subprocess.DEVNULL

[MrJean1]

BPO	5870
Nosy	@birkenfeld, @gpshead, @pitrou, @giampaolo, @socketpair
Files	5870_v1.patch: Patch + test for issue

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2011-03-16.17:38:53.804>
created_at = <Date 2009-04-28.19:24:02.743>
labels = ['extension-modules', 'type-feature']
title = 'subprocess.DEVNULL'
updated_at = <Date 2011-03-16.17:38:53.803>
user = 'https://bugs.python.org/MrJean1'

bugs.python.org fields:

activity = <Date 2011-03-16.17:38:53.803>
actor = 'rosslagerwall'
assignee = 'rosslagerwall'
closed = True
closed_date = <Date 2011-03-16.17:38:53.804>
closer = 'rosslagerwall'
components = ['Extension Modules']
creation = <Date 2009-04-28.19:24:02.743>
creator = 'MrJean1'
dependencies = []
files = ['20215']
hgrepos = []
issue_num = 5870
keywords = ['patch']
message_count = 9.0
messages = ['86761', '86771', '90444', '90446', '125030', '125057', '125058', '125063', '131146']
nosy_count = 11.0
nosy_names = ['georg.brandl', 'gregory.p.smith', 'pitrou', 'giampaolo.rodola', 'OG7', 'MrJean1', 'lucaspmelo', 'santoso.wijaya', 'rosslagerwall', 'socketpair', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue5870'
versions = ['Python 3.3']

[MrJean1]

The subprocess module exposes the PIPE and STDOUT constants to be used
for the stdin, stdout or stderr keyword arguments.

Often, stderr needs to be redirected to /dev/null (on posix). It would
nice if another constant was available for that purpose, e.g.
subprocess.DEVNULL.

Perhaps, that might be as simple as

DEVNULL = os.open(os.devnull, os.OS_RDWR)

and re-use the same, single file descriptor.

[MrJean1]

Typo. That should be ..., os.O_RDWR)

[lucaspmelo]

-1 on this one.
It is not a portable decision (only *nix OSes do have /dev/null).
Also, why would we want it as a default constant? The subprocess module
would need to open /dev/null every time. Despite that, I can't see how
would someone use the redirection of errors to /dev/null through a
python script and, at the same time, make it seem not a bad practice at all.

[pitrou]

Lucas, Windows has a /dev/null-like device. I think Jean's suggestion is
reasonable, but it should be a special constant instead of creating a
file descriptor unconditionnally (that is, the file should only be open
if needed). Also, a patch should be provided (with tests :-)).

[rosslagerwall]

Here is a fairly simple patch that adds the subprocess.DEVNULL constant.

[birkenfeld]

Hmm, we don't like these open-for-eternity file descriptors; we had such a thing for os.urandom() but removed it (see bpo-1177468).

I'm okay with DEVNULL (or even just NULL) as a shorthand, but it should open (and close) the devnull device each time just as if a normal fd was given.

[rosslagerwall]

I think if you look closely at the patch, the fd does not stay open the whole time. It is opened if necessary in _get_handles() with e.g.:

elif stdin == DEVNULL:
p2cread = self._get_devnull()

and then closed in _execute_child() with:

if hasattr(self, '_devnull'):
    os.close(self._devnull)

which is executed from __init__(). So I don't think it stays open for eternity :-)

[birkenfeld]

Right, sorry then :)

[python-dev]

New changeset eaf93e156dff by Ross Lagerwall in branch 'default':
Issue bpo-5870: Add subprocess.DEVNULL constant.
http://hg.python.org/cpython/rev/eaf93e156dff