Description

mattwang44

opened on Sep 5, 2025

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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

Activity

1. mattwang44

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Sep 5, 2025
2. StanFromIreland

   added this to Argparse issueson Sep 5, 2025
3. StanFromIreland

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Sep 5, 2025
4. serhiy-storchaka commented on Sep 5, 2025

   

   serhiy-storchaka

   on Sep 5, 2025

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   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.

   Reacted by W. H. Wang
5. serhiy-storchaka

   added

   type-featureA feature request or enhancementA feature request or enhancement

   and removed

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Sep 5, 2025
6. serhiy-storchaka

   added a commit that references this issue on Sep 9, 2025

   pythongh-138525: Support single-dash long options and prefix_chars in…

   fe05304
7. serhiy-storchaka commented on Sep 9, 2025

   

   serhiy-storchaka

   on Sep 9, 2025

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   #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.
8. serhiy-storchaka

   added 2 commits that reference this issue on Sep 9, 2025

   pythongh-138525: Support single-dash long options and prefix_chars in…

   bd8247f

   pythongh-138525: Support single-dash long options and prefix_chars in…

   4fa348b
9. mattwang44

   added a commit that references this issue on Nov 20, 2025

   Merge branch 'main' into pythongh-138525

   c6221f5
10. serhiy-storchaka

    added a commit that references this issue on Nov 22, 2025

    gh-138525: Support single-dash long options and prefix_chars in Boole…

    425fd85
11. serhiy-storchaka

    closed this as completedon Nov 22, 2025
12. github-project-automation

    moved this to Doc issues in Argparse issueson Nov 22, 2025
13. serhiy-storchaka

    moved this from Doc issues to Features in Argparse issueson Nov 22, 2025
14. StanFromIreland

    added a commit that references this issue on Dec 6, 2025

    pythongh-138525: Support single-dash long options and prefix_chars in…

    076e529
15. ashm-dev

    added a commit that references this issue on Dec 8, 2025

    pythongh-138525: Support single-dash long options and prefix_chars in…

    4edec83

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