Fix string_processing crash for one-item tuple string literals

[Harish-Nesrekar]

Description

Fix crash in string-processing preview for unassigned one-item tuples of string literals with a trailing comma.

• Problem: Black crashed when formatting code like:

"A very long string that exceeds line length and has a trailing comma",

because it tried to split the string but did not wrap it in parentheses, producing code that is not AST-equivalent.

• Cause: _prefer_paren_wrap_match() decides whether to wrap a literal in parentheses. Previously, syms.testlist (one-item tuples) was missing, so parentheses weren’t added.

• Fix: Add syms.testlist to _prefer_paren_wrap_match():

matching_nodes = [
    syms.listmaker,
    syms.dictsetmaker,
    syms.testlist_gexp,
    syms.testlist,  # added to handle one-item tuples
]

• This is a minimal, safe fix that preserves AST semantics.

• Added regression test:
  tests/data/preview/string_processing_trailing_comma.py

• Verified with:

black --preview --enable-unstable-feature=string_processing tests/data/preview/string_processing_trailing_comma.py

Checklist - did you ...

• Implement code changes under --preview style?
• Add / update tests?
• Add an entry in CHANGES.md if necessary?
• Add / update documentation if necessary?

[cobaltt7]

The test cases don't seem to be picked up by our tests runner. Please put them in tests/data/cases (not preview). The three test cases can be put in the same file.
Is this bug only reproducible with string_processing? If so, please add this comment to the beginning of the tests file so it's correctly run on the preview style: # flags: --unstable (The test runner will make sure it doesn't crash on preview regardless, but it's good to check any change in formatting is intentional)

Additionally, can you add a changelog entry to CHANGES.md, and please run Black on the codebase (python src/black .)

Thanks for working on this!

[github-actions]

diff-shades reports zero changes comparing this PR (88abdab) to main (c3cc5a9).

---

What is this? | Workflow run | diff-shades documentation

[Harish-Nesrekar]

Hi @cobaltt7

Thanks for the review!

I’ve moved the test cases to tests/data/cases/string_trailing_comma.py and combined all three related cases into a single file.

Added the # flags: --unstable comment at the top so it runs correctly under preview mode.

Added a changelog entry in CHANGES.md.

Ran Black on the codebase for consistent formatting.

All changes are now pushed to the string-processing branch.

Please let me know if there’s anything else needed!

Thanks again for the guidance.

[cobaltt7]

I'm inclined to believe that this PR is AI-generated. While using AI in itself isn't forbidden, we do expect you to look over what the AI did, that you've reviewed the code it writes, and that submitted PRs are up to our code standards and in line with the project expectations and requirements. Unfortunately, your contribution does not meet this bar.

Please review the docs on writing test cases; it's still not correct. https://black.readthedocs.io/en/stable/contributing/the_basics.html#testing
Please also correct the changelog entry.

[Harish-Nesrekar]

This is my first contribution to Black, and I understand now that while the issue and fix approach were valid, my tests and changelog entry don’t yet meet the project’s conventions. I’ll review the testing documentation more carefully, rework the tests to follow the expected structure, and correct the changelog entry accordingly. I appreciate you pointing this out and will update the PR once it’s aligned with Black’s standards.

…

________________________________ From: cobalt ***@***.***> Sent: Thursday, December 25, 2025 5:30 AM To: psf/black ***@***.***> Cc: Harish Nesrekar ***@***.***>; Author ***@***.***> Subject: Re: [psf/black] Fix string_processing crash for one-item tuple string literals (PR #4917) [https://avatars.githubusercontent.com/u/61329810?s=20&v=4]cobaltt7 left a comment (psf/black#4917)<#4917 (comment)> I'm inclined to believe that this PR is AI-generated. While using AI in itself isn't forbidden, we do expect you to look over what the AI did, that you've reviewed the code it writes, and that submitted PRs are up to our code standards and in line with the project expectations and requirements. Unfortunately, your contribution does not meet this bar. Please review the docs on writing test cases; it's still not correct. https://black.readthedocs.io/en/stable/contributing/the_basics.html#testing Please also correct the changelog entry. — Reply to this email directly, view it on GitHub<#4917 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/BTMIVW6YKGEROEDKH4CDJQL4DMSLHAVCNFSM6AAAAACP6UWSDKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTMOJQGYZDMOBRHE>. You are receiving this because you authored the thread.Message ID: ***@***.***>