python-peps/pep-0324.txt

PEP: 324
Title: subprocess - New process module
Version: $Revision$
Last-Modified: $Date$
Author: Peter Astrand <astrand@lysator.liu.se>
Status: Final
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.


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 subprocess 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 subprocess 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:

    - subprocess 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.  subprocess, however,
      contains one single class.  Since the subprocess 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.

    - One small utility function is provided: subprocess.call(). 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 an 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.

      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 subprocess.call(), this would look like:

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

      or, if executing through the shell:

          subprocess.call("stty sane -F " + device, shell=True)

    - 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.

     - On POSIX platforms, no extension module is required: the module
       uses os.fork(), os.execvp() etc.

     - On Windows platforms, the module requires either Mark Hammond's
       Windows extensions[5], or a small extension module called
       _subprocess.


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, 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 file, 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 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 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 meta-characters, 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).


Replacing older functions with the subprocess module

    In this section, "a ==> 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
    OSError exception.

    In the following examples, we assume that the subprocess module is
    imported with "from subprocess import *".


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

    output=`mycmd myarg`
    ==>
    output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]


    Replacing shell pipe line
    -------------------------

    output=`dmesg | grep hda`
    ==>
    p1 = Popen(["dmesg"], stdout=PIPE)
    p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
    output = p2.communicate()[0]


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

    sts = os.system("mycmd" + " myarg")
    ==>
    p = Popen("mycmd" + " myarg", shell=True)
    sts = os.waitpid(p.pid, 0)

    Note:

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

    * It's easier to look at the returncode attribute than the
      exit status.

    A more real-world 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, e:
        print >>sys.stderr, "Execution failed:", e


    Replacing os.spawn*
    -------------------

    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*
    -------------------

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

    pipe = os.popen(cmd, mode='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)


    Replacing popen2.*
    ------------------

    Note: If the cmd argument to popen2 functions is a string, the
    command is executed through /bin/sh.  If it is a list, the command
    is directly executed.

    (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)


    (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)

    The popen2.Popen3 and popen3.Popen4 basically works as
    subprocess.Popen, except that:

    * subprocess.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 subprocess.Popen.


Open Issues

    Some features have been requested but is not yet implemented.
    This includes:

    * Support for managing a whole flock of subprocesses

    * Support for managing "daemon" processes

    * Built-in method for killing subprocesses

    While these are useful features, it's expected that these can be
    added later without problems.

    * expect-like functionality, including pty support.

    pty support is highly platform-dependent, which is a
    problem.  Also, there are already other modules that provide this
    kind of functionality[6].


Backwards Compatibility

    Since this is a new module, no major backward compatible issues
    are expected.  The module name "subprocess" might collide with
    other, previous modules[3] with the same name, but the name
    "subprocess" seems to be the best suggested name so far.  The
    first name of this module was "popen5", but this name was
    considered too unintuitive.  For a while, the module was called
    "process", but this name is already used by Trent Mick's
    module[4].

    The functions and modules that this new module is trying to
    replace (os.system, os.spawn*, os.popen*, popen2.*, commands.*)
    are expected to be available in future Python versions for a long
    time, to preserve backwards compatibility.


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/

    [3] http://www.iol.ie/~padraiga/libs/subProcess.py

    [4] http://starship.python.net/crew/tmick/

    [5] http://starship.python.net/crew/mhammond/win32/

    [6] http://www.lysator.liu.se/~ceder/pcl-expect/


Copyright

    This document has been placed in the public domain.


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