ruff W605: fix changes program behavior

Hypothesis Nemotron 3 Super predicted to fire fired 1×

If the string contains mixed valid and invalid escapes (e.g., "\n\_"), fixing only the invalid part may create an inconsistent interpretation where the valid escape remains unchanged but the invalid o

Program

Output

(no stdout)
/w/prog.py:1: SyntaxWarning: invalid escape sequence '\_'

(no stdout)

Judge Nemotron 3 Ultra advisory intended-change · low

The rule rewrites the invalid escape sequence '\_' to the valid escaped backslash '\\_', which changes the string value from a newline followed by '_' to a newline followed by '\_'. This is exactly the semantic change the rule documents: eliminating the deprecated SyntaxWarning by making the escape sequence valid, even though the resulting string content differs.

Explanation Nemotron 3 Super unverified

The original string literal contains an invalid escape sequence `\_` which Python treats as two characters backslash and underscore while emitting a warning; replacing it with `\\_` yields a single escaped backslash followed by underscore changing the runtime value from `'\\_'` to `'\\\\_'`. This alters the string’s content so the program’s behavior differs.

Reproduce

file

prog.py (the before program)

minimized program

r1-p004.py

evidence sheet

r1-p004.md

regression fixture

r1-p004.py the minimized program with a header stating the observed change, for the tool's test suite

python3 prog.py
ruff check --preview --isolated --select W605 --fix prog.py
python3 prog.py

Upstream search GitHub issues Tavily Nemotron 3 Super + Ultra

W605 fix · W605 invalid escape backslash underscore · ruff W605 escape sequence warning fix · invalid escape \_ to \\_ correction

• related astral-sh/ruff/issues/11491 Existing report is about W605 fix in f-strings; new finding is a plain string literal.
• related astral-sh/ruff/issues/8616 Mentions W605 fixer creating unnecessary escapes; same rule but different context.
• duplicate astral-sh/ruff/issues/11461 Existing report 'W605 fix is unsafe' describes the same defect: the autofix changes '\_' to '\\_' in a plain string, altering runtime behavior.
• related astral-sh/ruff/issues/10434 W605 panic in f-strings; same rule but different trigger.
• related astral-sh/ruff/issues/6987 Autofix error involving W605; same rule but different construct.
• related astral-sh/ruff/issues/5061 Feature request to use raw strings for W605; same rule but not the same defect.
• related astral-sh/ruff/issues/19543 About W605 on byte strings; same rule but different string type.
• related astral-sh/ruff/issues/5352 PR for raw-string fix for W605; same rule but not the same defect.

Details

program

Mixed valid and invalid escapes

hazard

If the string contains mixed valid and invalid escapes (e.g., "\n\_"), fixing only the invalid part may create an inconsistent interpretation where the valid escape remains unchanged but the invalid o

oracle

stderr before: /w/prog.py:1: SyntaxWarning: invalid escape sequence '\_' stderr after: (none)

run

2026-10-17T20-50-05-853Z-2tuo-ruff-W605

--- orig.py	2026-10-17 16:50:20.778529321 -0400
+++ fixed.py	2026-10-17 16:50:20.992533481 -0400
@@ -1,4 +1,4 @@
-s = '\n\_'
+s = '\n\\_'
 print(repr(s))
 print(len(s))
 for c in s:

# invalid-escape-sequence (W605)

Derived from the **pycodestyle** linter.

Fix is always available.

## What it does
Checks for invalid escape sequences.

## Why is this bad?
Invalid escape sequences are deprecated in Python 3.6.

## Example
```python
regex = "\.png$"
```

Use instead:
```python
regex = r"\.png$"
```

Or, if the string already contains a valid escape sequence:
```python
value = "new line\nand invalid escape \_ here"
```

Use instead:
```python
value = "new line\nand invalid escape \\_ here"
```

## References
- [Python documentation: String and Bytes literals](https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals)