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<div class="section" id="module-popen2">
<h1>18.5. <tt class="xref docutils literal"><span class="pre">popen2</span></tt> — Subprocesses with accessible I/O streams<a class="headerlink" href="#module-popen2" title="Permalink to this headline">¶</a></h1>
<p class="deprecated">
<span class="versionmodified">Deprecated since version 2.6: </span>This module is obsolete. Use the <a title="Subprocess management." class="reference external" href="subprocess.html#module-subprocess"><tt class="xref docutils literal"><span class="pre">subprocess</span></tt></a> module. Check
especially the <a class="reference external" href="subprocess.html#subprocess-replacements"><em>Replacing Older Functions with the subprocess Module</em></a> section.</p>
<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 title="Subprocess management." class="reference external" href="subprocess.html#module-subprocess"><tt class="xref docutils literal"><span class="pre">subprocess</span></tt></a> module provides more powerful facilities for spawning new
processes and retrieving their results. Using the <a title="Subprocess management." class="reference external" href="subprocess.html#module-subprocess"><tt class="xref docutils literal"><span class="pre">subprocess</span></tt></a> module is
preferable to using the <tt class="xref docutils literal"><span class="pre">popen2</span></tt> 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 <tt class="docutils literal"><span class="pre">'b'</span></tt> or <tt class="docutils literal"><span class="pre">'t'</span></tt>; 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 <tt class="docutils literal"><span class="pre">'t'</span></tt>.</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 title="os.spawnv" class="reference external" href="os.html#os.spawnv"><tt class="xref docutils literal"><span class="pre">os.spawnv()</span></tt></a>).
If <em>cmd</em> is a string it will be passed to the shell (as with <a title="os.system" class="reference external" href="os.html#os.system"><tt class="xref docutils literal"><span class="pre">os.system()</span></tt></a>).</p>
<p>The only way to retrieve the return codes for the child processes is by using
the <tt class="xref docutils literal"><span class="pre">poll()</span></tt> or <tt class="xref docutils literal"><span class="pre">wait()</span></tt> methods on the <a title="popen2.Popen3" class="reference internal" href="#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">Popen3</span></tt></a> and
<a title="popen2.Popen4" class="reference internal" href="#popen2.Popen4"><tt class="xref docutils literal"><span class="pre">Popen4</span></tt></a> classes; these are only available on Unix. This information is
not available when using the <a title="popen2.popen2" class="reference internal" href="#popen2.popen2"><tt class="xref docutils literal"><span class="pre">popen2()</span></tt></a>, <a title="popen2.popen3" class="reference internal" href="#popen2.popen3"><tt class="xref docutils literal"><span class="pre">popen3()</span></tt></a>, and <a title="popen2.popen4" class="reference internal" href="#popen2.popen4"><tt class="xref docutils literal"><span class="pre">popen4()</span></tt></a>
functions, or the equivalent functions in the <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module. (Note that the
tuples returned by the <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module’s functions are in a different order
from the ones returned by the <tt class="xref docutils literal"><span class="pre">popen2</span></tt> module.)</p>
<dl class="function">
<dt id="popen2.popen2">
<tt class="descclassname">popen2.</tt><tt class="descname">popen2</tt><big>(</big><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><big>)</big><a class="headerlink" href="#popen2.popen2" title="Permalink to this definition">¶</a></dt>
<dd>Executes <em>cmd</em> as a sub-process. Returns the file objects <tt class="docutils literal"><span class="pre">(child_stdout,</span>
<span class="pre">child_stdin)</span></tt>.</dd></dl>
<dl class="function">
<dt id="popen2.popen3">
<tt class="descclassname">popen2.</tt><tt class="descname">popen3</tt><big>(</big><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><big>)</big><a class="headerlink" href="#popen2.popen3" title="Permalink to this definition">¶</a></dt>
<dd>Executes <em>cmd</em> as a sub-process. Returns the file objects <tt class="docutils literal"><span class="pre">(child_stdout,</span>
<span class="pre">child_stdin,</span> <span class="pre">child_stderr)</span></tt>.</dd></dl>
<dl class="function">
<dt id="popen2.popen4">
<tt class="descclassname">popen2.</tt><tt class="descname">popen4</tt><big>(</big><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><big>)</big><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
<tt class="docutils literal"><span class="pre">(child_stdout_and_stderr,</span> <span class="pre">child_stdin)</span></tt>.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.0.</span></p>
</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><tt class="descclassname">popen2.</tt><tt class="descname">Popen3</tt><big>(</big><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><big>)</big><a class="headerlink" href="#popen2.Popen3" title="Permalink to this definition">¶</a></dt>
<dd><p>This class represents a child process. Normally, <a title="popen2.Popen3" class="reference internal" href="#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">Popen3</span></tt></a> instances are
created using the <a title="popen2.popen2" class="reference internal" href="#popen2.popen2"><tt class="xref docutils literal"><span class="pre">popen2()</span></tt></a> and <a title="popen2.popen3" class="reference internal" href="#popen2.popen3"><tt class="xref docutils literal"><span class="pre">popen3()</span></tt></a> factory functions described
above.</p>
<p>If not using one of the helper functions to create <a title="popen2.Popen3" class="reference internal" href="#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">Popen3</span></tt></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><tt class="descclassname">popen2.</tt><tt class="descname">Popen4</tt><big>(</big><em>cmd</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#popen2.Popen4" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <a title="popen2.Popen3" class="reference internal" href="#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">Popen3</span></tt></a>, but always captures standard error into the same
file object as standard output. These are typically created using
<a title="popen2.popen4" class="reference internal" href="#popen2.popen4"><tt class="xref docutils literal"><span class="pre">popen4()</span></tt></a>.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.0.</span></p>
</dd></dl>
<div class="section" id="popen3-and-popen4-objects">
<span id="popen3-objects"></span><h2>18.5.1. Popen3 and Popen4 Objects<a class="headerlink" href="#popen3-and-popen4-objects" title="Permalink to this headline">¶</a></h2>
<p>Instances of the <a title="popen2.Popen3" class="reference internal" href="#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">Popen3</span></tt></a> and <a title="popen2.Popen4" class="reference internal" href="#popen2.Popen4"><tt class="xref docutils literal"><span class="pre">Popen4</span></tt></a> classes have the following
methods:</p>
<dl class="method">
<dt id="popen2.Popen3.poll">
<tt class="descclassname">Popen3.</tt><tt class="descname">poll</tt><big>(</big><big>)</big><a class="headerlink" href="#popen2.Popen3.poll" title="Permalink to this definition">¶</a></dt>
<dd>Returns <tt class="docutils literal"><span class="pre">-1</span></tt> if child process hasn’t completed yet, or its status code
(see <a title="popen2.Popen3.wait" class="reference internal" href="#popen2.Popen3.wait"><tt class="xref docutils literal"><span class="pre">wait()</span></tt></a>) otherwise.</dd></dl>
<dl class="method">
<dt id="popen2.Popen3.wait">
<tt class="descclassname">Popen3.</tt><tt class="descname">wait</tt><big>(</big><big>)</big><a class="headerlink" href="#popen2.Popen3.wait" title="Permalink to this definition">¶</a></dt>
<dd>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 <tt class="xref docutils literal"><span class="pre">exit()</span></tt> system call or died due to a signal. Functions
to help interpret the status code are defined in the <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module; see
section <a class="reference external" href="os.html#os-process"><em>Process Management</em></a> for the <tt class="xref docutils literal"><span class="pre">W*()</span></tt> family of functions.</dd></dl>
<p>The following attributes are also available:</p>
<dl class="attribute">
<dt id="popen2.Popen3.fromchild">
<tt class="descclassname">Popen3.</tt><tt class="descname">fromchild</tt><a class="headerlink" href="#popen2.Popen3.fromchild" title="Permalink to this definition">¶</a></dt>
<dd>A file object that provides output from the child process. For <a title="popen2.Popen4" class="reference internal" href="#popen2.Popen4"><tt class="xref docutils literal"><span class="pre">Popen4</span></tt></a>
instances, this will provide both the standard output and standard error
streams.</dd></dl>
<dl class="attribute">
<dt id="popen2.Popen3.tochild">
<tt class="descclassname">Popen3.</tt><tt class="descname">tochild</tt><a class="headerlink" href="#popen2.Popen3.tochild" title="Permalink to this definition">¶</a></dt>
<dd>A file object that provides input to the child process.</dd></dl>
<dl class="attribute">
<dt id="popen2.Popen3.childerr">
<tt class="descclassname">Popen3.</tt><tt class="descname">childerr</tt><a class="headerlink" href="#popen2.Popen3.childerr" title="Permalink to this definition">¶</a></dt>
<dd>A file object that provides error output from the child process, if
<em>capturestderr</em> was true for the constructor, otherwise <tt class="xref docutils literal"><span class="pre">None</span></tt>. This will
always be <tt class="xref docutils literal"><span class="pre">None</span></tt> for <a title="popen2.Popen4" class="reference internal" href="#popen2.Popen4"><tt class="xref docutils literal"><span class="pre">Popen4</span></tt></a> instances.</dd></dl>
<dl class="attribute">
<dt id="popen2.Popen3.pid">
<tt class="descclassname">Popen3.</tt><tt class="descname">pid</tt><a class="headerlink" href="#popen2.Popen3.pid" title="Permalink to this definition">¶</a></dt>
<dd>The process ID of the child process.</dd></dl>
</div>
<div class="section" id="flow-control-issues">
<span id="popen2-flow-control"></span><h2>18.5.2. 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 title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></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 <tt class="xref docutils literal"><span class="pre">_PC_PIPE_BUF</span></tt> 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-python"><div class="highlight"><pre><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="s">'python slave.py'</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-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># note that each of these print statements</span>
<span class="c"># writes a single long string</span>
<span class="k">print</span> <span class="o">>></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="s">'this is a test</span><span class="se">\n</span><span class="s">'</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="k">print</span> <span class="o">>></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="s">'this is another test</span><span class="se">\n</span><span class="s">'</span>
</pre></div>
</div>
<p>In particular, note that <tt class="docutils literal"><span class="pre">sys.stderr</span></tt> must be closed after writing all data,
or <tt class="xref docutils literal"><span class="pre">readlines()</span></tt> won’t return. Also note that <a title="os.close" class="reference external" href="os.html#os.close"><tt class="xref docutils literal"><span class="pre">os.close()</span></tt></a> must be
used, as <tt class="docutils literal"><span class="pre">sys.stderr.close()</span></tt> won’t close <tt class="docutils literal"><span class="pre">stderr</span></tt> (otherwise assigning to
<tt class="docutils literal"><span class="pre">sys.stderr</span></tt> 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 <tt class="xref docutils literal"><span class="pre">select()</span></tt> loops, or use separate threads to read each
of the individual files provided by whichever <tt class="xref docutils literal"><span class="pre">popen*()</span></tt> function or
<tt class="xref docutils literal"><span class="pre">Popen*</span></tt> class was used.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a title="Subprocess management." class="reference external" href="subprocess.html#module-subprocess"><tt class="xref docutils literal"><span class="pre">subprocess</span></tt></a></dt>
<dd>Module for spawning and managing subprocesses.</dd>
</dl>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">18.5. <tt class="docutils literal"><span class="pre">popen2</span></tt> — Subprocesses with accessible I/O streams</a><ul>
<li><a class="reference external" href="#popen3-and-popen4-objects">18.5.1. Popen3 and Popen4 Objects</a></li>
<li><a class="reference external" href="#flow-control-issues">18.5.2. Flow Control Issues</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
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