[Python] Exclusive group of arguments with optional values

The following code

from argparse import ArgumentParser

parser = ArgumentParser()
parser.add_argument('--foo', nargs='?')
parsed_args = parser.parse_args()


creates an optional argument with an optional value. It means that --foo 123 stores 123, --foo stores None, leaving the argument altogether stores None too.

I now create a mutually exclusive group of two such arguments:

from argparse import ArgumentParser

parser = ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('--foo', nargs='?')
group.add_argument('--bar', nargs='?')
parsed_args = parser.parse_args()


As expected, --foo 123 --bar 456 results in error argument --bar: not allowed with argument --foo. But running --foo --bar finishes without errors. Tested with Python 3.13. Is it a bug in Python or just my misunderstanding of the concept?

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