Issue 40527: Multiple "unknown option" errors when passing unknown arguments to the interpreter

A minor issue I just discovered today: When e.g. doing "python3 --foo", the output is:

  unknown option --foo
  unknown option --foo
  unknown option --foo
  usage: /usr/bin/python3 [option] ... [-c cmd | -m mod | file | -] [arg] ...

With more dashes in the options, it seems to get worse:

  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  unknown option --foo-bar-baz
  usage: /usr/bin/python3 [option] ... [-c cmd | -m mod | file | -] [arg] ...

This seems very similar to issue16306 (thus why I added people involved there to the nosy list), except that it happens with long rather than short arguments.

This only happens with Python 3.8/3.9, not 3.7 or older. I was able to bisect this to the following commit:

commit 6dcb54228e7520abd058897440c26e323f62afcd
Author: Victor Stinner <vstinner@redhat.com>
Date:   Tue Mar 5 02:44:12 2019 +0100

    bpo-36142: Add _PyPreConfig_ReadFromArgv() (GH-12173)
    
    The new function is now responsible to parse -E and -I command line
    arguments.

Oh right, that's because Python parses the command line at least 3 times :-) The first 2 times, it is not supposed to log errors. It's a bug: attached PR 19955 fix it.

> I was able to bisect this to the following commit: (...)

Oh, well done! Yeah, it's all my fault ;-) It's a regression of the PEP 587 (PyConfig) implementation.

I chose to share code between Python/preconfig.c and Python/initconfig.c rather than duplicate code. The implementation of _PyPreConfig_Read() and PyConfig_Read() is quite complex.

New changeset 2668a9a5aa506a048aef7b4881c8dcf6b81c6870 by Victor Stinner in branch 'master':
bpo-40527: Fix command line argument parsing (GH-19955)
https://github.com/python/cpython/commit/2668a9a5aa506a048aef7b4881c8dcf6b81c6870

New changeset bce4ddafdd188cc6deb1584728b67b9e149ca6a4 by Miss Islington (bot) in branch '3.8':
bpo-40527: Fix command line argument parsing (GH-19955)
https://github.com/python/cpython/commit/bce4ddafdd188cc6deb1584728b67b9e149ca6a4

Thanks for the bug report Florian and for work Victor!

I am now closing this issue :)

That was an insanely fast fix - thanks everyone! :)

> That was an insanely fast fix - thanks everyone! :)

Sorry about that. We are working on this issue to ensure that next bugs will not fixed as quickly a this done ;-)

stage: patch review -> resolved