python-peps/peps/pep-3145.rst

PEP: 3145
Title: Asynchronous I/O For subprocess.Popen
Author: Eric Pruitt, Charles R. McCreary, Josiah Carlson
Status: Withdrawn
Type: Standards Track
Content-Type: text/x-rst
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.


PEP Withdrawal
==============

This can be dealt with in the bug tracker.  A specific proposal is
attached to [11]_.


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 to 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`` 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
       https://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/problems-with-subprocess.html

.. [3] How can I run an external command asynchronously from Python? - Stack
       Overflow
       https://stackoverflow.com/q/636561

.. [4] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation
       https://docs.python.org/2.6/library/subprocess.html#subprocess.Popen.wait

.. [5] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation
       https://docs.python.org/2.6/library/subprocess.html#subprocess.Popen.kill

.. [6] Issue 1191964: asynchronous Subprocess - Python tracker
       https://github.com/python/cpython/issues/41922

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

[8] subprocess.rst - subprocdev - Project Hosting on Google Code
\   https://web.archive.org/web/20130306074135/http://code.google.com/p/subprocdev/source/browse/doc/subprocess.rst?spec=svn2c925e935cad0166d5da85e37c742d8e7f609de5&r=2c925e935cad0166d5da85e37c742d8e7f609de5

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

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

.. [11] https://github.com/python/cpython/issues/63023 -- Idle: use pipes instead of
        sockets to talk with user subprocess

Copyright
=========

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