ruff UP038: tool crashes

Hypothesis Nemotron 3 Super predicted to fire did not fire

generators vs lists, iterators consumed once, lazy vs eager evaluation

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 of any rewrite applied to the program. The program itself is trivial and unrelated to the rule's pattern; the failure is entirely in the tooling environment.

Explanation Nemotron 3 Super unverified

The original function returns a generator object when called; calling it yields values one at a time via iteration. If the fix incorrectly rewrites this to return the literal value 1 directly (e.g., replacing the generator with a simple return), then invoking the function would produce an integer instead of a generator. Subsequent code expecting to iterate over the result—such as a for‑loop or next() call—would fail with a TypeError because integers are not iterable. Thus the observable behavior changes from producing a sequence of values to either returning a single value or raising an error, breaking semantics.

Reproduce

file

prog.py (the before program)

minimized program

r2-p005.py

evidence sheet

r2-p005.md

regression fixture

r2-p005.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 generator function · rule UP038 yield expression · unused import UP038

• related astral-sh/ruff/issues/13051 Existing report is about outdated documentation for UP038 fix safety, not a tool crash on a generator function.
• related astral-sh/ruff/issues/7871 Existing report is about UP038 rewriting isinstance checks to slower code, not a tool crash on a generator function.
• related astral-sh/ruff/issues/18349 Existing report is about a warning for a deprecated UP038 rule, not a tool crash on a generator function.

Details

program

Generators vs lists, iterators consumed once

hazard

generators vs lists, iterators consumed once, lazy vs eager evaluation

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/