python-peps/pep-3145.txt

PEP: 3145
Title: Asynchronous I/O For subprocess.Popen
Version: $Revision$
Last-Modified: $Date$
Author: (James) Eric Pruitt, Charles R. McCreary, Josiah Carlson
Status: Deferred
Type: Standards Track
Content-Type: text/plain
Created: 04-Aug-2009
Python-Version: 3.2
Post-History:

Abstract:

    In its present form, the subprocess.Popen implementation is prone to
    dead-locking and blocking of the parent Python script while waiting on data
    from the child process. This PEP proposes to make
    subprocess.Popen more asynchronous to help alleviate these
    problems.


PEP Deferral:

    Further exploration of the concepts covered in this PEP has been deferred
    at least until after PEP 3156 has been resolved.


Motivation:

    A search for "python asynchronous subprocess" will turn up numerous
    accounts of people wanting to execute a child process and communicate with
    it from time to time reading only the data that is available instead of
    blocking to wait for the program to produce data [1] [2] [3].  The current
    behavior of the subprocess module is that when a user sends or receives
    data via the stdin, stderr and stdout file objects, dead locks are common
    and documented [4] [5].  While communicate can be used to alleviate some of
    the buffering issues, it will still cause the parent process to block while
    attempting to read data when none is available to be read from the child
    process.  

Rationale:

    There is a documented need for asynchronous, non-blocking functionality in
    subprocess.Popen [6] [7] [2] [3].  Inclusion of the code would improve the
    utility of the Python standard library that can be used on Unix based and
    Windows builds of Python.  Practically every I/O object in Python has a
    file-like wrapper of some sort.  Sockets already act as such and for
    strings there is StringIO.  Popen can be made to act like a file by simply
    using the methods attached the the subprocess.Popen.stderr, stdout and
    stdin file-like objects.  But when using the read and write methods of
    those options, you do not have the benefit of asynchronous I/O.  In the
    proposed solution the wrapper wraps the asynchronous methods to mimic a
    file object.

Reference Implementation:

    I have been maintaining a Google Code repository that contains all of my
    changes including tests and documentation [9] as well as blog detailing
    the problems I have come across in the development process [10].  

    I have been working on implementing non-blocking asynchronous I/O in the
    subprocess.Popen module as well as a wrapper class for subprocess.Popen
    that makes it so that an executed process can take the place of a file by
    duplicating all of the methods and attributes that file objects have.  

    There are two base functions that have been added to the subprocess.Popen
    class: Popen.send and Popen._recv, each with two separate implementations,
    one for Windows and one for Unix based systems.  The Windows
    implementation uses ctypes to access the functions needed to control pipes
    in the kernel 32 DLL in an asynchronous manner.  On Unix based systems,
    the Python interface for file control serves the same purpose.  The
    different implementations of Popen.send and Popen._recv have identical
    arguments to make code that uses these functions work across multiple
    platforms.  

    When calling the Popen._recv function, it requires the pipe name be
    passed as an argument so there exists the Popen.recv function that passes
    selects stdout as the pipe for Popen._recv by default. Popen.recv_err
    selects stderr as the pipe by default. Popen.recv and Popen.recv_err
    are much easier to read and understand than Popen._recv('stdout' ...) and
    Popen._recv('stderr' ...) respectively.  

    Since the Popen._recv function does not wait on data to be produced
    before returning a value, it may return empty bytes. Popen.asyncread
    handles this issue by returning all data read over a given time
    interval.  

    The ProcessIOWrapper class uses the asyncread and asyncwrite functions to
    allow a process to act like a file so that there are no blocking issues
    that can arise from using the stdout and stdin file objects produced from
    a subprocess.Popen call.
    

References:

    [1] [ python-Feature Requests-1191964 ] asynchronous Subprocess
        http://mail.python.org/pipermail/python-bugs-list/2006-December/
          036524.html

    [2] Daily Life in an Ivory Basement : /feb-07/problems-with-subprocess
        http://ivory.idyll.org/blog/feb-07/problems-with-subprocess

    [3] How can I run an external command asynchronously from Python? - Stack 
        Overflow
        http://stackoverflow.com/questions/636561/how-can-i-run-an-external-
          command-asynchronously-from-python

    [4] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation
        http://docs.python.org/library/subprocess.html#subprocess.Popen.wait

    [5] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation
        http://docs.python.org/library/subprocess.html#subprocess.Popen.kill

    [6] Issue 1191964: asynchronous Subprocess - Python tracker
        http://bugs.python.org/issue1191964

    [7] Module to allow Asynchronous subprocess use on Windows and Posix
        platforms - ActiveState Code
        http://code.activestate.com/recipes/440554/

    [8] subprocess.rst - subprocdev - Project Hosting on Google Code
        http://code.google.com/p/subprocdev/source/browse/doc/subprocess.rst?spec=svn2c925e935cad0166d5da85e37c742d8e7f609de5&r=2c925e935cad0166d5da85e37c742d8e7f609de5#437

    [9] subprocdev - Project Hosting on Google Code
        http://code.google.com/p/subprocdev

    [10] Python Subprocess Dev
         http://subdev.blogspot.com/

Copyright:

    This P.E.P. is licensed under the Open Publication License;
    http://www.opencontent.org/openpub/.