Bug report

When store_true, store_false, or store_const is set as an action of positional argument, the behaviour is counterintuitive. Running this small program:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('pos', action='store_true')
print(parser.parse_args([]))

outputs Namespace(pos=True). Changing last line to print(parser.parse_args(['foo'])) gives an error:

usage: prog.py [-h]
prog.py: error: unrecognized arguments: foo

Your environment

• CPython versions tested on: 3.10.2
• Operating system and architecture: Linux x86-64

Expected behaviour

I believe there is no good reason to allow setting those actions to positional arguments. Trying to do so should result in raising an exception (as is the case when trying to set required).

Linked PRs

• gh-97848: argparse: Disallow misbehaving actions for positional arguments #98367