`argparse.BooleanOptionalAction` silently failed when used with parser whose `prefix_chars` has no `'-'`

[mattwang44]

Bug report

Bug description:

argparse.BooleanOptionalAction (doc) handles the creation of the boolean flags with positive and negative options by having user only needs to add the positive argument. However, it only works with those parsers whose prefix_chars contains '-', which is the default value, and otherwise failed to create the negative option silently.

>>> from argparse import BooleanOptionalAction, ArgumentParser
>>> # normal usage
>>> p = ArgumentParser()  # prefix_chars defaults to '-'
>>> p.add_argument("--foo", action=BooleanOptionalAction)
BooleanOptionalAction(option_strings=['--foo', '--no-foo'], dest='foo', nargs=0, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None, deprecated=False)
>>> p.parse_args(['--no-foo'])
Namespace(foo=False)
>>>
>>> # parser with customized prefix_chars
>>> p = ArgumentParser(prefix_chars='+')
>>> p.add_argument("++foo", action=BooleanOptionalAction)
BooleanOptionalAction(option_strings=['++foo'], dest='foo', nargs=0, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None, deprecated=False)

Raising warning/error is expected when argparse.BooleanOptionalAction is used with parsers whose prefix_chars has no '-'.

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-138525: raise error when BooleanOptionalAction used with prefix_chars without '-' #138526
• gh-138525: Support single-dash long options and prefix_chars in BooleanOptionalAction #138692

[serhiy-storchaka]

Alternatively, it could support alternative prefix chars. For example, ++no-foo if ++foo was specified. It is not easy, because prefix_chars is not available in the BooleanOptionalAction constructor.

See also #119377.

[serhiy-storchaka]

#138692 is an alternative PR that adds support for alternate prefix_chars (and for single-dash long options, since they are common when alternate prefix_chars are used). So ++no-foo will be generated for ++foo, and /no-foo for /foo.

There is still an issue for the case when different prefix chars are used (e.g. -+foo), but this is uncommon case, I never encountered such options in wild. There are no tests for such case, so this is not intentionally added feature. I think it should not be supported, but this is a different issue.