astral-sh/ruff

[`Pylint`] `PLE2510` fix causes syntax error inside pre-3.12 f-strings

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#18,816 opened on Jun 20, 2025

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bugfixeshelp wanted

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Description

Summary

If a backspace is inside an f-string replacement while a pre-3.12 version is targeted, the fix for invalid-character-backspace (PLE2510) will introduce a syntax error. This happens both inside a string inside the replacement, and inside the format specs. playground

PS D:\python_projects> Get-Content issue.py

f"{'␈'}"

^ note: I'm cheating here, PowerShell does not actually display the backspace character as ␈, it actually gets displayed as f"{'}"

PS D:\python_projects> uvx ruff check issue.py --isolated --select PLE --target-version py39

issue.py:1:5: PLE2510 [*] Invalid unescaped character backspace, use "\b" instead
  |
1 | f"{'␈'}"
  |     ^ PLE2510
  |
  = help: Replace with escape sequence

Found 1 error.
[*] 1 fixable with the `--fix` option.

PS D:\python_projects> uvx ruff check issue.py --isolated --select PLE --target-version py39 --fix

issue.py:1:5: SyntaxError: Cannot use an escape sequence (backslash) in f-strings on Python 3.9 (syntax was added in Python 3.12)
  |
1 | f"{'\b'}"
  |     ^
  |

Found 2 errors (1 fixed, 1 remaining).

This also applies to PLE2512, PLE2513, and PLE2515 playground, as well as PLE2514, but the playground does not support null chars.

Version

ruff 0.12.0 (87f0feb 2025-06-17) + playground

Contributor guide

Research direction
The bug is in the fix for rule PLE2510 (invalid character backspace) in Ruff. The fix incorrectly inserts a backslash escape inside f string replacements when targeting Python <3.12. Look at the rule implementation in crates/ruff linter/src/rules/pylint/rules/invalid character.rs. The fix logic needs to detect whether the replacement is inside an f string and if the target version does not support backslash escapes in f strings. Also check similar rules PLE2512, PLE2513, PLE2514, PLE2515. Consider adding a check to avoid applying the fix in such contexts, or generate an alternative fix like using a Unicode escape. Review existing test cases in tests/ to cover this scenario.
Tech stack
pythonrust
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Basic understanding of Python f stringsFamiliarity with Ruff's codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60