
``subprocess`` --- Subprocess management
****************************************

New in version 2.4.

The ``subprocess`` 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:

   os.system
   os.spawn*
   os.popen*
   popen2.*
   commands.*

Information about how the ``subprocess`` module can be used to replace
these modules and functions can be found in the following sections.

See also:

   **PEP 324** -- PEP proposing the subprocess module


Using the ``subprocess`` Module
===============================

The recommended approach to invoking subprocesses is to use the
following convenience functions for all use cases they can handle. For
more advanced use cases, the underlying ``Popen`` interface can be
used directly.

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False)

   Run the command described by *args*.  Wait for command to complete,
   then return the ``returncode`` attribute.

   The arguments shown above are merely the most common ones,
   described below in *Frequently Used Arguments* (hence the slightly
   odd notation in the abbreviated signature). The full function
   signature is the same as that of the ``Popen`` constructor - this
   functions passes all supplied arguments directly through to that
   interface.

   Examples:

      >>> subprocess.call(["ls", "-l"])
      0

      >>> subprocess.call("exit 1", shell=True)
      1

   Warning: Invoking the system shell with ``shell=True`` can be a security
     hazard if combined with untrusted input. See the warning under
     *Frequently Used Arguments* for details.

   Note: Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function.
     As the pipes are not being read in the current process, the child
     process may block if it generates enough output to a pipe to fill
     up the OS pipe buffer.

subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False)

   Run command with arguments.  Wait for command to complete. If the
   return code was zero then return, otherwise raise
   ``CalledProcessError``. The ``CalledProcessError`` object will have
   the return code in the ``returncode`` attribute.

   The arguments shown above are merely the most common ones,
   described below in *Frequently Used Arguments* (hence the slightly
   odd notation in the abbreviated signature). The full function
   signature is the same as that of the ``Popen`` constructor - this
   functions passes all supplied arguments directly through to that
   interface.

   Examples:

      >>> subprocess.check_call(["ls", "-l"])
      0

      >>> subprocess.check_call("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

   New in version 2.5.

   Warning: Invoking the system shell with ``shell=True`` can be a security
     hazard if combined with untrusted input. See the warning under
     *Frequently Used Arguments* for details.

   Note: Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function.
     As the pipes are not being read in the current process, the child
     process may block if it generates enough output to a pipe to fill
     up the OS pipe buffer.

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)

   Run command with arguments and return its output as a byte string.

   If the return code was non-zero it raises a ``CalledProcessError``.
   The ``CalledProcessError`` object will have the return code in the
   ``returncode`` attribute and any output in the ``output``
   attribute.

   The arguments shown above are merely the most common ones,
   described below in *Frequently Used Arguments* (hence the slightly
   odd notation in the abbreviated signature). The full function
   signature is largely the same as that of the ``Popen`` constructor,
   except that *stdout* is not permitted as it is used internally. All
   other supplied arguments are passed directly through to the
   ``Popen`` constructor.

   Examples:

      >>> subprocess.check_output(["echo", "Hello World!"])
      'Hello World!\n'

      >>> subprocess.check_output("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

   To also capture standard error in the result, use
   ``stderr=subprocess.STDOUT``:

      >>> subprocess.check_output(
      ...     "ls non_existent_file; exit 0",
      ...     stderr=subprocess.STDOUT,
      ...     shell=True)
      'ls: non_existent_file: No such file or directory\n'

   New in version 2.7.

   Warning: Invoking the system shell with ``shell=True`` can be a security
     hazard if combined with untrusted input. See the warning under
     *Frequently Used Arguments* for details.

   Note: Do not use ``stderr=PIPE`` with this function. As the pipe is not
     being read in the current process, the child process may block if
     it generates enough output to the pipe to fill up the OS pipe
     buffer.

subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to ``Popen`` and indicates that a pipe to the standard
   stream should be opened.

subprocess.STDOUT

   Special value that can be used as the *stderr* argument to
   ``Popen`` and indicates that standard error should go into the same
   handle as standard output.

exception exception subprocess.CalledProcessError

   Exception raised when a process run by ``check_call()`` or
   ``check_output()`` returns a non-zero exit status.

   returncode

      Exit status of the child process.

   cmd

      Command that was used to spawn the child process.

   output

      Output of the child process if this exception is raised by
      ``check_output()``.  Otherwise, ``None``.


Frequently Used Arguments
-------------------------

To support a wide variety of use cases, the ``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* is required for all calls and 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). If passing a single string, either *shell* must be
   ``True`` (see below) or else the string must simply name the
   program to be executed without specifying any arguments.

   *stdin*, *stdout* and *stderr* specify the executed program's
   standard input, standard output and standard error file handles,
   respectively.  Valid values are ``PIPE``, an existing file
   descriptor (a positive integer), an existing file object, and
   ``None``.  ``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 ``STDOUT``, which indicates
   that the stderr data from the child process should be captured into
   the same file handle as for stdout.

   When *stdout* or *stderr* are pipes and *universal_newlines* is
   ``True`` then all line endings will be converted to ``'\n'`` as
   described for the *universal newlines* ``'U'`` mode argument to
   ``open()``.

   If *shell* is ``True``, the specified command will be executed
   through the shell.  This can be useful if you are using Python
   primarily for the enhanced control flow it offers over most system
   shells and still want convenient access to other shell features
   such as shell pipes, filename wildcards, environment variable
   expansion, and expansion of ``~`` to a user's home directory.
   However, note that Python itself offers implementations of many
   shell-like features (in particular, ``glob``, ``fnmatch``,
   ``os.walk()``, ``os.path.expandvars()``, ``os.path.expanduser()``,
   and ``shutil``).

   Warning: Executing shell commands that incorporate unsanitized input from
     an untrusted source makes a program vulnerable to shell
     injection, a serious security flaw which can result in arbitrary
     command execution. For this reason, the use of ``shell=True`` is
     **strongly discouraged** in cases where the command string is
     constructed from external input:

        >>> from subprocess import call
        >>> filename = input("What file would you like to display?\n")
        What file would you like to display?
        non_existent; rm -rf / #
        >>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...

     ``shell=False`` disables all shell based features, but does not
     suffer from this vulnerability; see the Note in the ``Popen``
     constructor documentation for helpful hints in getting
     ``shell=False`` to work.When using ``shell=True``,
     ``pipes.quote()`` can be used to properly escape whitespace and
     shell metacharacters in strings that are going to be used to
     construct shell commands.

These options, along with all of the other options, are described in
more detail in the ``Popen`` constructor documentation.


Popen Constructor
-----------------

The underlying process creation and management in this module is
handled by the ``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 class subprocess.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)

   Execute a child program in a new process.  On Unix, the class uses
   ``os.execvp()``-like behavior to execute the child program.  On
   Windows, the class uses the Windows ``CreateProcess()`` function.
   The arguments to ``Popen`` are as follows.

   *args* should be a sequence of program arguments or else a single
   string. By default, the program to execute is the first item in
   *args* if *args* is a sequence.  If *args* is a string, the
   interpretation is platform-dependent and described below.  See the
   *shell* and *executable* arguments for additional differences from
   the default behavior.  Unless otherwise stated, it is recommended
   to pass *args* as a sequence.

   On Unix, if *args* is a string, the string is interpreted as the
   name or path of the program to execute.  However, this can only be
   done if not passing arguments to the program.

   Note: ``shlex.split()`` can be useful when determining the correct
     tokenization for *args*, especially in complex cases:

        >>> import shlex, subprocess
        >>> command_line = raw_input()
        /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
        >>> args = shlex.split(command_line)
        >>> print args
        ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
        >>> p = subprocess.Popen(args) # Success!

     Note in particular that options (such as *-input*) and arguments
     (such as *eggs.txt*) 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 *echo* command shown above)
     are single list elements.

   On Windows, if *args* is a sequence, it will be converted to a
   string in a manner described in *Converting an argument sequence to
   a string on Windows*.  This is because the underlying
   ``CreateProcess()`` operates on strings.

   The *shell* argument (which defaults to *False*) specifies whether
   to use the shell as the program to execute.  If *shell* is *True*,
   it is recommended to pass *args* as a string rather than as a
   sequence.

   On Unix with ``shell=True``, the shell defaults to ``/bin/sh``.  If
   *args* is a string, the string specifies the command 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 *args* 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,
   ``Popen`` does the equivalent of:

      Popen(['/bin/sh', '-c', args[0], args[1], ...])

   On Windows with ``shell=True``, the ``COMSPEC`` environment
   variable specifies the default shell.  The only time you need to
   specify ``shell=True`` on Windows is when the command you wish to
   execute is built into the shell (e.g. **dir** or **copy**).  You do
   not need ``shell=True`` to run a batch file or console-based
   executable.

   Warning: Passing ``shell=True`` can be a security hazard if combined with
     untrusted input.  See the warning under *Frequently Used
     Arguments* for details.

   *bufsize*, if given, has the same meaning as the corresponding
   argument to the built-in open() function: ``0`` means unbuffered,
   ``1`` means line buffered, any other positive value means use a
   buffer of (approximately) that size.  A negative *bufsize* means to
   use the system default, which usually means fully buffered.  The
   default value for *bufsize* is ``0`` (unbuffered).

   Note: If you experience performance issues, it is recommended that you
     try to enable buffering by setting *bufsize* to either -1 or a
     large enough positive value (such as 4096).

   The *executable* argument specifies a replacement program to
   execute.   It is very seldom needed.  When ``shell=False``,
   *executable* replaces the program to execute specified by *args*.
   However, the original *args* is still passed to the program.  Most
   programs treat the program specified by *args* as the command name,
   which can then be different from the program actually executed.  On
   Unix, the *args* name becomes the display name for the executable
   in utilities such as **ps**.  If ``shell=True``, on Unix the
   *executable* argument specifies a replacement shell for the default
   ``/bin/sh``.

   *stdin*, *stdout* and *stderr* specify the executed program's
   standard input, standard output and standard error file handles,
   respectively.  Valid values are ``PIPE``, an existing file
   descriptor (a positive integer), an existing file object, and
   ``None``.  ``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 ``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)

   If *close_fds* is true, all file descriptors except ``0``, ``1``
   and ``2`` will be closed before the child process is executed.
   (Unix only). Or, on Windows, if *close_fds* is true then no handles
   will be inherited by the child process.  Note that on Windows, you
   cannot set *close_fds* to true and also redirect the standard
   handles by setting *stdin*, *stdout* or *stderr*.

   If *cwd* is not ``None``, the child's current directory will be
   changed to *cwd* 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 *cwd*.

   If *env* is not ``None``, 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.

   Note: If specified, *env* must provide any variables required for the
     program to execute.  On Windows, in order to run a side-by-side
     assembly the specified *env* **must** include a valid
     ``SystemRoot``.

   If *universal_newlines* is ``True``, the file objects *stdout* and
   *stderr* are opened as text files in *universal newlines* mode.
   Lines may be terminated by any of ``'\n'``, the Unix end-of-line
   convention, ``'\r'``, the old Macintosh convention or ``'\r\n'``,
   the Windows convention. All of these external representations are
   seen as ``'\n'`` by the Python program.

   Note: This feature is only available if Python is built with universal
     newline support (the default).  Also, the newlines attribute of
     the file objects ``stdout``, ``stdin`` and ``stderr`` are not
     updated by the communicate() method.

   If given, *startupinfo* will be a ``STARTUPINFO`` object, which is
   passed to the underlying ``CreateProcess`` function.
   *creationflags*, if given, can be ``CREATE_NEW_CONSOLE`` or
   ``CREATE_NEW_PROCESS_GROUP``. (Windows only)


Exceptions
----------

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
``child_traceback``, which is a string containing traceback
information from the child's point of view.

The most common exception raised is ``OSError``.  This occurs, for
example, when trying to execute a non-existent file.  Applications
should prepare for ``OSError`` exceptions.

A ``ValueError`` will be raised if ``Popen`` is called with invalid
arguments.

``check_call()`` and ``check_output()`` will raise
``CalledProcessError`` if the called process returns a non-zero return
code.


Security
--------

Unlike some other popen functions, this implementation will never call
a system shell implicitly.  This means that all characters, including
shell metacharacters, can safely be passed to child processes.
Obviously, if the shell is invoked explicitly, then it is the
application's responsibility to ensure that all whitespace and
metacharacters are quoted appropriately.


Popen Objects
=============

Instances of the ``Popen`` class have the following methods:

Popen.poll()

   Check if child process has terminated.  Set and return
   ``returncode`` attribute.

Popen.wait()

   Wait for child process to terminate.  Set and return ``returncode``
   attribute.

   Warning: 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.  Use ``communicate()`` to avoid that.

Popen.communicate(input=None)

   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 *input* argument should be a string to be
   sent to the child process, or ``None``, if no data should be sent
   to the child.

   ``communicate()`` returns a tuple ``(stdoutdata, stderrdata)``.

   Note that if you want to send data to the process's stdin, you need
   to create the Popen object with ``stdin=PIPE``.  Similarly, to get
   anything other than ``None`` in the result tuple, you need to give
   ``stdout=PIPE`` and/or ``stderr=PIPE`` too.

   Note: The data read is buffered in memory, so do not use this method if
     the data size is large or unlimited.

Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   Note: On Windows, SIGTERM is an alias for ``terminate()``. CTRL_C_EVENT
     and CTRL_BREAK_EVENT can be sent to processes started with a
     *creationflags* parameter which includes
     *CREATE_NEW_PROCESS_GROUP*.

   New in version 2.6.

Popen.terminate()

   Stop the child. On Posix OSs the method sends SIGTERM to the child.
   On Windows the Win32 API function ``TerminateProcess()`` is called
   to stop the child.

   New in version 2.6.

Popen.kill()

   Kills the child. On Posix OSs the function sends SIGKILL to the
   child. On Windows ``kill()`` is an alias for ``terminate()``.

   New in version 2.6.

The following attributes are also available:

Warning: Use ``communicate()`` rather than ``.stdin.write``, ``.stdout.read``
  or ``.stderr.read`` to avoid deadlocks due to any of the other OS
  pipe buffers filling up and blocking the child process.

Popen.stdin

   If the *stdin* argument was ``PIPE``, this attribute is a file
   object that provides input to the child process.  Otherwise, it is
   ``None``.

Popen.stdout

   If the *stdout* argument was ``PIPE``, this attribute is a file
   object that provides output from the child process.  Otherwise, it
   is ``None``.

Popen.stderr

   If the *stderr* argument was ``PIPE``, this attribute is a file
   object that provides error output from the child process.
   Otherwise, it is ``None``.

Popen.pid

   The process ID of the child process.

   Note that if you set the *shell* argument to ``True``, this is the
   process ID of the spawned shell.

Popen.returncode

   The child return code, set by ``poll()`` and ``wait()`` (and
   indirectly by ``communicate()``).  A ``None`` value indicates that
   the process hasn't terminated yet.

   A negative value ``-N`` indicates that the child was terminated by
   signal ``N`` (Unix only).


Windows Popen Helpers
=====================

The ``STARTUPINFO`` class and following constants are only available
on Windows.

class class subprocess.STARTUPINFO

   Partial support of the Windows STARTUPINFO structure is used for
   ``Popen`` creation.

   dwFlags

      A bit field that determines whether certain ``STARTUPINFO``
      attributes are used when the process creates a window.

         si = subprocess.STARTUPINFO()
         si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW

   hStdInput

      If ``dwFlags`` specifies ``STARTF_USESTDHANDLES``, this
      attribute is the standard input handle for the process. If
      ``STARTF_USESTDHANDLES`` is not specified, the default for
      standard input is the keyboard buffer.

   hStdOutput

      If ``dwFlags`` specifies ``STARTF_USESTDHANDLES``, this
      attribute is the standard output handle for the process.
      Otherwise, this attribute is ignored and the default for
      standard output is the console window's buffer.

   hStdError

      If ``dwFlags`` specifies ``STARTF_USESTDHANDLES``, this
      attribute is the standard error handle for the process.
      Otherwise, this attribute is ignored and the default for
      standard error is the console window's buffer.

   wShowWindow

      If ``dwFlags`` specifies ``STARTF_USESHOWWINDOW``, this
      attribute can be any of the values that can be specified in the
      ``nCmdShow`` parameter for the ShowWindow function, except for
      ``SW_SHOWDEFAULT``. Otherwise, this attribute is ignored.

      ``SW_HIDE`` is provided for this attribute. It is used when
      ``Popen`` is called with ``shell=True``.


Constants
---------

The ``subprocess`` module exposes the following constants.

subprocess.STD_INPUT_HANDLE

   The standard input device. Initially, this is the console input
   buffer, ``CONIN$``.

subprocess.STD_OUTPUT_HANDLE

   The standard output device. Initially, this is the active console
   screen buffer, ``CONOUT$``.

subprocess.STD_ERROR_HANDLE

   The standard error device. Initially, this is the active console
   screen buffer, ``CONOUT$``.

subprocess.SW_HIDE

   Hides the window. Another window will be activated.

subprocess.STARTF_USESTDHANDLES

   Specifies that the ``STARTUPINFO.hStdInput``,
   ``STARTUPINFO.hStdOutput``, and ``STARTUPINFO.hStdError``
   attributes contain additional information.

subprocess.STARTF_USESHOWWINDOW

   Specifies that the ``STARTUPINFO.wShowWindow`` attribute contains
   additional information.

subprocess.CREATE_NEW_CONSOLE

   The new process has a new console, instead of inheriting its
   parent's console (the default).

   This flag is always set when ``Popen`` is created with
   ``shell=True``.

subprocess.CREATE_NEW_PROCESS_GROUP

   A ``Popen`` ``creationflags`` parameter to specify that a new
   process group will be created. This flag is necessary for using
   ``os.kill()`` on the subprocess.

   This flag is ignored if ``CREATE_NEW_CONSOLE`` is specified.


Replacing Older Functions with the ``subprocess`` Module
========================================================

In this section, "a becomes b" means that b can be used as a
replacement for a.

Note: All "a" functions in this section fail (more or less) silently if
  the executed program cannot be found; the "b" replacements raise
  ``OSError`` instead.In addition, the replacements using
  ``check_output()`` will fail with a ``CalledProcessError`` if the
  requested operation produces a non-zero return code. The output is
  still available as the ``output`` attribute of the raised exception.

In the following examples, we assume that the relevant functions have
already been imported from the ``subprocess`` module.


Replacing /bin/sh shell backquote
---------------------------------

   output=`mycmd myarg`
   # 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.
   output = p2.communicate()[0]

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 own pipeline support may
still be used directly:

   output=`dmesg | grep hda`
   # becomes
   output=check_output("dmesg | grep hda", shell=True)


Replacing ``os.system()``
-------------------------

   sts = os.system("mycmd" + " myarg")
   # becomes
   sts = call("mycmd" + " myarg", shell=True)

Notes:

* Calling the program through the shell is usually not required.

A more realistic example would look like this:

   try:
       retcode = call("mycmd" + " myarg", shell=True)
       if retcode < 0:
           print >>sys.stderr, "Child was terminated by signal", -retcode
       else:
           print >>sys.stderr, "Child returned", retcode
   except OSError as e:
       print >>sys.stderr, "Execution failed:", e


Replacing the ``os.spawn`` family
---------------------------------

P_NOWAIT example:

   pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT example:

   retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   retcode = call(["/bin/mycmd", "myarg"])

Vector example:

   os.spawnvp(os.P_NOWAIT, path, args)
   ==>
   Popen([path] + args[1:])

Environment example:

   os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
   ==>
   Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})


Replacing ``os.popen()``, ``os.popen2()``, ``os.popen3()``
----------------------------------------------------------

   pipe = os.popen("cmd", 'r', bufsize)
   ==>
   pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout

   pipe = os.popen("cmd", 'w', bufsize)
   ==>
   pipe = Popen("cmd", shell=True, bufsize=bufsize, stdin=PIPE).stdin

   (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)

   (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)

   (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)

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:

   (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)

Return code handling translates as follows:

   pipe = os.popen("cmd", 'w')
   ...
   rc = pipe.close()
   if rc is not None and rc >> 8:
       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"


Replacing functions from the ``popen2`` module
----------------------------------------------

   (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)

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:

   (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)

``popen2.Popen3`` and ``popen2.Popen4`` basically work as
``subprocess.Popen``, except that:

* ``Popen`` raises an exception if the execution fails.

* the *capturestderr* argument is replaced with the *stderr* argument.

* ``stdin=PIPE`` and ``stdout=PIPE`` must be specified.

* popen2 closes all file descriptors by default, but you have to
  specify ``close_fds=True`` with ``Popen``.


Notes
=====


Converting an argument sequence to a string on Windows
------------------------------------------------------

On Windows, an *args* sequence is converted to a string that can be
parsed using the following rules (which correspond to the rules used
by the MS C runtime):

1. Arguments are delimited by white space, which is either a space or
   a tab.

2. A string surrounded by double quotation marks is interpreted as a
   single argument, regardless of white space contained within.  A
   quoted string can be embedded in an argument.

3. A double quotation mark preceded by a backslash is interpreted as a
   literal double quotation mark.

4. Backslashes are interpreted literally, unless they immediately
   precede a double quotation mark.

5. If backslashes immediately precede a double quotation mark, every
   pair of backslashes is interpreted as a literal backslash.  If the
   number of backslashes is odd, the last backslash escapes the next
   double quotation mark as described in rule 3.
