https://bugs.python.org/issue8538 recently added to Py3.9 a much welcome addition to argparse, namely the capability to generate --foo/--no-foo flag pairs.  A small issue with the implementation is that it *always* appends the default value to the help string (if any):

    if help is not None and default is not None:
        help += f" (default: {default})"

This is inconsistent with other action classes, and results in the defaults being printed twice if using ArgumentsDefaultHelpFormatter (which is the documented way to include the defaults in the help text):

    from argparse import *
    parser = ArgumentParser(formatter_class=ArgumentDefaultsHelpFormatter)
    parser.add_argument("--foo", action=BooleanOptionalAction, help="Whether to foo it", default=True)
    parser.add_argument("--quux", help="Set the quux", default=42)
    print(parser.parse_args())

yields

    usage: foo.py [-h] [--foo | --no-foo] [--quux QUUX]

    optional arguments:
      -h, --help       show this help message and exit
      --foo, --no-foo  Whether to foo it (default: True) (default: True)  # <--- HERE
      --quux QUUX      Set the quux (default: 42)

I think the fix is just a matter of not adding the default value to the help string.

Hi, if it's possible can I take this issue?

Hi I created a new pull request on Github

New changeset 1512bc21d60f098a9e9f37b44a2f6a9b49a3fd4f by Maximilian Hils in branch 'main':
bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672)
https://github.com/python/cpython/commit/1512bc21d60f098a9e9f37b44a2f6a9b49a3fd4f

New changeset 27fd31311097ab7326f084b5e0e5f388f8dc6b13 by Łukasz Langa in branch '3.9':
[3.9] bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672) (GH-27788)
https://github.com/python/cpython/commit/27fd31311097ab7326f084b5e0e5f388f8dc6b13

New changeset 6f6648e436d02bce0e49ba82f4377c0d2f586f0f by Miss Islington (bot) in branch '3.10':
bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672)
https://github.com/python/cpython/commit/6f6648e436d02bce0e49ba82f4377c0d2f586f0f

Thanks, Max for pushing this across the finish line! ✨ 🍰 ✨  

Closing this one. Further improvement of BooleanOptionalAction will be discussed in BPO-44587.