argparse eats initial '=' from short option argument

Bug report

Bug description:

When configured with a short option that requires an argument, and an input commandline has the option argument at the same argv item, argparse deletes initial '=' from it. This behavior is inconsistent with other implementations of the same parsing (at least: getopt, getopt_long from glibc or FreeBSD libc; getopt and optparse from Python standard library).

Actual result: initial '=' is deleted under the described conditions.

Expected result: initial '=' is kept in the collected option value.

Example code:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-Q', type=str)
args = parser.parse_args()
print(getattr(args, 'Q', None))

Running it:

$ ./t_eq_argparse.py -Q22
22
$ ./t_eq_argparse.py -Q=22
22

With a separate argument, truncating has no place:

$ ./t_eq_argparse.py -Q =22
=22

Compare to getopt module:

$ ./t_eq_getopt.py -Q22
22
$ ./t_eq_getopt.py -Q=22
=22

The same result with libc getopt (Ubuntu 22.04) and optparse module. Test code for getopt and optparse is obvious.

The issue is present with today's Lib/argparse.py from github trunk.

CPython versions tested on:

3.10, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-124332