[Python-Dev] PEP 3145 (With Contents) (original) (raw)

exarkun at twistedmatrix.com exarkun at twistedmatrix.com
Tue Sep 15 20:24:56 CEST 2009

On 04:25 pm, eric.pruitt at gmail.com wrote:

I'm bumping this PEP again in hopes of getting some feedback.

Thanks, Eric On Tue, Sep 8, 2009 at 23:52, Eric Pruitt <eric.pruitt at gmail.com> wrote: PEP: 3145 Title: Asynchronous I/O For subprocess.Popen Author: (James) Eric Pruitt, Charles R. McCreary, Josiah Carlson Type: Standards Track Content-Type: text/plain Created: 04-Aug-2009 Python-Version: 3.2

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. 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.

"Non-blocking" and "asynchronous" are actually two different things. From the rest of this PEP, I think only a non-blocking API is being introduced. I haven't looked beyond the PEP, though, so I might be missing something.

 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.

Why does the method for non-blocking read from a pipe start with an "_"? This is the convention (widely used) for a private API. The name also doesn't suggest that this is the non-blocking version of reading. Similarly, the name "send" doesn't suggest that this is the non-blocking version of writing.

 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.recverr  selects stderr as the pipe by default. "Popen.recv" and "Popen.recverr"  are much easier to read and understand than "Popen.recv('stdout' ..." and  "Popen.recv('stderr' ..." respectively.

What about reading from other file descriptors? subprocess.Popen allows arbitrary file descriptors to be used. Is there any provision here for reading and writing non-blocking from or to those?

 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.

Oh. Popen.asyncread? What's that? This is the first time the PEP mentions it.

 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.

What's the ProcessIOWrapper class? And what's the asyncwrite function? Again, this is the first time it's mentioned.

So, to sum up, I think my main comment is that the PEP seems to be missing a significant portion of the details of what it's actually proposing. I suspect that this information is present in the implementation, which I have not looked at, but it probably belongs in the PEP.

Jean-Paul

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