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  &lt;div class=&quot;section&quot; id=&quot;module-popen2&quot;&gt;
&lt;span id=&quot;popen2-subprocesses-with-accessible-i-o-streams&quot;&gt;&lt;/span&gt;&lt;h1&gt;17.5. &lt;a class=&quot;reference internal&quot; href=&quot;#module-popen2&quot; title=&quot;popen2: Subprocesses with accessible standard I/O streams. (deprecated)&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; &amp;#8212; Subprocesses with accessible I/O streams&lt;a class=&quot;headerlink&quot; href=&quot;#module-popen2&quot; title=&quot;Permalink to this headline&quot;&gt;¶&lt;/a&gt;&lt;/h1&gt;
&lt;p class=&quot;deprecated&quot;&gt;
&lt;span class=&quot;versionmodified&quot;&gt;Deprecated since version 2.6: &lt;/span&gt;This module is obsolete.  Use the &lt;a class=&quot;reference internal&quot; href=&quot;subprocess.html#module-subprocess&quot; title=&quot;subprocess: Subprocess management.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;subprocess&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module.  Check
especially the &lt;a class=&quot;reference internal&quot; href=&quot;subprocess.html#subprocess-replacements&quot;&gt;&lt;em&gt;Replacing Older Functions with the subprocess Module&lt;/em&gt;&lt;/a&gt; section.&lt;/p&gt;
&lt;p&gt;This module allows you to spawn processes and connect to their
input/output/error pipes and obtain their return codes under Unix and Windows.&lt;/p&gt;
&lt;p&gt;The &lt;a class=&quot;reference internal&quot; href=&quot;subprocess.html#module-subprocess&quot; title=&quot;subprocess: Subprocess management.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;subprocess&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module provides more powerful facilities for spawning new
processes and retrieving their results.  Using the &lt;a class=&quot;reference internal&quot; href=&quot;subprocess.html#module-subprocess&quot; title=&quot;subprocess: Subprocess management.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;subprocess&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module is
preferable to using the &lt;a class=&quot;reference internal&quot; href=&quot;#module-popen2&quot; title=&quot;popen2: Subprocesses with accessible standard I/O streams. (deprecated)&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module.&lt;/p&gt;
&lt;p&gt;The primary interface offered by this module is a trio of factory functions.
For each of these, if &lt;em&gt;bufsize&lt;/em&gt; is specified,  it specifies the buffer size for
the I/O pipes.  &lt;em&gt;mode&lt;/em&gt;, if provided, should be the string &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;'b'&lt;/span&gt;&lt;/tt&gt; or &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;'t'&lt;/span&gt;&lt;/tt&gt;; on
Windows this is needed to determine whether the file objects should be opened in
binary or text mode.  The default value for &lt;em&gt;mode&lt;/em&gt; is &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;'t'&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;p&gt;On Unix, &lt;em&gt;cmd&lt;/em&gt; may be a sequence, in which case arguments will be passed
directly to the program without shell intervention (as with &lt;a class=&quot;reference internal&quot; href=&quot;os.html#os.spawnv&quot; title=&quot;os.spawnv&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os.spawnv()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;).
If &lt;em&gt;cmd&lt;/em&gt; is a string it will be passed to the shell (as with &lt;a class=&quot;reference internal&quot; href=&quot;os.html#os.system&quot; title=&quot;os.system&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os.system()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;).&lt;/p&gt;
&lt;p&gt;The only way to retrieve the return codes for the child processes is by using
the &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;poll()&lt;/span&gt;&lt;/tt&gt; or &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;wait()&lt;/span&gt;&lt;/tt&gt; methods on the &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen3&quot; title=&quot;popen2.Popen3&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen3&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; and
&lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen4&quot; title=&quot;popen2.Popen4&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen4&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; classes; these are only available on Unix.  This information is
not available when using the &lt;a class=&quot;reference internal&quot; href=&quot;#module-popen2&quot; title=&quot;popen2: Subprocesses with accessible standard I/O streams. (deprecated)&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;, &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.popen3&quot; title=&quot;popen2.popen3&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen3()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;, and &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.popen4&quot; title=&quot;popen2.popen4&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen4()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;
functions, or the equivalent functions in the &lt;a class=&quot;reference internal&quot; href=&quot;os.html#module-os&quot; title=&quot;os: Miscellaneous operating system interfaces.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module. (Note that the
tuples returned by the &lt;a class=&quot;reference internal&quot; href=&quot;os.html#module-os&quot; title=&quot;os: Miscellaneous operating system interfaces.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module&amp;#8217;s functions are in a different order
from the ones returned by the &lt;a class=&quot;reference internal&quot; href=&quot;#module-popen2&quot; title=&quot;popen2: Subprocesses with accessible standard I/O streams. (deprecated)&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module.)&lt;/p&gt;
&lt;dl class=&quot;function&quot;&gt;
&lt;dt id=&quot;popen2.popen2&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;popen2.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;popen2&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;cmd&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;bufsize&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;mode&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.popen2&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Executes &lt;em&gt;cmd&lt;/em&gt; as a sub-process.  Returns the file objects &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;(child_stdout,&lt;/span&gt;
&lt;span class=&quot;pre&quot;&gt;child_stdin)&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;dl class=&quot;function&quot;&gt;
&lt;dt id=&quot;popen2.popen3&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;popen2.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;popen3&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;cmd&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;bufsize&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;mode&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.popen3&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Executes &lt;em&gt;cmd&lt;/em&gt; as a sub-process.  Returns the file objects &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;(child_stdout,&lt;/span&gt;
&lt;span class=&quot;pre&quot;&gt;child_stdin,&lt;/span&gt; &lt;span class=&quot;pre&quot;&gt;child_stderr)&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;dl class=&quot;function&quot;&gt;
&lt;dt id=&quot;popen2.popen4&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;popen2.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;popen4&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;cmd&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;bufsize&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;mode&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.popen4&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Executes &lt;em&gt;cmd&lt;/em&gt; as a sub-process.  Returns the file objects
&lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;(child_stdout_and_stderr,&lt;/span&gt; &lt;span class=&quot;pre&quot;&gt;child_stdin)&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;p class=&quot;versionadded&quot;&gt;
&lt;span class=&quot;versionmodified&quot;&gt;New in version 2.0.&lt;/span&gt;&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;p&gt;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.&lt;/p&gt;
&lt;dl class=&quot;class&quot;&gt;
&lt;dt id=&quot;popen2.Popen3&quot;&gt;
&lt;em class=&quot;property&quot;&gt;class &lt;/em&gt;&lt;tt class=&quot;descclassname&quot;&gt;popen2.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;Popen3&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;cmd&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;capturestderr&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;[&lt;/span&gt;, &lt;em&gt;bufsize&lt;/em&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;optional&quot;&gt;]&lt;/span&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.Popen3&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;This class represents a child process.  Normally, &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen3&quot; title=&quot;popen2.Popen3&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen3&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; instances are
created using the &lt;a class=&quot;reference internal&quot; href=&quot;#module-popen2&quot; title=&quot;popen2: Subprocesses with accessible standard I/O streams. (deprecated)&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; and &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.popen3&quot; title=&quot;popen2.popen3&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen3()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; factory functions described
above.&lt;/p&gt;
&lt;p&gt;If not using one of the helper functions to create &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen3&quot; title=&quot;popen2.Popen3&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen3&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; objects, the
parameter &lt;em&gt;cmd&lt;/em&gt; is the shell command to execute in a sub-process.  The
&lt;em&gt;capturestderr&lt;/em&gt; flag, if true, specifies that the object should capture standard
error output of the child process. The default is false.  If the &lt;em&gt;bufsize&lt;/em&gt;
parameter is specified, it specifies the size of the I/O buffers to/from the
child process.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

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

&lt;div class=&quot;section&quot; id=&quot;popen3-and-popen4-objects&quot;&gt;
&lt;span id=&quot;popen3-objects&quot;&gt;&lt;/span&gt;&lt;h2&gt;17.5.1. Popen3 and Popen4 Objects&lt;a class=&quot;headerlink&quot; href=&quot;#popen3-and-popen4-objects&quot; title=&quot;Permalink to this headline&quot;&gt;¶&lt;/a&gt;&lt;/h2&gt;
&lt;p&gt;Instances of the &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen3&quot; title=&quot;popen2.Popen3&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen3&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; and &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen4&quot; title=&quot;popen2.Popen4&quot;&gt;&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen4&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; classes have the following
methods:&lt;/p&gt;
&lt;dl class=&quot;method&quot;&gt;
&lt;dt id=&quot;popen2.Popen3.poll&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;Popen3.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;poll&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.Popen3.poll&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Returns &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;-1&lt;/span&gt;&lt;/tt&gt; if child process hasn&amp;#8217;t completed yet, or its status code
(see &lt;a class=&quot;reference internal&quot; href=&quot;#popen2.Popen3.wait&quot; title=&quot;popen2.Popen3.wait&quot;&gt;&lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;wait()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;) otherwise.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;dl class=&quot;method&quot;&gt;
&lt;dt id=&quot;popen2.Popen3.wait&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;Popen3.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;wait&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.Popen3.wait&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;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 &lt;tt class=&quot;xref c c-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;exit()&lt;/span&gt;&lt;/tt&gt; system call or died due to a signal.  Functions
to help interpret the status code are defined in the &lt;a class=&quot;reference internal&quot; href=&quot;os.html#module-os&quot; title=&quot;os: Miscellaneous operating system interfaces.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module; see
section &lt;a class=&quot;reference internal&quot; href=&quot;os.html#os-process&quot;&gt;&lt;em&gt;Process Management&lt;/em&gt;&lt;/a&gt; for the &lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;W*()&lt;/span&gt;&lt;/tt&gt; family of functions.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

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

&lt;dl class=&quot;attribute&quot;&gt;
&lt;dt id=&quot;popen2.Popen3.tochild&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;Popen3.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;tochild&lt;/tt&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.Popen3.tochild&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;A file object that provides input to the child process.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

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

&lt;dl class=&quot;attribute&quot;&gt;
&lt;dt id=&quot;popen2.Popen3.pid&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;Popen3.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;pid&lt;/tt&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#popen2.Popen3.pid&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;The process ID of the child process.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;/div&gt;
&lt;div class=&quot;section&quot; id=&quot;flow-control-issues&quot;&gt;
&lt;span id=&quot;popen2-flow-control&quot;&gt;&lt;/span&gt;&lt;h2&gt;17.5.2. Flow Control Issues&lt;a class=&quot;headerlink&quot; href=&quot;#flow-control-issues&quot; title=&quot;Permalink to this headline&quot;&gt;¶&lt;/a&gt;&lt;/h2&gt;
&lt;p&gt;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 &lt;a class=&quot;reference internal&quot; href=&quot;os.html#module-os&quot; title=&quot;os: Miscellaneous operating system interfaces.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module equivalents).&lt;/p&gt;
&lt;p&gt;When reading output from a child process that writes a lot of data to standard
error while the parent is reading from the child&amp;#8217;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 &lt;tt class=&quot;xref py py-const docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;_PC_PIPE_BUF&lt;/span&gt;&lt;/tt&gt; 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.&lt;/p&gt;
&lt;p&gt;There are several ways to deal with this situation.&lt;/p&gt;
&lt;p&gt;The simplest application change, in many cases, will be to follow this model in
the parent process:&lt;/p&gt;
&lt;div class=&quot;highlight-python&quot;&gt;&lt;div class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;span class=&quot;kn&quot;&gt;import&lt;/span&gt; &lt;span class=&quot;nn&quot;&gt;popen2&lt;/span&gt;

&lt;span class=&quot;n&quot;&gt;r&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;w&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;e&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;popen2&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;popen3&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;s&quot;&gt;&amp;#39;python slave.py&amp;#39;&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;)&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;e&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;readlines&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;r&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;readlines&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;r&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;close&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;e&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;close&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;w&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;close&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;p&gt;with code like this in the child:&lt;/p&gt;
&lt;div class=&quot;highlight-python&quot;&gt;&lt;div class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;span class=&quot;kn&quot;&gt;import&lt;/span&gt; &lt;span class=&quot;nn&quot;&gt;os&lt;/span&gt;
&lt;span class=&quot;kn&quot;&gt;import&lt;/span&gt; &lt;span class=&quot;nn&quot;&gt;sys&lt;/span&gt;

&lt;span class=&quot;c&quot;&gt;# note that each of these print statements&lt;/span&gt;
&lt;span class=&quot;c&quot;&gt;# writes a single long string&lt;/span&gt;

&lt;span class=&quot;k&quot;&gt;print&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;&amp;gt;&amp;gt;&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;sys&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;stderr&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;mi&quot;&gt;400&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;*&lt;/span&gt; &lt;span class=&quot;s&quot;&gt;&amp;#39;this is a test&lt;/span&gt;&lt;span class=&quot;se&quot;&gt;\n&lt;/span&gt;&lt;span class=&quot;s&quot;&gt;&amp;#39;&lt;/span&gt;
&lt;span class=&quot;n&quot;&gt;os&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;close&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;sys&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;stderr&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;fileno&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;())&lt;/span&gt;
&lt;span class=&quot;k&quot;&gt;print&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;&amp;gt;&amp;gt;&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;sys&lt;/span&gt;&lt;span class=&quot;o&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;stdout&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;mi&quot;&gt;400&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;*&lt;/span&gt; &lt;span class=&quot;s&quot;&gt;&amp;#39;this is another test&lt;/span&gt;&lt;span class=&quot;se&quot;&gt;\n&lt;/span&gt;&lt;span class=&quot;s&quot;&gt;&amp;#39;&lt;/span&gt;
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;p&gt;In particular, note that &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;sys.stderr&lt;/span&gt;&lt;/tt&gt; must be closed after writing all data,
or &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;readlines()&lt;/span&gt;&lt;/tt&gt; won&amp;#8217;t return.  Also note that &lt;a class=&quot;reference internal&quot; href=&quot;os.html#os.close&quot; title=&quot;os.close&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;os.close()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; must be
used, as &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;sys.stderr.close()&lt;/span&gt;&lt;/tt&gt; won&amp;#8217;t close &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;stderr&lt;/span&gt;&lt;/tt&gt; (otherwise assigning to
&lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;sys.stderr&lt;/span&gt;&lt;/tt&gt; will silently close it, so no further errors can be printed).&lt;/p&gt;
&lt;p&gt;Applications which need to support a more general approach should integrate I/O
over pipes with their &lt;a class=&quot;reference internal&quot; href=&quot;select.html#module-select&quot; title=&quot;select: Wait for I/O completion on multiple streams.&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;select()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; loops, or use separate threads to read each
of the individual files provided by whichever &lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen*()&lt;/span&gt;&lt;/tt&gt; function or
&lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;Popen*&lt;/span&gt;&lt;/tt&gt; class was used.&lt;/p&gt;
&lt;div class=&quot;admonition-see-also admonition seealso&quot;&gt;
&lt;p class=&quot;first admonition-title&quot;&gt;See also&lt;/p&gt;
&lt;dl class=&quot;last docutils&quot;&gt;
&lt;dt&gt;Module &lt;a class=&quot;reference internal&quot; href=&quot;subprocess.html#module-subprocess&quot; title=&quot;subprocess: Subprocess management.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;subprocess&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;Module for spawning and managing subprocesses.&lt;/dd&gt;
&lt;/dl&gt;
&lt;/div&gt;
&lt;/div&gt;
&lt;/div&gt;


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  &lt;h3&gt;&lt;a href=&quot;../contents.html&quot;&gt;Table Of Contents&lt;/a&gt;&lt;/h3&gt;
  &lt;ul&gt;
&lt;li&gt;&lt;a class=&quot;reference internal&quot; href=&quot;#&quot;&gt;17.5. &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;popen2&lt;/span&gt;&lt;/tt&gt; &amp;#8212; Subprocesses with accessible I/O streams&lt;/a&gt;&lt;ul&gt;
&lt;li&gt;&lt;a class=&quot;reference internal&quot; href=&quot;#popen3-and-popen4-objects&quot;&gt;17.5.1. Popen3 and Popen4 Objects&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a class=&quot;reference internal&quot; href=&quot;#flow-control-issues&quot;&gt;17.5.2. Flow Control Issues&lt;/a&gt;&lt;/li&gt;
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