# subprocess - Subprocesses with accessible I/O streams

#

# For more information about this module, see PEP 324.

#

# This module should remain compatible with Python 2.2, see PEP 291.

#

# Copyright (c) 2003-2005 by Peter Astrand <astrand@lysator.liu.se>

#

# Licensed to PSF under a Contributor Agreement.

# See http://www.python.org/2.4/license for licensing details.

r"""subprocess - Subprocesses with accessible I/O streams

This module allows you to spawn processes, connect to their

input/output/error pipes, and obtain their return codes.  This module

intends to replace several other, older modules and functions, like:

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 below.

Using the subprocess module

===========================

This module defines one class called Popen:

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):

Arguments are:

args should be a string, or a sequence of program arguments.  The

program to execute is normally the first item in the args sequence or

string, but can be explicitly set by using the executable argument.

On UNIX, with shell=False (default): In this case, the Popen class

uses os.execvp() to execute the child program.  args should normally

be a sequence.  A string will be treated as a sequence with the string

as the only item (the program to execute).

On UNIX, with shell=True: If args is a string, it specifies the

command string to execute through the shell.  If args is a sequence,

the first item specifies the command string, and any additional items

will be treated as additional shell arguments.

On Windows: the Popen class uses CreateProcess() to execute the child

program, which operates on strings.  If args is a sequence, it will be

converted to a string using the list2cmdline method.  Please note that

not all MS Windows applications interpret the command line the same

way: The list2cmdline is designed for applications using the same

rules as the MS C runtime.

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).

stdin, stdout and stderr specify the executed programs' 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 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 applications 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.

If close_fds is true, all file descriptors except 0, 1 and 2 will be

closed before the child process is executed.

if shell is true, the specified command will be executed through the

shell.

If cwd is not None, the current directory will be changed to cwd

before the child is executed.

If env is not None, it defines the environment variables for the new

process.

If universal_newlines is true, the file objects stdout and stderr are

opened as a text files, but lines may be terminated by any of '\n',

the Unix end-of-line convention, '\r', the 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.

The startupinfo and creationflags, 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)

This module also defines two shortcut functions:

call(*args, **kwargs):

    Run command with arguments.  Wait for command to complete, then

    return the returncode attribute.

    The arguments are the same as for the Popen constructor.  Example:

    retcode = call(["ls", "-l"])

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 childs 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 OSErrors.

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

Security

--------

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.

Popen objects

=============

Instances of the Popen class have the following methods:

poll()

    Check if child process has terminated.  Returns returncode

    attribute.

wait()

    Wait for child process to terminate.  Returns returncode attribute.

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 stdin argument should be a string to be

    sent to the child process, or None, if no data should be sent to

    the child.