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<div class="section" id="module-subprocess">
<h1>18.1. <tt class="xref docutils literal"><span class="pre">subprocess</span></tt> — Subprocess management<a class="headerlink" href="#module-subprocess" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.4.</span></p>
<p>The <tt class="xref docutils literal"><span class="pre">subprocess</span></tt> module allows you to spawn new processes, connect to their
input/output/error pipes, and obtain their return codes. This module intends to
replace several other, older modules and functions, such as:</p>
<div class="highlight-python"><pre>os.system
os.spawn*
os.popen*
popen2.*
commands.*</pre>
</div>
<p>Information about how the <tt class="xref docutils literal"><span class="pre">subprocess</span></tt> module can be used to replace these
modules and functions can be found in the following sections.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><span class="target" id="index-629"></span><a class="reference external" href="http://www.python.org/dev/peps/pep-0324"><strong>PEP 324</strong></a> – PEP proposing the subprocess module</p>
</div>
<div class="section" id="using-the-subprocess-module">
<h2>18.1.1. Using the subprocess Module<a class="headerlink" href="#using-the-subprocess-module" title="Permalink to this headline">¶</a></h2>
<p>This module defines one class called <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a>:</p>
<dl class="class">
<dt id="subprocess.Popen">
<em class="property">class </em><tt class="descclassname">subprocess.</tt><tt class="descname">Popen</tt><big>(</big><em>args</em>, <em>bufsize=0</em>, <em>executable=None</em>, <em>stdin=None</em>, <em>stdout=None</em>, <em>stderr=None</em>, <em>preexec_fn=None</em>, <em>close_fds=False</em>, <em>shell=False</em>, <em>cwd=None</em>, <em>env=None</em>, <em>universal_newlines=False</em>, <em>startupinfo=None</em>, <em>creationflags=0</em><big>)</big><a class="headerlink" href="#subprocess.Popen" title="Permalink to this definition">¶</a></dt>
<dd><p>Arguments are:</p>
<p><em>args</em> should be a string, or a sequence of program arguments. The program
to execute is normally the first item in the args sequence or the string if
a string is given, but can be explicitly set by using the <em>executable</em>
argument. When <em>executable</em> is given, the first item in the args sequence
is still treated by most programs as the command name, which can then be
different from the actual executable name. On Unix, it becomes the display
name for the executing program in utilities such as <strong>ps</strong>.</p>
<p>On Unix, with <em>shell=False</em> (default): In this case, the Popen class uses
<a title="os.execvp" class="reference external" href="os.html#os.execvp"><tt class="xref docutils literal"><span class="pre">os.execvp()</span></tt></a> to execute the child program. <em>args</em> should normally be a
sequence. If a string is specified for <em>args</em>, it will be used as the name
or path of the program to execute; this will only work if the program is
being given no arguments.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p><a title="shlex.split" class="reference external" href="shlex.html#shlex.split"><tt class="xref docutils literal"><span class="pre">shlex.split()</span></tt></a> can be useful when determining the correct
tokenization for <em>args</em>, especially in complex cases:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">shlex</span><span class="o">,</span> <span class="nn">subprocess</span>
<span class="gp">>>> </span><span class="n">command_line</span> <span class="o">=</span> <span class="nb">raw_input</span><span class="p">()</span>
<span class="go">/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"</span>
<span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="n">shlex</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="n">command_line</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">print</span> <span class="n">args</span>
<span class="go">['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]</span>
<span class="gp">>>> </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">args</span><span class="p">)</span> <span class="c"># Success!</span>
</pre></div>
</div>
<p class="last">Note in particular that options (such as <em>-input</em>) and arguments (such
as <em>eggs.txt</em>) that are separated by whitespace in the shell go in separate
list elements, while arguments that need quoting or backslash escaping when
used in the shell (such as filenames containing spaces or the <em>echo</em> command
shown above) are single list elements.</p>
</div>
<p>On Unix, with <em>shell=True</em>: If args is a string, it specifies the command
string to execute through the shell. This means that the string must be
formatted exactly as it would be when typed at the shell prompt. This
includes, for example, quoting or backslash escaping filenames with spaces in
them. If <em>args</em> is a sequence, the first item specifies the command string, and
any additional items will be treated as additional arguments to the shell
itself. That is to say, <em>Popen</em> does the equivalent of:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">Popen</span><span class="p">([</span><span class="s">'/bin/sh'</span><span class="p">,</span> <span class="s">'-c'</span><span class="p">,</span> <span class="n">args</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">args</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="o">...</span><span class="p">])</span>
</pre></div>
</div>
<p>On Windows: the <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> class uses CreateProcess() to execute the child
program, which operates on strings. If <em>args</em> is a sequence, it will be
converted to a string using the <tt class="xref docutils literal"><span class="pre">list2cmdline()</span></tt> method. Please note that
not all MS Windows applications interpret the command line the same way:
<tt class="xref docutils literal"><span class="pre">list2cmdline()</span></tt> is designed for applications using the same rules as the MS
C runtime.</p>
<p><em>bufsize</em>, if given, has the same meaning as the corresponding argument to the
built-in open() function: <tt class="xref docutils literal"><span class="pre">0</span></tt> means unbuffered, <tt class="xref docutils literal"><span class="pre">1</span></tt> means line
buffered, any other positive value means use a buffer of (approximately) that
size. A negative <em>bufsize</em> means to use the system default, which usually means
fully buffered. The default value for <em>bufsize</em> is <tt class="xref docutils literal"><span class="pre">0</span></tt> (unbuffered).</p>
<p>The <em>executable</em> argument specifies the program to execute. It is very seldom
needed: Usually, the program to execute is defined by the <em>args</em> argument. If
<tt class="docutils literal"><span class="pre">shell=True</span></tt>, the <em>executable</em> argument specifies which shell to use. On Unix,
the default shell is <tt class="docutils literal"><span class="pre">/bin/sh</span></tt>. On Windows, the default shell is
specified by the <span class="target" id="index-630"></span><strong class="xref">COMSPEC</strong> environment variable. The only reason you
would need to specify <tt class="docutils literal"><span class="pre">shell=True</span></tt> on Windows is where the command you
wish to execute is actually built in to the shell, eg <tt class="docutils literal"><span class="pre">dir</span></tt>, <tt class="docutils literal"><span class="pre">copy</span></tt>.
You don’t need <tt class="docutils literal"><span class="pre">shell=True</span></tt> to run a batch file, nor to run a console-based
executable.</p>
<p><em>stdin</em>, <em>stdout</em> and <em>stderr</em> specify the executed programs’ standard input,
standard output and standard error file handles, respectively. Valid values
are <a title="subprocess.PIPE" class="reference internal" href="#subprocess.PIPE"><tt class="xref docutils literal"><span class="pre">PIPE</span></tt></a>, an existing file descriptor (a positive integer), an
existing file object, and <tt class="xref docutils literal"><span class="pre">None</span></tt>. <a title="subprocess.PIPE" class="reference internal" href="#subprocess.PIPE"><tt class="xref docutils literal"><span class="pre">PIPE</span></tt></a> indicates that a new pipe
to the child should be created. With <tt class="xref docutils literal"><span class="pre">None</span></tt>, no redirection will occur;
the child’s file handles will be inherited from the parent. Additionally,
<em>stderr</em> can be <a title="subprocess.STDOUT" class="reference internal" href="#subprocess.STDOUT"><tt class="xref docutils literal"><span class="pre">STDOUT</span></tt></a>, which indicates that the stderr data from the
applications should be captured into the same file handle as for stdout.</p>
<p>If <em>preexec_fn</em> is set to a callable object, this object will be called in the
child process just before the child is executed. (Unix only)</p>
<p>If <em>close_fds</em> is true, all file descriptors except <tt class="xref docutils literal"><span class="pre">0</span></tt>, <tt class="xref docutils literal"><span class="pre">1</span></tt> and
<tt class="xref docutils literal"><span class="pre">2</span></tt> will be closed before the child process is executed. (Unix only).
Or, on Windows, if <em>close_fds</em> is true then no handles will be inherited by the
child process. Note that on Windows, you cannot set <em>close_fds</em> to true and
also redirect the standard handles by setting <em>stdin</em>, <em>stdout</em> or <em>stderr</em>.</p>
<p>If <em>shell</em> is <a title="True" class="reference external" href="constants.html#True"><tt class="xref xref docutils literal"><span class="pre">True</span></tt></a>, the specified command will be executed through the
shell.</p>
<p>If <em>cwd</em> is not <tt class="xref docutils literal"><span class="pre">None</span></tt>, the child’s current directory will be changed to <em>cwd</em>
before it is executed. Note that this directory is not considered when
searching the executable, so you can’t specify the program’s path relative to
<em>cwd</em>.</p>
<p>If <em>env</em> is not <tt class="xref docutils literal"><span class="pre">None</span></tt>, it must be a mapping that defines the environment
variables for the new process; these are used instead of inheriting the current
process’ environment, which is the default behavior.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If specified, <em>env</em> must provide any variables required
for the program to execute. On Windows, in order to run a
<a class="reference external" href="http://en.wikipedia.org/wiki/Side-by-Side_Assembly">side-by-side assembly</a> the specified <em>env</em> <strong>must</strong> include a valid
<span class="target" id="index-631"></span><strong class="xref">SystemRoot</strong>.</p>
</div>
<p>If <em>universal_newlines</em> is <a title="True" class="reference external" href="constants.html#True"><tt class="xref xref docutils literal"><span class="pre">True</span></tt></a>, the file objects stdout and stderr are
opened as text files, but lines may be terminated by any of <tt class="docutils literal"><span class="pre">'\n'</span></tt>, the Unix
end-of-line convention, <tt class="docutils literal"><span class="pre">'\r'</span></tt>, the old Macintosh convention or <tt class="docutils literal"><span class="pre">'\r\n'</span></tt>, the
Windows convention. All of these external representations are seen as <tt class="docutils literal"><span class="pre">'\n'</span></tt>
by the Python program.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This feature is only available if Python is built with universal newline support
(the default). Also, the newlines attribute of the file objects <a title="subprocess.Popen.stdout" class="reference internal" href="#subprocess.Popen.stdout"><tt class="xref docutils literal"><span class="pre">stdout</span></tt></a>,
<a title="subprocess.Popen.stdin" class="reference internal" href="#subprocess.Popen.stdin"><tt class="xref docutils literal"><span class="pre">stdin</span></tt></a> and <a title="subprocess.Popen.stderr" class="reference internal" href="#subprocess.Popen.stderr"><tt class="xref docutils literal"><span class="pre">stderr</span></tt></a> are not updated by the communicate() method.</p>
</div>
<p>The <em>startupinfo</em> and <em>creationflags</em>, if given, will be passed to the
underlying CreateProcess() function. They can specify things such as appearance
of the main window and priority for the new process. (Windows only)</p>
</dd></dl>
<dl class="data">
<dt id="subprocess.PIPE">
<tt class="descclassname">subprocess.</tt><tt class="descname">PIPE</tt><a class="headerlink" href="#subprocess.PIPE" title="Permalink to this definition">¶</a></dt>
<dd>Special value that can be used as the <em>stdin</em>, <em>stdout</em> or <em>stderr</em> argument
to <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> and indicates that a pipe to the standard stream should be
opened.</dd></dl>
<dl class="data">
<dt id="subprocess.STDOUT">
<tt class="descclassname">subprocess.</tt><tt class="descname">STDOUT</tt><a class="headerlink" href="#subprocess.STDOUT" title="Permalink to this definition">¶</a></dt>
<dd>Special value that can be used as the <em>stderr</em> argument to <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> and
indicates that standard error should go into the same handle as standard
output.</dd></dl>
<div class="section" id="convenience-functions">
<h3>18.1.1.1. Convenience Functions<a class="headerlink" href="#convenience-functions" title="Permalink to this headline">¶</a></h3>
<p>This module also defines two shortcut functions:</p>
<dl class="function">
<dt id="subprocess.call">
<tt class="descclassname">subprocess.</tt><tt class="descname">call</tt><big>(</big><em>*popenargs</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#subprocess.call" title="Permalink to this definition">¶</a></dt>
<dd><p>Run command with arguments. Wait for command to complete, then return the
<tt class="xref docutils literal"><span class="pre">returncode</span></tt> attribute.</p>
<p>The arguments are the same as for the Popen constructor. Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">retcode</span> <span class="o">=</span> <span class="n">call</span><span class="p">([</span><span class="s">"ls"</span><span class="p">,</span> <span class="s">"-l"</span><span class="p">])</span>
</pre></div>
</div>
</dd></dl>
<dl class="function">
<dt id="subprocess.check_call">
<tt class="descclassname">subprocess.</tt><tt class="descname">check_call</tt><big>(</big><em>*popenargs</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#subprocess.check_call" title="Permalink to this definition">¶</a></dt>
<dd><p>Run command with arguments. Wait for command to complete. If the exit code was
zero then return, otherwise raise <tt class="xref docutils literal"><span class="pre">CalledProcessError</span></tt>. The
<tt class="xref docutils literal"><span class="pre">CalledProcessError</span></tt> object will have the return code in the
<tt class="xref docutils literal"><span class="pre">returncode</span></tt> attribute.</p>
<p>The arguments are the same as for the Popen constructor. Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">check_call</span><span class="p">([</span><span class="s">"ls"</span><span class="p">,</span> <span class="s">"-l"</span><span class="p">])</span>
</pre></div>
</div>
<p class="versionadded">
<span class="versionmodified">New in version 2.5.</span></p>
</dd></dl>
</div>
<div class="section" id="exceptions">
<h3>18.1.1.2. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h3>
<p>Exceptions raised in the child process, before the new program has started to
execute, will be re-raised in the parent. Additionally, the exception object
will have one extra attribute called <tt class="xref docutils literal"><span class="pre">child_traceback</span></tt>, which is a string
containing traceback information from the childs point of view.</p>
<p>The most common exception raised is <a title="exceptions.OSError" class="reference external" href="exceptions.html#exceptions.OSError"><tt class="xref docutils literal"><span class="pre">OSError</span></tt></a>. This occurs, for example,
when trying to execute a non-existent file. Applications should prepare for
<a title="exceptions.OSError" class="reference external" href="exceptions.html#exceptions.OSError"><tt class="xref docutils literal"><span class="pre">OSError</span></tt></a> exceptions.</p>
<p>A <a title="exceptions.ValueError" class="reference external" href="exceptions.html#exceptions.ValueError"><tt class="xref docutils literal"><span class="pre">ValueError</span></tt></a> will be raised if <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> is called with invalid
arguments.</p>
<p>check_call() will raise <tt class="xref docutils literal"><span class="pre">CalledProcessError</span></tt>, if the called process returns
a non-zero return code.</p>
</div>
<div class="section" id="security">
<h3>18.1.1.3. Security<a class="headerlink" href="#security" title="Permalink to this headline">¶</a></h3>
<p>Unlike some other popen functions, this implementation will never call /bin/sh
implicitly. This means that all characters, including shell metacharacters, can
safely be passed to child processes.</p>
</div>
</div>
<div class="section" id="popen-objects">
<h2>18.1.2. Popen Objects<a class="headerlink" href="#popen-objects" title="Permalink to this headline">¶</a></h2>
<p>Instances of the <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> class have the following methods:</p>
<dl class="method">
<dt id="subprocess.Popen.poll">
<tt class="descclassname">Popen.</tt><tt class="descname">poll</tt><big>(</big><big>)</big><a class="headerlink" href="#subprocess.Popen.poll" title="Permalink to this definition">¶</a></dt>
<dd>Check if child process has terminated. Set and return <a title="subprocess.Popen.returncode" class="reference internal" href="#subprocess.Popen.returncode"><tt class="xref docutils literal"><span class="pre">returncode</span></tt></a>
attribute.</dd></dl>
<dl class="method">
<dt id="subprocess.Popen.wait">
<tt class="descclassname">Popen.</tt><tt class="descname">wait</tt><big>(</big><big>)</big><a class="headerlink" href="#subprocess.Popen.wait" title="Permalink to this definition">¶</a></dt>
<dd><p>Wait for child process to terminate. Set and return <a title="subprocess.Popen.returncode" class="reference internal" href="#subprocess.Popen.returncode"><tt class="xref docutils literal"><span class="pre">returncode</span></tt></a>
attribute.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">This will deadlock if the child process generates enough output to a
stdout or stderr pipe such that it blocks waiting for the OS pipe buffer
to accept more data. Use <a title="subprocess.Popen.communicate" class="reference internal" href="#subprocess.Popen.communicate"><tt class="xref docutils literal"><span class="pre">communicate()</span></tt></a> to avoid that.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="subprocess.Popen.communicate">
<tt class="descclassname">Popen.</tt><tt class="descname">communicate</tt><big>(</big><em>input=None</em><big>)</big><a class="headerlink" href="#subprocess.Popen.communicate" title="Permalink to this definition">¶</a></dt>
<dd><p>Interact with process: Send data to stdin. Read data from stdout and stderr,
until end-of-file is reached. Wait for process to terminate. The optional
<em>input</em> argument should be a string to be sent to the child process, or
<tt class="xref docutils literal"><span class="pre">None</span></tt>, if no data should be sent to the child.</p>
<p><a title="subprocess.Popen.communicate" class="reference internal" href="#subprocess.Popen.communicate"><tt class="xref docutils literal"><span class="pre">communicate()</span></tt></a> returns a tuple <tt class="docutils literal"><span class="pre">(stdoutdata,</span> <span class="pre">stderrdata)</span></tt>.</p>
<p>Note that if you want to send data to the process’s stdin, you need to create
the Popen object with <tt class="docutils literal"><span class="pre">stdin=PIPE</span></tt>. Similarly, to get anything other than
<tt class="xref docutils literal"><span class="pre">None</span></tt> in the result tuple, you need to give <tt class="docutils literal"><span class="pre">stdout=PIPE</span></tt> and/or
<tt class="docutils literal"><span class="pre">stderr=PIPE</span></tt> too.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The data read is buffered in memory, so do not use this method if the data
size is large or unlimited.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="subprocess.Popen.send_signal">
<tt class="descclassname">Popen.</tt><tt class="descname">send_signal</tt><big>(</big><em>signal</em><big>)</big><a class="headerlink" href="#subprocess.Popen.send_signal" title="Permalink to this definition">¶</a></dt>
<dd><p>Sends the signal <em>signal</em> to the child.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">On Windows only SIGTERM is supported so far. It’s an alias for
<a title="subprocess.Popen.terminate" class="reference internal" href="#subprocess.Popen.terminate"><tt class="xref docutils literal"><span class="pre">terminate()</span></tt></a>.</p>
</div>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="method">
<dt id="subprocess.Popen.terminate">
<tt class="descclassname">Popen.</tt><tt class="descname">terminate</tt><big>(</big><big>)</big><a class="headerlink" href="#subprocess.Popen.terminate" title="Permalink to this definition">¶</a></dt>
<dd><p>Stop the child. On Posix OSs the method sends SIGTERM to the
child. On Windows the Win32 API function <tt class="xref docutils literal"><span class="pre">TerminateProcess()</span></tt> is called
to stop the child.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="method">
<dt id="subprocess.Popen.kill">
<tt class="descclassname">Popen.</tt><tt class="descname">kill</tt><big>(</big><big>)</big><a class="headerlink" href="#subprocess.Popen.kill" title="Permalink to this definition">¶</a></dt>
<dd><p>Kills the child. On Posix OSs the function sends SIGKILL to the child.
On Windows <a title="subprocess.Popen.kill" class="reference internal" href="#subprocess.Popen.kill"><tt class="xref docutils literal"><span class="pre">kill()</span></tt></a> is an alias for <a title="subprocess.Popen.terminate" class="reference internal" href="#subprocess.Popen.terminate"><tt class="xref docutils literal"><span class="pre">terminate()</span></tt></a>.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<p>The following attributes are also available:</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Use <tt class="xref docutils literal"><span class="pre">communicate()</span></tt> rather than <tt class="xref docutils literal"><span class="pre">.stdin.write</span></tt>,
<tt class="xref docutils literal"><span class="pre">.stdout.read</span></tt> or <tt class="xref docutils literal"><span class="pre">.stderr.read</span></tt> to avoid
deadlocks due to any of the other OS pipe buffers filling up and blocking the
child process.</p>
</div>
<dl class="attribute">
<dt id="subprocess.Popen.stdin">
<tt class="descclassname">Popen.</tt><tt class="descname">stdin</tt><a class="headerlink" href="#subprocess.Popen.stdin" title="Permalink to this definition">¶</a></dt>
<dd>If the <em>stdin</em> argument was <a title="subprocess.PIPE" class="reference internal" href="#subprocess.PIPE"><tt class="xref docutils literal"><span class="pre">PIPE</span></tt></a>, this attribute is a file object
that provides input to the child process. Otherwise, it is <tt class="xref docutils literal"><span class="pre">None</span></tt>.</dd></dl>
<dl class="attribute">
<dt id="subprocess.Popen.stdout">
<tt class="descclassname">Popen.</tt><tt class="descname">stdout</tt><a class="headerlink" href="#subprocess.Popen.stdout" title="Permalink to this definition">¶</a></dt>
<dd>If the <em>stdout</em> argument was <a title="subprocess.PIPE" class="reference internal" href="#subprocess.PIPE"><tt class="xref docutils literal"><span class="pre">PIPE</span></tt></a>, this attribute is a file object
that provides output from the child process. Otherwise, it is <tt class="xref docutils literal"><span class="pre">None</span></tt>.</dd></dl>
<dl class="attribute">
<dt id="subprocess.Popen.stderr">
<tt class="descclassname">Popen.</tt><tt class="descname">stderr</tt><a class="headerlink" href="#subprocess.Popen.stderr" title="Permalink to this definition">¶</a></dt>
<dd>If the <em>stderr</em> argument was <a title="subprocess.PIPE" class="reference internal" href="#subprocess.PIPE"><tt class="xref docutils literal"><span class="pre">PIPE</span></tt></a>, this attribute is a file object
that provides error output from the child process. Otherwise, it is
<tt class="xref docutils literal"><span class="pre">None</span></tt>.</dd></dl>
<dl class="attribute">
<dt id="subprocess.Popen.pid">
<tt class="descclassname">Popen.</tt><tt class="descname">pid</tt><a class="headerlink" href="#subprocess.Popen.pid" title="Permalink to this definition">¶</a></dt>
<dd>The process ID of the child process.</dd></dl>
<dl class="attribute">
<dt id="subprocess.Popen.returncode">
<tt class="descclassname">Popen.</tt><tt class="descname">returncode</tt><a class="headerlink" href="#subprocess.Popen.returncode" title="Permalink to this definition">¶</a></dt>
<dd><p>The child return code, set by <a title="subprocess.Popen.poll" class="reference internal" href="#subprocess.Popen.poll"><tt class="xref docutils literal"><span class="pre">poll()</span></tt></a> and <a title="subprocess.Popen.wait" class="reference internal" href="#subprocess.Popen.wait"><tt class="xref docutils literal"><span class="pre">wait()</span></tt></a> (and indirectly
by <a title="subprocess.Popen.communicate" class="reference internal" href="#subprocess.Popen.communicate"><tt class="xref docutils literal"><span class="pre">communicate()</span></tt></a>). A <tt class="xref docutils literal"><span class="pre">None</span></tt> value indicates that the process
hasn’t terminated yet.</p>
<p>A negative value <tt class="docutils literal"><span class="pre">-N</span></tt> indicates that the child was terminated by signal
<tt class="docutils literal"><span class="pre">N</span></tt> (Unix only).</p>
</dd></dl>
</div>
<div class="section" id="replacing-older-functions-with-the-subprocess-module">
<span id="subprocess-replacements"></span><h2>18.1.3. Replacing Older Functions with the subprocess Module<a class="headerlink" href="#replacing-older-functions-with-the-subprocess-module" title="Permalink to this headline">¶</a></h2>
<p>In this section, “a ==> b” means that b can be used as a replacement for a.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">All functions in this section fail (more or less) silently if the executed
program cannot be found; this module raises an <a title="exceptions.OSError" class="reference external" href="exceptions.html#exceptions.OSError"><tt class="xref docutils literal"><span class="pre">OSError</span></tt></a> exception.</p>
</div>
<p>In the following examples, we assume that the subprocess module is imported with
“from subprocess import *”.</p>
<div class="section" id="replacing-bin-sh-shell-backquote">
<h3>18.1.3.1. Replacing /bin/sh shell backquote<a class="headerlink" href="#replacing-bin-sh-shell-backquote" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><pre>output=`mycmd myarg`
==>
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]</pre>
</div>
</div>
<div class="section" id="replacing-shell-pipeline">
<h3>18.1.3.2. Replacing shell pipeline<a class="headerlink" href="#replacing-shell-pipeline" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><pre>output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]</pre>
</div>
</div>
<div class="section" id="replacing-os-system">
<h3>18.1.3.3. Replacing <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><a class="headerlink" href="#replacing-os-system" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><pre>sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
sts = os.waitpid(p.pid, 0)[1]</pre>
</div>
<p>Notes:</p>
<ul class="simple">
<li>Calling the program through the shell is usually not required.</li>
<li>It’s easier to look at the <tt class="xref docutils literal"><span class="pre">returncode</span></tt> attribute than the exit status.</li>
</ul>
<p>A more realistic example would look like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">retcode</span> <span class="o">=</span> <span class="n">call</span><span class="p">(</span><span class="s">"mycmd"</span> <span class="o">+</span> <span class="s">" myarg"</span><span class="p">,</span> <span class="n">shell</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">if</span> <span class="n">retcode</span> <span class="o"><</span> <span class="mi">0</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">stderr</span><span class="p">,</span> <span class="s">"Child was terminated by signal"</span><span class="p">,</span> <span class="o">-</span><span class="n">retcode</span>
<span class="k">else</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">stderr</span><span class="p">,</span> <span class="s">"Child returned"</span><span class="p">,</span> <span class="n">retcode</span>
<span class="k">except</span> <span class="ne">OSError</span><span class="p">,</span> <span class="n">e</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">stderr</span><span class="p">,</span> <span class="s">"Execution failed:"</span><span class="p">,</span> <span class="n">e</span>
</pre></div>
</div>
</div>
<div class="section" id="replacing-the-os-spawn-family">
<h3>18.1.3.4. Replacing the <a title="os.spawnl" class="reference external" href="os.html#os.spawnl"><tt class="xref docutils literal"><span class="pre">os.spawn</span></tt></a> family<a class="headerlink" href="#replacing-the-os-spawn-family" title="Permalink to this headline">¶</a></h3>
<p>P_NOWAIT example:</p>
<div class="highlight-python"><pre>pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid</pre>
</div>
<p>P_WAIT example:</p>
<div class="highlight-python"><pre>retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])</pre>
</div>
<p>Vector example:</p>
<div class="highlight-python"><pre>os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])</pre>
</div>
<p>Environment example:</p>
<div class="highlight-python"><pre>os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})</pre>
</div>
</div>
<div class="section" id="replacing-os-popen-os-popen2-os-popen3">
<h3>18.1.3.5. Replacing <a title="os.popen" class="reference external" href="os.html#os.popen"><tt class="xref docutils literal"><span class="pre">os.popen()</span></tt></a>, <a title="os.popen2" class="reference external" href="os.html#os.popen2"><tt class="xref docutils literal"><span class="pre">os.popen2()</span></tt></a>, <a title="os.popen3" class="reference external" href="os.html#os.popen3"><tt class="xref docutils literal"><span class="pre">os.popen3()</span></tt></a><a class="headerlink" href="#replacing-os-popen-os-popen2-os-popen3" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><pre>pipe = os.popen("cmd", 'r', bufsize)
==>
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout</pre>
</div>
<div class="highlight-python"><pre>pipe = os.popen("cmd", 'w', bufsize)
==>
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdin=PIPE).stdin</pre>
</div>
<div class="highlight-python"><pre>(child_stdin, child_stdout) = os.popen2("cmd", mode, bufsize)
==>
p = Popen("cmd", shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)</pre>
</div>
<div class="highlight-python"><pre>(child_stdin,
child_stdout,
child_stderr) = os.popen3("cmd", mode, bufsize)
==>
p = Popen("cmd", shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
child_stdout,
child_stderr) = (p.stdin, p.stdout, p.stderr)</pre>
</div>
<div class="highlight-python"><pre>(child_stdin, child_stdout_and_stderr) = os.popen4("cmd", mode,
bufsize)
==>
p = Popen("cmd", shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)</pre>
</div>
<p>On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as
the command to execute, in which case arguments will be passed
directly to the program without shell intervention. This usage can be
replaced as follows:</p>
<div class="highlight-python"><pre>(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
bufsize)
==>
p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
(child_stdin, child_stdout) = (p.stdin, p.stdout)</pre>
</div>
<p>Return code handling translates as follows:</p>
<div class="highlight-python"><pre>pipe = os.popen("cmd", 'w')
...
rc = pipe.close()
if rc != None and rc % 256:
print "There were some errors"
==>
process = Popen("cmd", 'w', shell=True, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
print "There were some errors"</pre>
</div>
</div>
<div class="section" id="replacing-functions-from-the-popen2-module">
<h3>18.1.3.6. Replacing functions from the <a title="Subprocesses with accessible standard I/O streams. (deprecated)" class="reference external" href="popen2.html#module-popen2"><tt class="xref docutils literal"><span class="pre">popen2</span></tt></a> module<a class="headerlink" href="#replacing-functions-from-the-popen2-module" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><pre>(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen(["somestring"], shell=True, bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)</pre>
</div>
<p>On Unix, popen2 also accepts a sequence as the command to execute, in
which case arguments will be passed directly to the program without
shell intervention. This usage can be replaced as follows:</p>
<div class="highlight-python"><pre>(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize,
mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)</pre>
</div>
<p><a title="popen2.Popen3" class="reference external" href="popen2.html#popen2.Popen3"><tt class="xref docutils literal"><span class="pre">popen2.Popen3</span></tt></a> and <a title="popen2.Popen4" class="reference external" href="popen2.html#popen2.Popen4"><tt class="xref docutils literal"><span class="pre">popen2.Popen4</span></tt></a> basically work as
<a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">subprocess.Popen</span></tt></a>, except that:</p>
<ul class="simple">
<li><a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a> raises an exception if the execution fails.</li>
<li>the <em>capturestderr</em> argument is replaced with the <em>stderr</em> argument.</li>
<li><tt class="docutils literal"><span class="pre">stdin=PIPE</span></tt> and <tt class="docutils literal"><span class="pre">stdout=PIPE</span></tt> must be specified.</li>
<li>popen2 closes all file descriptors by default, but you have to specify
<tt class="docutils literal"><span class="pre">close_fds=True</span></tt> with <a title="subprocess.Popen" class="reference internal" href="#subprocess.Popen"><tt class="xref docutils literal"><span class="pre">Popen</span></tt></a>.</li>
</ul>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">18.1. <tt class="docutils literal"><span class="pre">subprocess</span></tt> — Subprocess management</a><ul>
<li><a class="reference external" href="#using-the-subprocess-module">18.1.1. Using the subprocess Module</a><ul>
<li><a class="reference external" href="#convenience-functions">18.1.1.1. Convenience Functions</a></li>
<li><a class="reference external" href="#exceptions">18.1.1.2. Exceptions</a></li>
<li><a class="reference external" href="#security">18.1.1.3. Security</a></li>
</ul>
</li>
<li><a class="reference external" href="#popen-objects">18.1.2. Popen Objects</a></li>
<li><a class="reference external" href="#replacing-older-functions-with-the-subprocess-module">18.1.3. Replacing Older Functions with the subprocess Module</a><ul>
<li><a class="reference external" href="#replacing-bin-sh-shell-backquote">18.1.3.1. Replacing /bin/sh shell backquote</a></li>
<li><a class="reference external" href="#replacing-shell-pipeline">18.1.3.2. Replacing shell pipeline</a></li>
<li><a class="reference external" href="#replacing-os-system">18.1.3.3. Replacing <tt class="docutils literal"><span class="pre">os.system()</span></tt></a></li>
<li><a class="reference external" href="#replacing-the-os-spawn-family">18.1.3.4. Replacing the <tt class="docutils literal"><span class="pre">os.spawn</span></tt> family</a></li>
<li><a class="reference external" href="#replacing-os-popen-os-popen2-os-popen3">18.1.3.5. Replacing <tt class="docutils literal"><span class="pre">os.popen()</span></tt>, <tt class="docutils literal"><span class="pre">os.popen2()</span></tt>, <tt class="docutils literal"><span class="pre">os.popen3()</span></tt></a></li>
<li><a class="reference external" href="#replacing-functions-from-the-popen2-module">18.1.3.6. Replacing functions from the <tt class="docutils literal"><span class="pre">popen2</span></tt> module</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="ipc.html"
title="previous chapter">18. Interprocess Communication and Networking</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="socket.html"
title="next chapter">18.2. <tt class="docutils literal docutils literal docutils literal"><span class="pre">socket</span></tt> — Low-level networking interface</a></p>
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