf">setup_function</span><span class="p">(</span><span class="n">function</span><span class="p">):</span> <span class="k">print</span> <span class="p">(</span><span class="s">"setting up </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="n">function</span><span class="p">)</span> <span class="k">def</span> <span class="nf">test_func1</span><span class="p">():</span> <span class="k">assert</span> <span class="bp">True</span> <span class="k">def</span> <span class="nf">test_func2</span><span class="p">():</span> <span class="k">assert</span> <span class="bp">False</span> </pre></div> </div> <p>and running this module will show you precisely the output of the failing function and hide the other one:</p> <div class="highlight-python"><pre>$ py.test =========================== test session starts ============================ platform linux2 -- Python 2.7.3 -- pytest-2.3.5 collected 2 items test_module.py .F ================================= FAILURES ================================= ________________________________ test_func2 ________________________________ def test_func2(): > assert False E assert False test_module.py:9: AssertionError ----------------------------- Captured stdout ------------------------------ setting up <function test_func2 at 0x2d79f50> ==================== 1 failed, 1 passed in 0.01 seconds ====================</pre> </div> </div> <div class="section" id="accessing-captured-output-from-a-test-function"> <h2>Accessing captured output from a test function<a class="headerlink" href="#accessing-captured-output-from-a-test-function" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="fixture.html#funcarg-mechanism"><em>Fixtures as Function arguments (funcargs)</em></a> allows test function a very easy way to access the captured output by simply using the names <tt class="docutils literal"><span class="pre">capsys</span></tt> or <tt class="docutils literal"><span class="pre">capfd</span></tt> in the test function signature. Here is an example test function that performs some output related checks:</p> <div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">test_myoutput</span><span class="p">(</span><span class="n">capsys</span><span class="p">):</span> <span class="c"># or use "capfd" for fd-level</span> <span class="k">print</span> <span class="p">(</span><span class="s">"hello"</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="s">"world</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span> <span class="n">out</span><span class="p">,</span> <span class="n">err</span> <span class="o">=</span> <span class="n">capsys</span><span class="o">.</span><span class="n">readouterr</span><span class="p">()</span> <span class="k">assert</span> <span class="n">out</span> <span class="o">==</span> <span class="s">"hello</span><span class="se">\n</span><span class="s">"</span> <span class="k">assert</span> <span class="n">err</span> <span class="o">==</span> <span class="s">"world</span><span class="se">\n</span><span class="s">"</span> <span class="k">print</span> <span class="s">"next"</span> <span class="n">out</span><span class="p">,</span> <span class="n">err</span> <span class="o">=</span> <span class="n">capsys</span><span class="o">.</span><span class="n">readouterr</span><span class="p">()</span> <span class="k">assert</span> <span class="n">out</span> <span class="o">==</span> <span class="s">"next</span><span class="se">\n</span><span class="s">"</span> </pre></div> </div> <p>The <tt class="docutils literal"><span class="pre">readouterr()</span></tt> call snapshots the output so far - and capturing will be continued. After the test function finishes the original streams will be restored. Using <tt class="docutils literal"><span class="pre">capsys</span></tt> this way frees your test from having to care about setting/resetting output streams and also interacts well with py.test’s own per-test capturing.</p> <p>If you want to capture on <tt class="docutils literal"><span class="pre">fd</span></tt> level you can use the <tt class="docutils literal"><span class="pre">capfd</span></tt> function argument which offers the exact same interface.</p> </div> </div> </div> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="monkeypatch.html" title="Monkeypatching/mocking modules and environments" >next</a></li> <li class="right" > <a href="xunit_setup.html" title="classic xunit-style setup" >previous</a> |</li> <li><a href="contents.html">pytest-2.3.4.1</a> »</li> <li><a href="apiref.html" >py.test reference documentation</a> »</li> <g:plusone></g:plusone> </ul> </div> <div class="footer"> © Copyright 2012, holger krekel. 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