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

Mike Meyer mwm at mired.org
Sun Jan 8 01:14:06 CET 2012

On Sat, 7 Jan 2012 21:25:37 +0000 (UTC) Vinay Sajip <vinay_sajip at yahoo.co.uk> wrote:

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

Note that this is documented behavior for these attributes.

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.

Since the only reason they exist is so you can access your end of a pipe, setting them to anything would seem to be a bug. I'd argue that their existence is more a pola violation than them having the value None. But None is easier than a call to hasattr.

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'.

You can close the object you passed in if it wasn't PIPE. If you passed in PIPE, the object has to be exposed some way, otherwise you can't close it.

This did raise one interesting question, which will go to ideas...

   <mike

-- Mike Meyer <mwm at mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information.

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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'.

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