[New-bugs-announce] [issue42648] subprocess: add a helper/parameter to catch exec() OSError exception

STINNER Victor report at bugs.python.org
Tue Dec 15 12:05:42 EST 2020

New submission from STINNER Victor <vstinner at python.org>:

While working on on bpo-42641 "Deprecate os.popen() function", I tried to replace os.popen() with a subprocess function, to avoid relying on an external shell. Example from test_posix:

        with os.popen('id -G 2>/dev/null') as idg:
            groups = idg.read().strip()
            ret = idg.close()

os.popen() uses a shell, and so returns an non-zero exit status if the "id" program is not available:

>>> import os; os.popen('nonexistent').close()
/bin/sh: nonexistent : commande introuvable

whereas the subprocess module raises an OSError:

>>> import subprocess; proc=subprocess.run('nonexistent')
FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent'

It's not convenient to have to write try/except OSError when replacing os.popen() with subprocess.run().

It would be convenient to have a subprocess API which avoid the need for try/except, if possible with keeping the ability to distinguish when exec() fails and when exec() completed but waitpid() returns a non-zero exit status (child process exit code is non-zero).

This issue becomes more interesting when subprocess uses os.posix_spawn(). The subprocess module only uses os.posix_spawn() if the libc implementation is known to report exec() failure: if os.posix_spawn() raises an OSError exception if exec() fails. See subprocess._use_posix_spawn() which uses os.confstr('CS_GNU_LIBC_VERSION') to check if the glibc 2.24+ is used.

Or maybe I simply missed a very obvious API in subprocess for this problem?

components: Library (Lib)
messages: 383078
nosy: gregory.p.smith, vstinner
priority: normal
severity: normal
status: open
title: subprocess: add a helper/parameter to catch exec() OSError exception
versions: Python 3.10

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