[Python-Dev] A question about the subprocess implementation (original) (raw)

Vinay Sajip vinay_sajip at yahoo.co.uk
Sat Jan 7 22:25:37 CET 2012

The subprocess.Popen constructor takes stdin, stdout and stderr keyword arguments which are supposed to represent the file handles of the child process. The object also has stdin, stdout and stderr attributes, which one would naively expect to correspond to the passed in values, except where you pass in e.g. subprocess.PIPE (in which case the corresponding attribute would be set to an actual stream or descriptor).

However, in common cases, even when keyword arguments are passed in, the corresponding attributes are set to None. The following script

import os from subprocess import Popen, PIPE import tempfile

cmd = 'ls /tmp'.split()

p = Popen(cmd, stdout=open(os.devnull, 'w+b')) print('process output streams: %s, %s' % (p.stdout, p.stderr)) p = Popen(cmd, stdout=tempfile.TemporaryFile()) print('process output streams: %s, %s' % (p.stdout, p.stderr))

prints

process output streams: None, None process output streams: None, None

under both Python 2.7 and 3.2. However, if subprocess.PIPE is passed in, then the corresponding attribute is set: if the last four lines are changed to

p = Popen(cmd, stdout=PIPE) print('process output streams: %s, %s' % (p.stdout, p.stderr)) p = Popen(cmd, stdout=open(os.devnull, 'w+b'), stderr=PIPE) print('process output streams: %s, %s' % (p.stdout, p.stderr))

then you get

process output streams: <open file '', mode 'rb' at 0x2088660>, None process output streams: None, <open file '', mode 'rb' at 0x2088e40>

under Python 2.7, and

process output streams: <_io.FileIO name=3 mode='rb'>, None process output streams: None, <_io.FileIO name=5 mode='rb'>

This seems to me to contradict the principle of least surprise. One would expect, when an file-like object is passed in as a keyword argument, that it be placed in the corresponding attribute. That way, if one wants to do p.stdout.close() (which is necessary in some cases), one doesn't hit an AttributeError because NoneType has no attribute 'close'.

This seems like it might be a bug, but if so it does seem rather egregious: can someone tell me if there is a good design reason for the current behaviour? If there isn't one, I'll raise an issue.

Regards,

Vinay Sajip

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