Argparse: inconsistent default handling between `nargs` values

Bug report

Bug description:

import argparse

parser = argparse.ArgumentParser(
    formatter_class=argparse.ArgumentDefaultsHelpFormatter
)

day_names = ["mo", "tu", "we", "th", "fr", "sa", "su"]
days = [1, 2, 3, 4, 5, 6, 7]

def name_day(value):
    return day_names.index(value) + 1

parser.add_argument(
    '--day',
    type=name_day,
    default="mo",
    nargs='?',  
    help="pick a day",
)
args = parser.parse_args()
print(args.day)

Actual results:

The above code block will print the value 1. However, if we change the arguments to

    default=["mo"],
    nargs='+',

The code will print the value ["mo"]. So it seems type conversion does not happen for default arguments when nargs specifies multiple arguments.

Expected results:

The supplied default values are converted, irrespective of the value of nargs.

CPython versions tested on:

3.13, 3.10

Operating systems tested on:

Linux

Linked PRs

• gh-132724

Start by reproducing the example with argparse and compare the default handling for nargs='?' and nargs='+'. Trace the argparse argument-processing entry point to see where type conversion differs, then verify that supplied defaults are converted consistently for both forms and preserve the expected parsed values.