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53<H1><A NAME="SECTION000800000000000000000">
547 PEP 324: New subprocess Module</A>
55</H1>
56
57<P>
58The standard library provides a number of ways to execute a
59subprocess, offering different features and different levels of
60complexity. <tt class="function">os.system(<var>command</var>)</tt> is easy to use, but
61slow (it runs a shell process which executes the command) and
62dangerous (you have to be careful about escaping the shell's
63metacharacters). The <tt class="module">popen2</tt> module offers classes that can
64capture standard output and standard error from the subprocess, but
65the naming is confusing. The <tt class="module">subprocess</tt> module cleans
66this up, providing a unified interface that offers all the features
67you might need.
68
69<P>
70Instead of <tt class="module">popen2</tt>'s collection of classes,
71<tt class="module">subprocess</tt> contains a single class called <tt class="class">Popen</tt>
72whose constructor supports a number of different keyword arguments.
73
74<P>
75<div class="verbatim"><pre>
76class Popen(args, bufsize=0, executable=None,
77 stdin=None, stdout=None, stderr=None,
78 preexec_fn=None, close_fds=False, shell=False,
79 cwd=None, env=None, universal_newlines=False,
80 startupinfo=None, creationflags=0):
81</pre></div>
82
83<P>
84<var>args</var> is commonly a sequence of strings that will be the
85arguments to the program executed as the subprocess. (If the
86<var>shell</var> argument is true, <var>args</var> can be a string which will
87then be passed on to the shell for interpretation, just as
88<tt class="function">os.system()</tt> does.)
89
90<P>
91<var>stdin</var>, <var>stdout</var>, and <var>stderr</var> specify what the
92subprocess's input, output, and error streams will be. You can
93provide a file object or a file descriptor, or you can use the
94constant <code>subprocess.PIPE</code> to create a pipe between the
95subprocess and the parent.
96
97<P>
98The constructor has a number of handy options:
99
100<P>
101
102<UL>
103<LI><var>close_fds</var> requests that all file descriptors be closed
104 before running the subprocess.
105
106<P>
107</LI>
108<LI><var>cwd</var> specifies the working directory in which the
109 subprocess will be executed (defaulting to whatever the parent's
110 working directory is).
111
112<P>
113</LI>
114<LI><var>env</var> is a dictionary specifying environment variables.
115
116<P>
117</LI>
118<LI><var>preexec_fn</var> is a function that gets called before the
119 child is started.
120
121<P>
122</LI>
123<LI><var>universal_newlines</var> opens the child's input and output
124 using Python's universal newline feature.
125
126<P>
127</LI>
128</UL>
129
130<P>
131Once you've created the <tt class="class">Popen</tt> instance,
132you can call its <tt class="method">wait()</tt> method to pause until the subprocess
133has exited, <tt class="method">poll()</tt> to check if it's exited without pausing,
134or <tt class="method">communicate(<var>data</var>)</tt> to send the string <var>data</var> to
135the subprocess's standard input. <tt class="method">communicate(<var>data</var>)</tt>
136then reads any data that the subprocess has sent to its standard output
137or standard error, returning a tuple <code>(<var>stdout_data</var>,
138<var>stderr_data</var>)</code>.
139
140<P>
141<tt class="function">call()</tt> is a shortcut that passes its arguments along to the
142<tt class="class">Popen</tt> constructor, waits for the command to complete, and
143returns the status code of the subprocess. It can serve as a safer
144analog to <tt class="function">os.system()</tt>:
145
146<P>
147<div class="verbatim"><pre>
148sts = subprocess.call(['dpkg', '-i', '/tmp/new-package.deb'])
149if sts == 0:
150 # Success
151 ...
152else:
153 # dpkg returned an error
154 ...
155</pre></div>
156
157<P>
158The command is invoked without use of the shell. If you really do want to
159use the shell, you can add <code>shell=True</code> as a keyword argument and provide
160a string instead of a sequence:
161
162<P>
163<div class="verbatim"><pre>
164sts = subprocess.call('dpkg -i /tmp/new-package.deb', shell=True)
165</pre></div>
166
167<P>
168The PEP takes various examples of shell and Python code and shows how
169they'd be translated into Python code that uses <tt class="module">subprocess</tt>.
170Reading this section of the PEP is highly recommended.
171
172<P>
173<div class="seealso">
174 <p class="heading">See Also:</p>
175
176<dl compact="compact" class="seerfc">
177 <dt><a href="http://www.python.org/peps/pep-0324.html"
178 title="subprocess - New process module"
179 >PEP 324, <em>subprocess - New process module</em></a>
180 <dd>Written and implemented by Peter &#197;strand, with assistance from Fredrik Lundh and others.
181 </dl>
182</div>
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