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Document comment policy around fix safety (#14300)

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## Summary

Closes https://github.com/astral-sh/ruff/issues/9790.
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Charlie Marsh 2024-11-13 08:03:58 -05:00 • committed by GitHub
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10
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@ -161,7 +161,11 @@ whether a rule supports fixing, see [_Rules_](rules.md).
### Fix safety
Ruff labels fixes as "safe" and "unsafe". The meaning and intent of your code will be retained when
applying safe fixes, but the meaning could be changed when applying unsafe fixes.
applying safe fixes, but the meaning could change when applying unsafe fixes.
Specifically, an unsafe fix could lead to a change in runtime behavior, the removal of comments, or both,
while safe fixes are intended to preserve runtime behavior and will only remove comments when deleting
entire statements or expressions (e.g., removing unused imports).
For example, [`unnecessary-iterable-allocation-for-first-element`](rules/unnecessary-iterable-allocation-for-first-element.md)
(`RUF015`) is a rule which checks for potentially unperformant use of `list(...)[0]`. The fix
@ -177,7 +181,7 @@ $ python -m timeit "head = next(iter(range(99999999)))"
5000000 loops, best of 5: 70.8 nsec per loop
```
However, when the collection is empty, this changes the raised exception from an `IndexError` to `StopIteration`:
However, when the collection is empty, this raised exception changes from an `IndexError` to `StopIteration`:
```console
$ python -c 'list(range(0))[0]'
@ -193,7 +197,7 @@ Traceback (most recent call last):
StopIteration
```
Since this could break error handling, this fix is categorized as unsafe.
Since the change in exception type could break error handling upstream, this fix is categorized as unsafe.
Ruff only enables safe fixes by default. Unsafe fixes can be enabled by settings [`unsafe-fixes`](settings.md#unsafe-fixes) in your configuration file or passing the `--unsafe-fixes` flag to `ruff check`: