gh-138525: raise error when BooleanOptionalAction used with prefix_chars without '-'

[mattwang44]

(this contribution is made during PyCon TW sprint)

fixes #138525. Here's the output of my local build with this patch.

>>> from argparse import BooleanOptionalAction, ArgumentParser
>>> p = ArgumentParser(prefix_chars='+')
>>> p.add_argument("++foo", action=BooleanOptionalAction)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    p.add_argument("++foo", action=BooleanOptionalAction)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/whw/Desktop/cpython/Lib/argparse.py", line 1554, in add_argument
    action._check_compatibility(self)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/whw/Desktop/cpython/Lib/argparse.py", line 969, in _check_compatibility
    raise ValueError(f"BooleanOptionalAction requires '-' in prefix_chars.")
ValueError: BooleanOptionalAction requires '-' in prefix_chars.

I believe the description of BooleanOptionalAction on the argparse doc has already implied that it should be used with prefix_chars containing '-', so I didn't make any change to the docs.

• Issue: argparse.BooleanOptionalAction silently failed when used with parser whose prefix_chars has no '-' #138525

[corona10]

Would like to add NEWS.d to use blurb tool?

[serhiy-storchaka]

Isn't it too late? It raises an error when the option is used, not when it is defined.

[mattwang44]

Isn't it too late? It raises an error when the option is used, not when it is defined.

I find it difficult to achieve this without changing the interface. I've added a generic _check_compatibility() validation hook in the Action base class that gets called after action creation in add_argument(), allowing BooleanOptionalAction to validate itself against the parser's prefix_chars without leaking action-specific knowledge into _ActionsContainer. PTAL.

[serhiy-storchaka]

Thank you for your PR, @mattwang44, but after adding support of alternate prefix chars in #138692 this check is no longer needed.

[savannahostrowski]

Hey @mattwang44! Thanks for opening the original issue and opening a PR!

If you're interested in continuing to contribute to argparse, we have a project board tracking open argparse issues that could use attention. There are issues ranging from small fixes/good first issues to larger enhancements, and I'm happy to help guide you through any of them if you'd like to pick something up.

Thanks again for your contribution, and I hope to see more PRs from you in the future!