ruff UP038: tool crashes

Hypothesis Nemotron 3 Super predicted to fire did not fire

Comments attached to the tuple or its elements (e.g., isinstance(x, (int, # comment float))) may be lost during the rewrite.

Program

Output

(no stdout)

(not run)

Judge Nemotron 3 Ultra advisory program-bug · low

The linter crashed because rule UP038 was removed from the codebase and cannot be selected, not because the rewrite changed program semantics. The oracle observed a tool crash due to the rule's removal, which is an environment/configuration issue unrelated to the fix's correctness.

Explanation Nemotron 3 Super unverified

The original expression uses parentheses to group arguments across lines; removing them changes parsing so that isinstance receives only one argument after x, causing a TypeError due to missing second argument, altering runtime behavior from valid type check to exception. This violates semantic equivalence despite identical source text appearance.

Reproduce

file

prog.py (the before program)

minimized program

r2-p003.py

evidence sheet

r2-p003.md

regression fixture

r2-p003.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 isinstance tuple formatting · rule UP038 multiline isinstance parentheses · linter crash UP038 removed rule selection

no duplicate or related report 0 hits triaged

Details

program

Comments lost during rewrite

hazard

Comments attached to the tuple or its elements (e.g., isinstance(x, (int, # comment float))) may be lost during the rewrite.

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/