For argparse add_argument with action='store_const', const should default to None.

[nanjekyejoannah]

BPO	37880
Nosy	@vsajip, @bitdancer, @tyomitch, @nanjekyejoannah, @jacobtylerwalls, @jdevries3133
PRs	bpo-37880: for argparse add_argument with action='store_const', const defaults to None. #26707

^{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 = 'https://github.com/vsajip'
closed_at = <Date 2021-08-01.06:20:10.974>
created_at = <Date 2019-08-18.00:41:15.421>
labels = ['type-bug', 'library', '3.11']
title = "For argparse add_argument with action='store_const', const should default to None."
updated_at = <Date 2021-08-01.06:20:10.974>
user = 'https://github.com/nanjekyejoannah'

bugs.python.org fields:

activity = <Date 2021-08-01.06:20:10.974>
actor = 'vinay.sajip'
assignee = 'vinay.sajip'
closed = True
closed_date = <Date 2021-08-01.06:20:10.974>
closer = 'vinay.sajip'
components = ['Library (Lib)']
creation = <Date 2019-08-18.00:41:15.421>
creator = 'nanjekyejoannah'
dependencies = []
files = []
hgrepos = []
issue_num = 37880
keywords = ['patch']
message_count = 5.0
messages = ['349911', '395699', '395700', '395701', '398646']
nosy_count = 6.0
nosy_names = ['vinay.sajip', 'r.david.murray', 'A. Skrobov', 'nanjekyejoannah', 'jacobtylerwalls', 'jack__d']
pr_nums = ['26707']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37880'
versions = ['Python 3.11']

[nanjekyejoannah]

Currently, when parser.add_argument() is given argument with action='store_const' and no const argument , it throws an exception :

    >>> from argparse import ArgumentParser
    >>> parser = ArgumentParser()
    >>> parser.add_argument("--foo", help="foo", action='store_const')
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/home/captain/projects/cpython/Lib/argparse.py", line 1350, in add_argument
    action = action_class(**kwargs)
    TypeError: __init__() missing 1 required positional argument: 'const'
    >>>

Specifying the const argument stops this exception:

>>> parser.add_argument("--foo", help="foo", action='store_const', const=None)
_StoreConstAction(option_strings=['--foo'], dest='foo', nargs=0, const=None, default=None, type=None, choices=None, help='foo', metavar=None)

Originally the docs, said when action was set to 'store_const' const defaulted to None which was not matching with the implementation at the time.

After this commit : https://github.com/python/cpython/commi/b4912b8ed367e540ee060fe912f841cc764fd293, The docs were updated to match the implementation to fix Bpo issues :

https://bugs.python.org/issue25299

https://bugs.python.org/issue24754 and

https://bugs.python.org/issue25314

I suggest that we make const default to None If action='store_const' as was intended originally before edits to the docs. If no one objects, I can open a PR for this.

[jacobtylerwalls]

Sounds reasonable to me.

[jdevries3133]

Hi Joannah, I'm a new contributor and happy to take a crack at this if you haven't already started on a fix / would like me to do that. Thanks!

[nanjekyejoannah]

@jack__d, please feel free to work on a PR.

Also, do not hesitate to ask any questions along the way.

Thanks for contributing to CPython.

[vsajip]

New changeset 0ad1732 by Jack DeVries in branch 'main':
bpo-37880: for argparse add_argument with action='store_const', const now defaults to None. (GH-26707)
0ad1732