ruff UP038: tool crashes

Hypothesis Nemotron 3 Super predicted to fire did not fire

walrus, comprehensions scoping, late binding in closures and lambdas

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 during linting, which is an environment or tooling issue unrelated to the fix's correctness on the source code.

Explanation Nemotron 3 Super unverified

The original loop creates two lambda functions that each capture the current value of x from the enclosing scope when called; since x is unchanged between iterations, both lambdas behave identically. However, if the rewrite attempted to hoist or inline the lambda definition outside the loop—such as defining it once and reusing it—the behavior could differ if x were mutated between calls, because all instances would then share the same closure over a potentially changing x. In this specific case, x is not mutated, so the behavior remains the same, but the rewrite is still unsafe because it assumes x is invariant, which cannot be guaranteed statically. Thus, the transformation may alter semantics in contexts where x varies, making it non‑preserving in general.

Reproduce

file

prog.py (the before program)

minimized program

r2-p008.py

evidence sheet

r2-p008.md

regression fixture

r2-p008.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 lambda isinstance · rule UP038 callable expression · UP038 type check pattern

• related astral-sh/ruff/issues/13051 Existing report concerns outdated documentation for UP038, not a tool crash when the rule is removed.
• related astral-sh/ruff/issues/7871 Existing report is about UP038 rewriting code to be slower, not about a crash due to the rule being removed.
• related astral-sh/ruff/issues/18349 Existing report is about a warning for a deprecated rule, not a crash when selecting a removed rule.

Details

program

Walrus, comprehensions scoping, late binding

hazard

walrus, comprehensions scoping, late binding in closures and lambdas

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/