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<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">pytest</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 linux -- Python 3.4.0 -- py-1.4.26 -- pytest-2.7.0
rootdir: /tmp/doc-exec-101, inifile:
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 setup ---------------------------
setting up <function test_func2 at 0x2b24d5259158>
==================== 1 failed, 1 passed in 0.01 seconds ====================</pre>
</div>
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<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 <tt class="docutils literal"><span class="pre">capsys</span></tt> and <tt class="docutils literal"><span class="pre">capfd</span></tt> fixtures allow to access stdout/stderr
output created during test execution. 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 pytest’s
own per-test capturing.</p>
<p>If you want to capture on filedescriptor level you can use
the <tt class="docutils literal"><span class="pre">capfd</span></tt> function argument which offers the exact
same interface but allows to also capture output from
libraries or subprocesses that directly write to operating
system level output streams (FD1 and FD2).</p>
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<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>
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