Wrong parse long argument

[chr0139]

BPO	37227
Nosy	@ericvsmith, @tirkarthi

^{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 2019-06-11.09:26:10.163>
created_at = <Date 2019-06-11.08:53:06.919>
labels = ['invalid', 'type-bug', 'library']
title = 'Wrong parse long argument'
updated_at = <Date 2019-06-13.05:11:18.962>
user = 'https://bugs.python.org/chr0139'

bugs.python.org fields:

activity = <Date 2019-06-13.05:11:18.962>
actor = 'paul.j3'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-11.09:26:10.163>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2019-06-11.08:53:06.919>
creator = 'chr0139'
dependencies = []
files = []
hgrepos = []
issue_num = 37227
keywords = []
message_count = 5.0
messages = ['345203', '345204', '345206', '345207', '345208']
nosy_count = 4.0
nosy_names = ['eric.smith', 'paul.j3', 'xtreak', 'chr0139']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37227'
versions = ['Python 2.7']

[chr0139]

I have this script

import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-l', '--list', action='store_true', help="show help")
group.add_argument('-u', '--upgrade', action='store_true', help="show help")
parser.parse_args(['--li'])

and this is the unexpected result:

Namespace(list=True, upgrade=False)

[ericvsmith]

And what is your expected result, and why?

[chr0139]

I think --li is not the same argument as --list

So I expect that

parser.parse_args(['-l']) or parser.parse_args(['--list'])

should be only two options which should leads to the result

Namespace(list=True, upgrade=False)

[tirkarthi]

argparse allows abbreviations by default so --li matches --list as a prefix match. This is documented at https://docs.python.org/2.7/library/argparse.html#argument-abbreviations-prefix-matching

From python 3.5 allow_abbrev=False can be passed to ArgumentParser to disable this. See also bpo-10981 and bpo-14910 that implemented this. https://docs.python.org/3/library/argparse.html#argument-abbreviations-prefix-matching.

[ericvsmith]

Although note that with allow_abbrev=False, -l will conflict with --long, which I'm not sure is a great design.

I'm closing this as not a bug, since it works as documented and there's a workaround (albeit not in 2.7, which is closed to new features).