Using the subprocess Module
---------------------------
-This module defines one class called :class:`Popen`:
+The recommended interface to this module is to use the following convenience
+functions for all use cases they can handle. For more advanced use cases, the
+underlying :class:`Popen` interface can be used directly.
+
+
+.. function:: call(args, *, stdin=None, stdout=None, stderr=None)
+
+ Run the command described by *args*. Wait for command to complete, then
+ return the :attr:`returncode` attribute.
+
+ The arguments shown above are merely the most common ones, described below
+ in :ref:`frequently-used-arguments`. The full function signature is the
+ same as that of the :class:`Popen` constructor - the convenience functions
+ pass all supplied arguments directly through to that interface.
+
+ Examples::
+
+ >>> subprocess.call(["ls", "-l"])
+ 0
+
+ >>> subprocess.call(["python", "-c", "import sys; sys.exit(1)"])
+ 1
+
+ .. warning::
+
+ Like :meth:`Popen.wait`, this will deadlock when using
+ ``stdout=PIPE`` and/or ``stderr=PIPE`` and the child process
+ generates enough output to a pipe such that it blocks waiting
+ for the OS pipe buffer to accept more data.
+
+
+.. function:: check_call(*callargs, **kwargs)
+
+ Run command with arguments. Wait for command to complete. If the return
+ code was zero then return, otherwise raise :exc:`CalledProcessError`. The
+ :exc:`CalledProcessError` object will have the return code in the
+ :attr:`returncode` attribute.
+
+ The arguments are the same as for :func:`call`. Examples::
+
+ >>> subprocess.check_call(["ls", "-l"])
+ 0
+
+ >>> subprocess.check_call(["python", "-c", "import sys; sys.exit(1)"])
+ Traceback (most recent call last):
+ ...
+ subprocess.CalledProcessError: Command '['python', '-c', 'import sys; sys.exit(1)']' returned non-zero exit status 1
+
+ .. versionadded:: 2.5
+
+ .. warning::
+
+ See the warning for :func:`call`.
+
+
+.. function:: check_output(*callargs, **kwargs)
+
+ Run command with arguments and return its output as a byte string.
+
+ If the return code was non-zero it raises a :exc:`CalledProcessError`. The
+ :exc:`CalledProcessError` object will have the return code in the
+ :attr:`returncode` attribute and any output in the :attr:`output`
+ attribute.
+
+ Examples::
+
+ >>> subprocess.check_output(["ls", "-l", "/dev/null"])
+ 'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n'
+
+ >>> subprocess.check_output(["python", "-c", "import sys; sys.exit(1)"])
+ Traceback (most recent call last):
+ ...
+ subprocess.CalledProcessError: Command '['python', '-c', 'import sys; sys.exit(1)']' returned non-zero exit status 1
+
+ The arguments are the same as for :func:`call`, except that *stdout* is
+ not allowed as it is used internally. To also capture standard error in
+ the result, use ``stderr=subprocess.STDOUT``::
+
+ >>> subprocess.check_output(
+ ... ["/bin/sh", "-c", "ls non_existent_file; exit 0"],
+ ... stderr=subprocess.STDOUT)
+ 'ls: non_existent_file: No such file or directory\n'
+
+ .. versionadded:: 2.7
+
+
+.. data:: PIPE
+
+ Special value that can be used as the *stdin*, *stdout* or *stderr* argument
+ to :class:`Popen` and indicates that a pipe to the standard stream should be
+ opened.
+
+
+.. data:: STDOUT
+
+ Special value that can be used as the *stderr* argument to :class:`Popen` and
+ indicates that standard error should go into the same handle as standard
+ output.
+
+
+.. _frequently-used-arguments:
+
+Frequently Used Arguments
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To support a wide variety of use cases, the :class:`Popen` constructor (and
+the convenience functions) accept a large number of optional arguments. For
+most typical use cases, many of these arguments can be safely left at their
+default values. The arguments that are most commonly needed are:
+
+ *args* should be a string, or a sequence of program arguments. Providing
+ a sequence of arguments is generally preferred, as it allows the module to
+ take care of any required escaping and quoting of arguments (e.g. to permit
+ spaces in file names)
+
+ *stdin*, *stdout* and *stderr* specify the executed program's standard input,
+ standard output and standard error file handles, respectively. Valid values
+ are :data:`PIPE`, an existing file descriptor (a positive integer), an
+ existing file object, and ``None``. :data:`PIPE` indicates that a new pipe
+ to the child should be created. With the default settings of ``None``, no
+ redirection will occur; the child's file handles will be inherited from the
+ parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates that
+ the stderr data from the child process should be captured into the same file
+ handle as for stdout.
+
+These options, along with all of the other options, are described in more
+detail in the :class:`Popen` constructor documentation.
+
+
+Popen Constuctor
+^^^^^^^^^^^^^^^^
+
+The underlying process creation and management in this module is handled by
+the :class:`Popen` class. It offers a lot of flexibility so that developers
+are able to handle the less common cases not covered by the convenience
+functions.
.. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
You don't need ``shell=True`` to run a batch file, nor to run a console-based
executable.
- *stdin*, *stdout* and *stderr* specify the executed programs' standard input,
+ *stdin*, *stdout* and *stderr* specify the executed program's standard input,
standard output and standard error file handles, respectively. Valid values
are :data:`PIPE`, an existing file descriptor (a positive integer), an
existing file object, and ``None``. :data:`PIPE` indicates that a new pipe
- to the child should be created. With ``None``, no redirection will occur;
- the child's file handles will be inherited from the parent. Additionally,
- *stderr* can be :data:`STDOUT`, which indicates that the stderr data from the
- applications should be captured into the same file handle as for stdout.
+ to the child should be created. With the default settings of ``None``, no
+ redirection will occur; the child's file handles will be inherited from the
+ parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates that
+ the stderr data from the child process should be captured into the same file
+ handle as for stdout.
If *preexec_fn* is set to a callable object, this object will be called in the
child process just before the child is executed. (Unix only)
:data:`CREATE_NEW_PROCESS_GROUP`. (Windows only)
-.. data:: PIPE
-
- Special value that can be used as the *stdin*, *stdout* or *stderr* argument
- to :class:`Popen` and indicates that a pipe to the standard stream should be
- opened.
-
-
-.. data:: STDOUT
-
- Special value that can be used as the *stderr* argument to :class:`Popen` and
- indicates that standard error should go into the same handle as standard
- output.
-
-
-Convenience Functions
-^^^^^^^^^^^^^^^^^^^^^
-
-This module also defines the following shortcut functions:
-
-
-.. function:: call(*popenargs, **kwargs)
-
- Run command with arguments. Wait for command to complete, then return the
- :attr:`returncode` attribute.
-
- The arguments are the same as for the :class:`Popen` constructor. Example::
-
- >>> retcode = subprocess.call(["ls", "-l"])
-
- .. warning::
-
- Like :meth:`Popen.wait`, this will deadlock when using
- ``stdout=PIPE`` and/or ``stderr=PIPE`` and the child process
- generates enough output to a pipe such that it blocks waiting
- for the OS pipe buffer to accept more data.
-
-
-.. function:: check_call(*popenargs, **kwargs)
-
- Run command with arguments. Wait for command to complete. If the exit code was
- zero then return, otherwise raise :exc:`CalledProcessError`. The
- :exc:`CalledProcessError` object will have the return code in the
- :attr:`returncode` attribute.
-
- The arguments are the same as for the :class:`Popen` constructor. Example::
-
- >>> subprocess.check_call(["ls", "-l"])
- 0
-
- .. versionadded:: 2.5
-
- .. warning::
-
- See the warning for :func:`call`.
-
-
-.. function:: check_output(*popenargs, **kwargs)
-
- Run command with arguments and return its output as a byte string.
-
- If the exit code was non-zero it raises a :exc:`CalledProcessError`. The
- :exc:`CalledProcessError` object will have the return code in the
- :attr:`returncode`
- attribute and output in the :attr:`output` attribute.
-
- The arguments are the same as for the :class:`Popen` constructor. Example::
-
- >>> subprocess.check_output(["ls", "-l", "/dev/null"])
- 'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n'
-
- The stdout argument is not allowed as it is used internally.
- To capture standard error in the result, use ``stderr=subprocess.STDOUT``::
-
- >>> subprocess.check_output(
- ... ["/bin/sh", "-c", "ls non_existent_file; exit 0"],
- ... stderr=subprocess.STDOUT)
- 'ls: non_existent_file: No such file or directory\n'
-
- .. versionadded:: 2.7
-
-
Exceptions
^^^^^^^^^^
Replacing Older Functions with the subprocess Module
----------------------------------------------------
-In this section, "a ==> b" means that b can be used as a replacement for a.
+In this section, "a becomes b" means that b can be used as a replacement for a.
.. note::
All functions in this section fail (more or less) silently if the executed
- program cannot be found; this module raises an :exc:`OSError` exception.
+ program cannot be found; this module raises an :exc:`OSError` exception. In
+ addition, the replacements using :func:`check_output` will fail with a
+ :exc:`CalledProcessError` if the requested operation produces a non-zero
+ return code.
In the following examples, we assume that the subprocess module is imported with
"from subprocess import \*".
::
output=`mycmd myarg`
- ==>
- output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
+ # becomes
+ output = check_output(["mycmd", "myarg"])
Replacing shell pipeline
::
output=`dmesg | grep hda`
- ==>
+ # becomes
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits.
The p1.stdout.close() call after starting the p2 is important in order for p1
to receive a SIGPIPE if p2 exits before p1.
+Alternatively, for trusted input, the shell's pipeline may still be used
+directly:
+
+ output=`dmesg | grep hda`
+ # becomes
+ output=check_output("dmesg | grep hda", shell=True)
+
+
Replacing :func:`os.system`
^^^^^^^^^^^^^^^^^^^^^^^^^^^
::
sts = os.system("mycmd" + " myarg")
- ==>
- p = Popen("mycmd" + " myarg", shell=True)
- sts = os.waitpid(p.pid, 0)[1]
+ # becomes
+ sts = call("mycmd" + " myarg", shell=True)
Notes:
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