python-peps/pep-0324.txt

PEP: 324
Title: popen5 - New POSIX process module
Version: $Revision$
Last-Modified: $Date$
Author: Peter Astrand <astrand@lysator.liu.se>
Status: Draft
Type: Standards Track (library)
Created: 19-Nov-2003
Content-Type: text/plain
Python-Version: 2.4


Abstract

    This PEP describes a new module for starting and communicating
    with processes on POSIX systems.


Motivation

    Starting new processes is a common task in any programming
    language, and very common in a high-level language like Python.
    Good support for this task is needed, because:

    - Inappropriate functions for starting processes could mean a
      security risk: If the program is started through the shell, and
      the arguments contain shell meta characters, the result can be
      disastrous. [1]

    - It makes Python an even better replacement language for
      over-complicated shell scripts.

    Currently, Python has a large number of different functions for
    process creation. This makes it hard for developers to choose.

    The popen5 modules provides the following enhancements over
    previous functions:

    - One "unified" module provides all functionality from previous
      functions.

    - Cross-process exceptions: Exceptions happening in the child
      before the new process has started to execute are re-raised in
      the parent.  This means that it's easy to handle exec()
      failures, for example.  With popen2, for example, it's
      impossible to detect if the execution failed.

    - A hook for executing custom code between fork and exec.  This
      can be used for, for example, changing uid.

    - No implicit call of /bin/sh.  This means that there is no need
      for escaping dangerous shell meta characters.

    - All combinations of file descriptor redirection is possible.
      For example, the "python-dialog" [2] needs to spawn a process
      and redirect stderr, but not stdout.  This is not possible with
      current functions, without using temporary files.

    - With popen5, it's possible to control if all open file
      descriptors should be closed before the new program is
      executed.

    - Support for connecting several subprocesses (shell "pipe").

    - Universal newline support.

    - A communicate() method, which makes it easy to send stdin data
      and read stdout and stderr data, without risking deadlocks.
      Most people are aware of the flow control issues involved with
      child process communication, but not all have the patience or
      skills to write a fully correct and deadlock-free select loop.
      This means that many Python applications contain race
      conditions.  A communicate() method in the standard library
      solves this problem.


Rationale

    The following points summarizes the design:

    - popen5 was based on popen2, which is tried-and-tested.

    - The factory functions in popen2 have been removed, because I
      consider the class constructor equally easy to work with.

    - popen2 contains several factory functions and classes for
      different combinations of redirection.  popen5, however,
      contains one single class.  Since popen5 supports 12 different
      combinations of redirection, providing a class or function for
      each of them would be cumbersome and not very intuitive.  Even
      with popen2, this is a readability problem.  For example, many
      people cannot tell the difference between popen2.popen2 and
      popen2.popen4 without using the documentation.

    - One small utility function is provided: popen5.run().  It aims
      to be an enhancement over os.system(), while still very easy to
      use:

        - It does not use the Standard C function system(), which has
          limitations.

        - It does not call the shell implicitly.

        - No need for quoting; using a variable argument list.

        - The return value is easier to work with.

    - The "preexec" functionality makes it possible to run arbitrary
      code between fork and exec.  One might ask why there are special
      arguments for setting the environment and current directory, but
      not for, for example, setting the uid.  The answer is:

        - Changing environment and working directory is considered
          fairly common.

        - Old functions like spawn() has support for an
          "env"-argument.

        - env and cwd are considered quite cross-platform: They make
          sense even on Windows.

     - No MS Windows support is available, currently.  To be able to
       provide more functionality than what is already available from
       the popen2 module, help from C modules is required.


Specification

    This module defines one class called Popen:

        class Popen(args, bufsize=0, argv0=None,
                    stdin=None, stdout=None, stderr=None,
                    preexec_fn=None, preexec_args=(), close_fds=0,
                    cwd=None, env=None, universal_newlines=0)

    Arguments are:

    - args should be a sequence of program arguments.  The program to
      execute is normally the first item in the args sequence, but can
      be explicitly set by using the argv0 argument.  The Popen class
      uses os.execvp() to execute the child program.

    - 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,
      with arguments preexec_args.

    - If close_fds is true, all file descriptors except 0, 1 and 2
      will be closed before the child process is executed.

    - 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 fromchild and
      childerr 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 fromchild, tochild and
      childerr are not updated by the communicate() method.

    The module also defines one shortcut function:

        run(*args):
            Run command with arguments.  Wait for command to complete,
            then return the returncode attribute.  Example:

                retcode = popen5.run("stty", "sane")


    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 OSErrors.

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


    Security
    --------
    popen5 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()
        Returns -1 if child process hasn't completed yet, or its exit
        status otherwise.  See below for a description of how the exit
        status is encoded.

    wait()
        Waits for and returns the exit status of the child process.
        The exit status encodes both the return code of the process
        and information about whether it exited using the exit()
        system call or died due to a signal.  Functions to help
        interpret the status code are defined in the os module (the
        W*() family of functions).

    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.

        communicate() returns a tuple (stdout, stderr).

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

    The following attributes are also available:

    fromchild
        A file object that provides output from the child process.

    tochild
        A file object that provides input to the child process.

    childerr
        A file object that provides error output from the child
        process.

    pid
        The process ID of the child process.

    returncode
        The child return code.  A None value indicates that the
        process hasn't terminated yet.  A negative value means that
        the process was terminated by a signal with number
        -returncode.


Open Issues

    Perhaps the module should be called something like "process",
    instead of "popen5".


Reference Implementation

    A reference implementation is available from
    http://www.lysator.liu.se/~astrand/popen5/.


References

    [1] Secure Programming for Linux and Unix HOWTO, section 8.3.
        http://www.dwheeler.com/secure-programs/

    [2] Python Dialog
        http://pythondialog.sourceforge.net/


Copyright

    This document has been placed in the public domain.


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