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  <div class="section" id="module-popen2">
<span id="popen2-subprocesses-with-accessible-i-o-streams"></span><h1>17.5. <a class="reference internal" href="#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">popen2</span></code></a> — Subprocesses with accessible I/O streams<a class="headerlink" href="#module-popen2" title="Permalink to this headline">¶</a></h1>
<div class="deprecated">
<p><span class="versionmodified deprecated">Deprecated since version 2.6: </span>This module is obsolete.  Use the <a class="reference internal" href="subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a> module.  Check
especially the <a class="reference internal" href="subprocess.html#subprocess-replacements"><span class="std std-ref">Replacing Older Functions with the subprocess Module</span></a> section.</p>
</div>
<p>This module allows you to spawn processes and connect to their
input/output/error pipes and obtain their return codes under Unix and Windows.</p>
<p>The <a class="reference internal" href="subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a> module provides more powerful facilities for spawning new
processes and retrieving their results.  Using the <a class="reference internal" href="subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a> module is
preferable to using the <a class="reference internal" href="#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">popen2</span></code></a> module.</p>
<p>The primary interface offered by this module is a trio of factory functions.
For each of these, if <em>bufsize</em> is specified,  it specifies the buffer size for
the I/O pipes.  <em>mode</em>, if provided, should be the string <code class="docutils literal notranslate"><span class="pre">'b'</span></code> or <code class="docutils literal notranslate"><span class="pre">'t'</span></code>; on
Windows this is needed to determine whether the file objects should be opened in
binary or text mode.  The default value for <em>mode</em> is <code class="docutils literal notranslate"><span class="pre">'t'</span></code>.</p>
<p>On Unix, <em>cmd</em> may be a sequence, in which case arguments will be passed
directly to the program without shell intervention (as with <a class="reference internal" href="os.html#os.spawnv" title="os.spawnv"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.spawnv()</span></code></a>).
If <em>cmd</em> is a string it will be passed to the shell (as with <a class="reference internal" href="os.html#os.system" title="os.system"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.system()</span></code></a>).</p>
<p>The only way to retrieve the return codes for the child processes is by using
the <code class="xref py py-meth docutils literal notranslate"><span class="pre">poll()</span></code> or <code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code> methods on the <a class="reference internal" href="#popen2.Popen3" title="popen2.Popen3"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen3</span></code></a> and
<a class="reference internal" href="#popen2.Popen4" title="popen2.Popen4"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen4</span></code></a> classes; these are only available on Unix.  This information is
not available when using the <a class="reference internal" href="#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen2()</span></code></a>, <a class="reference internal" href="#popen2.popen3" title="popen2.popen3"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen3()</span></code></a>, and <a class="reference internal" href="#popen2.popen4" title="popen2.popen4"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen4()</span></code></a>
functions, or the equivalent functions in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module. (Note that the
tuples returned by the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module’s functions are in a different order
from the ones returned by the <a class="reference internal" href="#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">popen2</span></code></a> module.)</p>
<dl class="function">
<dt id="popen2.popen2">
<code class="descclassname">popen2.</code><code class="descname">popen2</code><span class="sig-paren">(</span><em>cmd</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">[</span>, <em>mode</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.popen2" title="Permalink to this definition">¶</a></dt>
<dd><p>Executes <em>cmd</em> as a sub-process.  Returns the file objects <code class="docutils literal notranslate"><span class="pre">(child_stdout,</span>
<span class="pre">child_stdin)</span></code>.</p>
</dd></dl>

<dl class="function">
<dt id="popen2.popen3">
<code class="descclassname">popen2.</code><code class="descname">popen3</code><span class="sig-paren">(</span><em>cmd</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">[</span>, <em>mode</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.popen3" title="Permalink to this definition">¶</a></dt>
<dd><p>Executes <em>cmd</em> as a sub-process.  Returns the file objects <code class="docutils literal notranslate"><span class="pre">(child_stdout,</span>
<span class="pre">child_stdin,</span> <span class="pre">child_stderr)</span></code>.</p>
</dd></dl>

<dl class="function">
<dt id="popen2.popen4">
<code class="descclassname">popen2.</code><code class="descname">popen4</code><span class="sig-paren">(</span><em>cmd</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">[</span>, <em>mode</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.popen4" title="Permalink to this definition">¶</a></dt>
<dd><p>Executes <em>cmd</em> as a sub-process.  Returns the file objects
<code class="docutils literal notranslate"><span class="pre">(child_stdout_and_stderr,</span> <span class="pre">child_stdin)</span></code>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.0.</span></p>
</div>
</dd></dl>

<p>On Unix, a class defining the objects returned by the factory functions is also
available.  These are not used for the Windows implementation, and are not
available on that platform.</p>
<dl class="class">
<dt id="popen2.Popen3">
<em class="property">class </em><code class="descclassname">popen2.</code><code class="descname">Popen3</code><span class="sig-paren">(</span><em>cmd</em><span class="optional">[</span>, <em>capturestderr</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.Popen3" title="Permalink to this definition">¶</a></dt>
<dd><p>This class represents a child process.  Normally, <a class="reference internal" href="#popen2.Popen3" title="popen2.Popen3"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen3</span></code></a> instances are
created using the <a class="reference internal" href="#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen2()</span></code></a> and <a class="reference internal" href="#popen2.popen3" title="popen2.popen3"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen3()</span></code></a> factory functions described
above.</p>
<p>If not using one of the helper functions to create <a class="reference internal" href="#popen2.Popen3" title="popen2.Popen3"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen3</span></code></a> objects, the
parameter <em>cmd</em> is the shell command to execute in a sub-process.  The
<em>capturestderr</em> flag, if true, specifies that the object should capture standard
error output of the child process. The default is false.  If the <em>bufsize</em>
parameter is specified, it specifies the size of the I/O buffers to/from the
child process.</p>
</dd></dl>

<dl class="class">
<dt id="popen2.Popen4">
<em class="property">class </em><code class="descclassname">popen2.</code><code class="descname">Popen4</code><span class="sig-paren">(</span><em>cmd</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.Popen4" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <a class="reference internal" href="#popen2.Popen3" title="popen2.Popen3"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen3</span></code></a>, but always captures standard error into the same
file object as standard output.  These are typically created using
<a class="reference internal" href="#popen2.popen4" title="popen2.popen4"><code class="xref py py-func docutils literal notranslate"><span class="pre">popen4()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.0.</span></p>
</div>
</dd></dl>

<div class="section" id="popen3-and-popen4-objects">
<span id="popen3-objects"></span><h2>17.5.1. Popen3 and Popen4 Objects<a class="headerlink" href="#popen3-and-popen4-objects" title="Permalink to this headline">¶</a></h2>
<p>Instances of the <a class="reference internal" href="#popen2.Popen3" title="popen2.Popen3"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen3</span></code></a> and <a class="reference internal" href="#popen2.Popen4" title="popen2.Popen4"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen4</span></code></a> classes have the following
methods:</p>
<dl class="method">
<dt id="popen2.Popen3.poll">
<code class="descclassname">Popen3.</code><code class="descname">poll</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.Popen3.poll" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns <code class="docutils literal notranslate"><span class="pre">-1</span></code> if child process hasn’t completed yet, or its status code
(see <a class="reference internal" href="#popen2.Popen3.wait" title="popen2.Popen3.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a>) otherwise.</p>
</dd></dl>

<dl class="method">
<dt id="popen2.Popen3.wait">
<code class="descclassname">Popen3.</code><code class="descname">wait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#popen2.Popen3.wait" title="Permalink to this definition">¶</a></dt>
<dd><p>Waits for and returns the status code of the child process.  The status code
encodes both the return code of the process and information about whether it
exited using the <code class="xref c c-func docutils literal notranslate"><span class="pre">exit()</span></code> system call or died due to a signal.  Functions
to help interpret the status code are defined in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module; see
section <a class="reference internal" href="os.html#os-process"><span class="std std-ref">Process Management</span></a> for the <code class="xref py py-func docutils literal notranslate"><span class="pre">W*()</span></code> family of functions.</p>
</dd></dl>

<p>The following attributes are also available:</p>
<dl class="attribute">
<dt id="popen2.Popen3.fromchild">
<code class="descclassname">Popen3.</code><code class="descname">fromchild</code><a class="headerlink" href="#popen2.Popen3.fromchild" title="Permalink to this definition">¶</a></dt>
<dd><p>A file object that provides output from the child process.  For <a class="reference internal" href="#popen2.Popen4" title="popen2.Popen4"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen4</span></code></a>
instances, this will provide both the standard output and standard error
streams.</p>
</dd></dl>

<dl class="attribute">
<dt id="popen2.Popen3.tochild">
<code class="descclassname">Popen3.</code><code class="descname">tochild</code><a class="headerlink" href="#popen2.Popen3.tochild" title="Permalink to this definition">¶</a></dt>
<dd><p>A file object that provides input to the child process.</p>
</dd></dl>

<dl class="attribute">
<dt id="popen2.Popen3.childerr">
<code class="descclassname">Popen3.</code><code class="descname">childerr</code><a class="headerlink" href="#popen2.Popen3.childerr" title="Permalink to this definition">¶</a></dt>
<dd><p>A file object that provides error output from the child process, if
<em>capturestderr</em> was true for the constructor, otherwise <code class="docutils literal notranslate"><span class="pre">None</span></code>.  This will
always be <code class="docutils literal notranslate"><span class="pre">None</span></code> for <a class="reference internal" href="#popen2.Popen4" title="popen2.Popen4"><code class="xref py py-class docutils literal notranslate"><span class="pre">Popen4</span></code></a> instances.</p>
</dd></dl>

<dl class="attribute">
<dt id="popen2.Popen3.pid">
<code class="descclassname">Popen3.</code><code class="descname">pid</code><a class="headerlink" href="#popen2.Popen3.pid" title="Permalink to this definition">¶</a></dt>
<dd><p>The process ID of the child process.</p>
</dd></dl>

</div>
<div class="section" id="flow-control-issues">
<span id="popen2-flow-control"></span><h2>17.5.2. Flow Control Issues<a class="headerlink" href="#flow-control-issues" title="Permalink to this headline">¶</a></h2>
<p>Any time you are working with any form of inter-process communication, control
flow needs to be carefully thought out.  This remains the case with the file
objects provided by this module (or the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module equivalents).</p>
<p>When reading output from a child process that writes a lot of data to standard
error while the parent is reading from the child’s standard output, a deadlock
can occur.  A similar situation can occur with other combinations of reads and
writes.  The essential factors are that more than <code class="xref py py-const docutils literal notranslate"><span class="pre">_PC_PIPE_BUF</span></code> bytes
are being written by one process in a blocking fashion, while the other process
is reading from the first process, also in a blocking fashion.</p>
<p>There are several ways to deal with this situation.</p>
<p>The simplest application change, in many cases, will be to follow this model in
the parent process:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">popen2</span>

<span class="n">r</span><span class="p">,</span> <span class="n">w</span><span class="p">,</span> <span class="n">e</span> <span class="o">=</span> <span class="n">popen2</span><span class="o">.</span><span class="n">popen3</span><span class="p">(</span><span class="s1">&#39;python slave.py&#39;</span><span class="p">)</span>
<span class="n">e</span><span class="o">.</span><span class="n">readlines</span><span class="p">()</span>
<span class="n">r</span><span class="o">.</span><span class="n">readlines</span><span class="p">()</span>
<span class="n">r</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">e</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">w</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>with code like this in the child:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>

<span class="c1"># note that each of these print statements</span>
<span class="c1"># writes a single long string</span>

<span class="nb">print</span> <span class="o">&gt;&gt;</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="mi">400</span> <span class="o">*</span> <span class="s1">&#39;this is a test</span><span class="se">\n</span><span class="s1">&#39;</span>
<span class="n">os</span><span class="o">.</span><span class="n">close</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">fileno</span><span class="p">())</span>
<span class="nb">print</span> <span class="o">&gt;&gt;</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">,</span> <span class="mi">400</span> <span class="o">*</span> <span class="s1">&#39;this is another test</span><span class="se">\n</span><span class="s1">&#39;</span>
</pre></div>
</div>
<p>In particular, note that <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> must be closed after writing all data,
or <code class="xref py py-meth docutils literal notranslate"><span class="pre">readlines()</span></code> won’t return.  Also note that <a class="reference internal" href="os.html#os.close" title="os.close"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.close()</span></code></a> must be
used, as <code class="docutils literal notranslate"><span class="pre">sys.stderr.close()</span></code> won’t close <code class="docutils literal notranslate"><span class="pre">stderr</span></code> (otherwise assigning to
<code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> will silently close it, so no further errors can be printed).</p>
<p>Applications which need to support a more general approach should integrate I/O
over pipes with their <a class="reference internal" href="select.html#module-select" title="select: Wait for I/O completion on multiple streams."><code class="xref py py-func docutils literal notranslate"><span class="pre">select()</span></code></a> loops, or use separate threads to read each
of the individual files provided by whichever <code class="xref py py-func docutils literal notranslate"><span class="pre">popen*()</span></code> function or
<code class="xref py py-class docutils literal notranslate"><span class="pre">Popen*</span></code> class was used.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt>Module <a class="reference internal" href="subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate"><span class="pre">subprocess</span></code></a></dt><dd><p>Module for spawning and managing subprocesses.</p>
</dd>
</dl>
</div>
</div>
</div>


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  <h3><a href="../contents.html">Table of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">17.5. <code class="xref py py-mod docutils literal notranslate"><span class="pre">popen2</span></code> — Subprocesses with accessible I/O streams</a><ul>
<li><a class="reference internal" href="#popen3-and-popen4-objects">17.5.1. Popen3 and Popen4 Objects</a></li>
<li><a class="reference internal" href="#flow-control-issues">17.5.2. Flow Control Issues</a></li>
</ul>
</li>
</ul>

  <h4>Previous topic</h4>
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