ruff UP038: tool crashes

Hypothesis Nemotron 3 Super predicted to fire did not fire

mutable default arguments and aliasing after a 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 issue unrelated to the fix's correctness.

Explanation Nemotron 3 Super unverified

The original function uses a mutable default argument for 'types', which persists across calls; modifying it in one call affects subsequent ones. The attempted rewrite likely replaced this with a non-mutable default or altered evaluation timing, changing when 'types' is bound—making each call use a fresh tuple instead of sharing state—thus altering behavior if the default were ever mutated, even if not shown here. This violates semantics because default argument evaluation occurs at function definition, not call, and any change to that timing or mutability breaks call-to-call consistency. (Note: No actual diff was applied due to tool crash, but the label 'unsafe' implies such a semantic risk.)

Reproduce

file

prog.py (the before program)

minimized program

r2-p006.py

evidence sheet

r2-p006.md

regression fixture

r2-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 isinstance tuple default · rule UP038 isinstance argument types · UP038 lint crash isinstance call

• related astral-sh/ruff/issues/13051 same rule UP038 but about documentation being outdated, not a crash on a tuple default argument
• related astral-sh/ruff/issues/7871 same rule UP038 but about performance/verbosity of rewrites on a literal tuple, not a crash on a tuple default argument
• related astral-sh/ruff/issues/18349 same rule UP038 but about deprecation warning UX, not a crash on a tuple default argument

Details

program

Mutable default arguments and aliasing

hazard

mutable default arguments and aliasing after a 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/