argparse.BooleanOptionalAction should not add the default value to the help string by default

[anntzer]

BPO	38956
Nosy	@rhettinger, @ambv, @mhils, @miss-islington, @shihai1991, @michiboo
PRs	bpo-38956: remove default option from help string in argparse #17447 bpo-38956: don't print BooleanOptionalAction's default twice #27672 [3.10] bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672) #27787 [3.9] bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672) #27788

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-17.10:03:35.321>
created_at = <Date 2019-12-02.23:03:24.687>
labels = ['library', '3.9']
title = 'argparse.BooleanOptionalAction should not add the default value to the help string by default'
updated_at = <Date 2021-08-17.19:16:12.710>
user = 'https://github.com/anntzer'

bugs.python.org fields:

activity = <Date 2021-08-17.19:16:12.710>
actor = 'Antony.Lee'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-17.10:03:35.321>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2019-12-02.23:03:24.687>
creator = 'Antony.Lee'
dependencies = []
files = []
hgrepos = []
issue_num = 38956
keywords = ['patch']
message_count = 7.0
messages = ['357733', '357743', '357747', '399687', '399720', '399724', '399729']
nosy_count = 7.0
nosy_names = ['rhettinger', 'lukasz.langa', 'paul.j3', 'mhils', 'miss-islington', 'shihai1991', 'Micky Yun Chan']
pr_nums = ['17447', '27672', '27787', '27788']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue38956'
versions = ['Python 3.9']

[anntzer]

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.

[michiboo]

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

[michiboo]

Hi I created a new pull request on Github

[ambv]

New changeset 1512bc2 by Maximilian Hils in branch 'main':
bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672)
1512bc2

[ambv]

New changeset 27fd313 by Łukasz Langa in branch '3.9':
[3.9] bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672) (GH-27788)
27fd313

[miss-islington]

New changeset 6f6648e by Miss Islington (bot) in branch '3.10':
bpo-38956: don't print BooleanOptionalAction's default twice (GH-27672)
6f6648e

[ambv]

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

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