PEP: 3145 Title: Asynchronous I/O For subprocess.Popen Version: $Revision$ Last-Modified: $Date$ Author: (James) Eric Pruitt, Charles R. McCreary, Josiah Carlson Status: Withdrawn Type: Standards Track Content-Type: text/x-rst Created: 04-Aug-2009 Python-Version: 3.2 Post-History: Abstract ======== In its present form, the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/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/ .. [11] https://bugs.python.org/issue18823 -- 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/.