Rewrite parse_intermixed_args() in argparse

[serhiy-storchaka]

parse_intermixed_args() and parse_known_intermixed_args() are implemented by parsing command lines twice -- first only optional arguments, then the remaining is parsed as positional arguments. This approach has some issues.

• The parser is temporary modified to suppress positional or required optional arguments in these two stages. This is not good, because the parser can no longer be used concurrently. This also smells bad in general, this can hide some bugs.
• Default values are handled twice.
• Unknown options in parse_known_intermixed_args() cannot be intermixed with positional arguments. Well, "parsing only known arguments" is a dubious feature, but still...

I tried to rewrite the implementation by moving the code deeper. No longer parser patching, defaults are handled only once, unknown options are excluded from positionals parsing. @hpaulj, @bitdancer, as the authors of the original implementation, could you please review this code?

Linked PRs

• gh-125355: Rewrite parse_intermixed_args() in argparse #125356
• [3.13] gh-125355: Rewrite parse_intermixed_args() in argparse (GH-125356) #125834
• [3.12] gh-125355: Rewrite parse_intermixed_args() in argparse (GH-125356) #125839

[serhiy-storchaka]

For reference, the original feature was implemented in bpo-14191(gh-58399), PR #3319, 0f6b9d2.

[serhiy-storchaka]

This also fixes support for required mutually exclusive groups containing positionals.