python-peps/pep-0324.txt

PEP: 324
Title: process - 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 process module 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 the process module, 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:

    - process 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.  process, however,
      contains one single class.  Since the process module 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.

    - Two small utility functions are provided: process.call() and
      process.callv().  These 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 call() utility function accepts an 'args' argument, just
      like the Popen class constructor. It waits for the command to
      complete, then returns the returncode attribute. The
      implementation is very simple:

      def call(*args, **kwargs):
          return Popen(*args, **kwargs).wait()

      The motivation behind the call() function is simple: Starting a
      process and wait for it to finish is a common task.

      The callv() function is identical to call(), except that each
      non-keyword argument is treated as a program argument. This
      gives a slightly nicer syntax. The drawback is that callv() does
      not allow specifying the program and it's arguments as a
      whitespace-separated string: The entire (first) string would be
      intepreted as the executable. The implementation of callv() is
      also very simple:

      def callv(*args, **kwargs):
          return Popen(args, **kwargs).wait()

      While Popen supports a wide range of options, many users have
      simple needs. Many people are using os.system() today, mainly
      because it provides a simple interface. Consider this example:

          os.system("stty sane -F " + device)

      With process.call(), this would look like:

          process.call(["stty", "sane", "-F", device])

      Some people feel that the list brackets are clumsy. With
      callv(), they are not needed:

          process.callv("stty", "sane", "-F", device)

    - 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.


Specification

    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,
                    cwd=None, env=None, universal_newlines=False,
                    startupinfo=None, creationflags=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 executable argument.
      
    - On UNIX: the Popen class uses os.execvp() to execute the child
      program, which operates on sequences.  If args is a string, it
      will be converted to a sequence using the cmdline2list method.
      Please note that syntax for quoting arguments is different from
      a typical UNIX shell.  See the documentation of the cmdline2list
      method for more information.
      
    - 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 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"])
      
      
    - callv(*args, **kwargs):
          Run command with arguments.  Wait for command to complete, then
          return the returncode attribute.
      
          This function is identical to call(), except that each non-keyword
          argument is treated as a program argument.  Example:
      
          retcode = callv("ls", "-l")
      
          This is equivalent to:
      
          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.

        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:
    
    stdin
        If the stdin argument is PIPE, this attribute is a file object
        that provides input to the child process.  Otherwise, it is None.
    
    stdout
        If the stdout argument is PIPE, this attribute is a file object
        that provides output from the child process.  Otherwise, it is
        None.
    
    stderr
        If the stderr argument is PIPE, this attribute is file object that
        provides error output from the child process.  Otherwise, it is
        None.
    
    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 -N indicates
        that the child was terminated by signal N (UNIX only).
          
      
Open Issues

    Currently, the reference implementation requires the "win32all"
    extensions when running on the Windows platform. This dependency
    could probably be eliminated by providing a small "glue" module
    written in C, just like the _winreg module. 


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.


Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 70
End: