ruff UP038: tool crashes

Hypothesis Nemotron 3 Super predicted not to fire did not fire

Rewriting isinstance(x, (int,)) to int | is invalid syntax; the | operator requires two operands, so the fix must not apply to 1-tuples.

Program

Output

(no stdout)

(not run)

Judge Nemotron 3 Ultra advisory program-bug · low

The rule UP038 has been removed from ruff entirely, so the linter crashes when trying to apply it. The program itself is trivial (x = 5) and contains no isinstance or issubclass calls, so the fix cannot meaningfully apply. The crash is a tooling/environment issue, not a semantic change from a rewrite.

Explanation Nemotron 3 Super unverified

The rewrite does nothing—it leaves x=5 unchanged—but claims to apply a fix that no longer exists (rule UP038 was removed). Since the tool attempts to enforce a non-existent rule, it crashes instead of producing any output. Thus, the behavior differs: the original program runs fine, while the "fixed" version triggers a tool crash due to missing rule selection, altering execution outcome despite identical source code. No actual code change occurred; the difference arises from the tool’s failure to handle the invalid rule reference. This makes the fix unsafe because it introduces instability without modifying semantics.

Reproduce

file

prog.py (the before program)

minimized program

r1-p006.py

evidence sheet

r1-p006.md

regression fixture

r1-p006.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 UP038 --fix prog.py
python3 prog.py

Upstream search GitHub issues Tavily Nemotron 3 Super + Ultra

UP038 fix · ruff UP038 assignment expression · rule UP038 variable initialization pattern · linter crash due to missing UP038

• related astral-sh/ruff/issues/13051 Existing report is about outdated documentation for UP038 fix safety, not a crash when selecting a removed rule.
• related astral-sh/ruff/issues/7871 Existing report is about UP038 rewriting code to be slower, not a crash due to rule removal.
• duplicate astral-sh/ruff/issues/18349 Existing report describes the same defect: selecting deprecated/removed rule UP038 causes an error/crash.

Details

program

Singleton Tuple Rewrite Produces Invalid Syntax

hazard

Rewriting isinstance(x, (int,)) to int | is invalid syntax; the | operator requires two operands, so the fix must not apply to 1-tuples.

oracle

lint crashed: ruff failed Cause: Rule `UP038` was removed and cannot be selected.

run

2026-10-17T21-20-52-142Z-httm-ruff-UP038

# non-pep604-isinstance (UP038)

Derived from the **pyupgrade** linter.

Fix is always available.

## Removed
This rule was removed as using [PEP 604] syntax in `isinstance` and `issubclass` calls
isn't recommended practice, and it incorrectly suggests that other typing syntaxes like [PEP 695]
would be supported by `isinstance` and `issubclass`. Using the [PEP 604] syntax
is also slightly slower.

## What it does
Checks for uses of `isinstance` and `issubclass` that take a tuple
of types for comparison.

## Why is this bad?
Since Python 3.10, `isinstance` and `issubclass` can be passed a
`|`-separated union of types, which is consistent
with the union operator introduced in [PEP 604].

Note that this results in slower code. Ignore this rule if the
performance of an `isinstance` or `issubclass` check is a
concern, e.g., in a hot loop.

## Example
```python
isinstance(x, (int, float))
```

Use instead:
```python
isinstance(x, int | float)
```

## Options
- `target-version`

## References
- [Python documentation: `isinstance`](https://docs.python.org/3/library/functions.html#isinstance)
- [Python documentation: `issubclass`](https://docs.python.org/3/library/functions.html#issubclass)

[PEP 604]: https://peps.python.org/pep-0604/
[PEP 695]: https://peps.python.org/pep-0695/