argparse.BooleanOptionalAction accept and silently discard its the const argument

[remilapeyre]

BPO	40862
Nosy	@rhettinger, @miss-islington, @remilapeyre
PRs	bpo-40862: Raise TypeError when const is given to argparse.BooleanOptionalAction #20623 [3.9] bpo-40862: Raise TypeError when const is given to argparse.BooleanOptionalAction (GH-20623) #20664

^{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 2020-06-06.02:31:45.829>
created_at = <Date 2020-06-04.11:59:48.464>
labels = ['library', '3.9', '3.10']
title = 'argparse.BooleanOptionalAction accept and silently discard its the const argument'
updated_at = <Date 2020-06-07.16:22:59.941>
user = 'https://github.com/remilapeyre'

bugs.python.org fields:

activity = <Date 2020-06-07.16:22:59.941>
actor = 'paul.j3'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-06.02:31:45.829>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2020-06-04.11:59:48.464>
creator = 'remi.lapeyre'
dependencies = []
files = []
hgrepos = []
issue_num = 40862
keywords = ['patch']
message_count = 6.0
messages = ['370703', '370772', '370776', '370777', '370792', '370811']
nosy_count = 4.0
nosy_names = ['rhettinger', 'paul.j3', 'miss-islington', 'remi.lapeyre']
pr_nums = ['20623', '20664']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue40862'
versions = ['Python 3.9', 'Python 3.10']

[remilapeyre]

The action is used to store None, True or False when an argument like --foo or --no-foo is given to the cli so it has no used for this action, but it is accepted without warning:

Python 3.10.0a0 (heads/bpo-wip:6e23a9c82b, Jun  4 2020, 13:41:35) 
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', const='this_is_not_used', action=argparse.BooleanOptionalAction)
BooleanOptionalAction(option_strings=['--foo', '--no-foo'], dest='foo', nargs=0, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>> args = parser.parse_args()
>>> args
Namespace(foo=None)

We could either always refuse this argument, or accepted and raise ValueError if it is different than None. The attached PR does the first.

[rhettinger]

ISTM this should be addressed through documentation rather than by breaking currently deployed code that may have specified a default that already matches what the action does.

[remilapeyre]

argparse.BooleanOptionalAction was introduced in Python3.9 so there is no code there should be no code that already rely on it.

[rhettinger]

Right. Go ahead :-)

[rhettinger]

New changeset b084d1b by Rémi Lapeyre in branch 'master':
bpo-40862: Raise TypeError when const is given to argparse.BooleanOptionalAction (GH-20623)
b084d1b

[rhettinger]

New changeset d5e7348 by Miss Islington (bot) in branch '3.9':
bpo-40862: Raise TypeError when const is given to argparse.BooleanOptionalAction (GH-20623) (GH-20664)
d5e7348