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GeezerVison is showing github.com — gh-138525: raise error when BooleanOptionalAction used with prefix_chars without '-' by mattwang44 · Pull Request #138526 · python/cpython

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@mattwang44

@mattwang44 mattwang44 commented Sep 5, 2025 •
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(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

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@mattwang44
fix(argparse): raise error when BooleanOptionalAction used with prefi…
fb362be 
…x_chars without '-'
@mattwang44
mattwang44 requested a review from savannahostrowski as a code owner September 5, 2025 06:57
@bedevere-app bedevere-app Bot added the awaiting review label Sep 5, 2025
@bedevere-app bedevere-app Bot mentioned this pull request Sep 5, 2025
argparse.BooleanOptionalAction silently failed when used with parser whose prefix_chars has no '-' #138525
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@mattwang44
fix linter warning
0d45340
@corona10

corona10 commented Sep 5, 2025

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Would like to add NEWS.d to use blurb tool?
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@corona10 corona10 assigned savannahostrowski Sep 5, 2025
@mattwang44
chore: Add NEWS entry
2932858
@serhiy-storchaka

serhiy-storchaka commented Sep 5, 2025

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Isn't it too late? It raises an error when the option is used, not when it is defined.
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@mattwang44
add action method for parser compatibility check
c14013f
@mattwang44

mattwang44 commented Sep 7, 2025 •
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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.
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sobolevn
sobolevn reviewed Sep 8, 2025
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Comment thread Lib/argparse.py Resolved

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Comment thread Misc/NEWS.d/next/Library/2025-09-05-15-42-16.gh-issue-138525.ngZ0qT.rst Resolved

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Comment thread Lib/test/test_argparse.py Resolved

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mattwang44 and others added 4 commits September 8, 2025 21:45
@mattwang44 @sobolevn
fix(argparse): apply suggestions from code review
69d2c98 
Co-authored-by: sobolevn <[email protected]>
@mattwang44 @sobolevn
fix(argparse): apply suggestions from code review
e8c1c94 
Co-authored-by: sobolevn <[email protected]>
@mattwang44
fix(argparse): match message with assertRaisesRegex
9faaac7
@mattwang44
Merge branch 'main' into pythongh-138525
c6221f5
@savannahostrowski savannahostrowski mentioned this pull request Nov 22, 2025
gh-138525: Support single-dash long options and prefix_chars in BooleanOptionalAction #138692
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@serhiy-storchaka

serhiy-storchaka commented Nov 22, 2025

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Thank you for your PR, @mattwang44, but after adding support of alternate prefix chars in #138692 this check is no longer needed.
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@serhiy-storchaka serhiy-storchaka closed this Nov 22, 2025
@savannahostrowski

savannahostrowski commented Nov 22, 2025

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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!
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@mattwang44
mattwang44 deleted the gh-138525 branch November 23, 2025 03:46
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@sobolevn sobolevn sobolevn left review comments
@savannahostrowski savannahostrowski Awaiting requested review from savannahostrowski savannahostrowski is a code owner

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@mattwang44 @corona10 @serhiy-storchaka @savannahostrowski @sobolevn
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