Bug report

Bug description:

When using argparse to define a BooleanOptionAction that starts with "--no-", it is not possible to set the flag as True. This is counterintuitive, as one would expect that passing the argument should set the flag to True, regardless of the name of the argument.

The bug appears to originate from this line of code:

setattr(namespace, self.dest, not option_string.startswith('--no-'))

Because BooleanOptionAction always generates another argument that starts with "--no-", the code here attempts to check for the case where the user passed in that generated argument. It doesn't consider the possibility that the original argument could also start with "--no-".

This code here demonstrates the bug:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--no-foo', action=argparse.BooleanOptionalAction)
assert(parser.parse_args(['--no-no-foo']).no_foo == False) # Passes
assert(parser.parse_args(['--no-foo']).no_foo == True) # Fails

While I tested this on 3.10, the argparse snippet above is from the main branch, which suggests the bug is in the latest version.

CPython versions tested on:

3.10

Operating systems tested on:

Linux

Linked PRs

• gh-117941: Fix argparse issue-Changed call method of BooleanOptionalAction class in argparse.py #117941 #118103
• gh-117941: Reject option names starting with "--no-" in argparse.BooleanOptionalAction #125894