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.

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

-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.

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 :-)).

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

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

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 :-)

Right, sorry then :)

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