argparse: allow_abbrev=False is ignored for alternative prefix characters

[kyleam]

BPO	39546
Nosy	@rhettinger, @miss-islington, @shihai1991, @kyleam
PRs	bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars #18337 [3.8] bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337) #18543

^{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 2020-03-03.16:30:56.744>
created_at = <Date 2020-02-03.20:11:28.709>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = 'argparse: allow_abbrev=False is ignored for alternative prefix characters'
updated_at = <Date 2020-03-03.16:30:56.744>
user = 'https://github.com/kyleam'

bugs.python.org fields:

activity = <Date 2020-03-03.16:30:56.744>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-03.16:30:56.744>
closer = 'petr.viktorin'
components = ['Library (Lib)']
creation = <Date 2020-02-03.20:11:28.709>
creator = 'kyleam'
dependencies = []
files = []
hgrepos = []
issue_num = 39546
keywords = ['patch']
message_count = 4.0
messages = ['361326', '361345', '362183', '362193']
nosy_count = 5.0
nosy_names = ['rhettinger', 'paul.j3', 'miss-islington', 'shihai1991', 'kyleam']
pr_nums = ['18337', '18543']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39546'
versions = ['Python 3.8', 'Python 3.9']

[kyleam]

As of Python v3.8.0 (specifically commit b1e4d1b), specifying
allow_abbrev=False does not disable abbreviation for prefix
characters other than '-'.

--8<---------------cut here---------------start------------->8---

import argparse

parser = argparse.ArgumentParser(prefix_chars='+', allow_abbrev=False)
parser.add_argument('++long')
print(parser.parse_args(['++lo=val']))
--8<---------------cut here---------------end--------------->8

---

Observed output (with b1e4d1b and current master):

    Namespace(long='val')

Expected (and observed with b1e4d1b^ and 3.7.3):

usage: scratch.py [+h] [++long LONG]
scratch.py: error: unrecognized arguments: ++lo=val

I will follow up with a PR to propose a fix.

[shihai1991]

Good catch, it apprears in my python env(master base) too.

[miss-islington]

New changeset 8edfc47 by Kyle Meyer in branch 'master':
bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337)
8edfc47

[miss-islington]

New changeset e412cbb by Miss Islington (bot) in branch '3.8':
[3.8] bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337) (GH-18543)
e412cbb