C415 doc and/or implementation is broken

[JelleZijlstra]

The documentation for C415 (https://docs.astral.sh/ruff/rules/unnecessary-subscript-reversal/) states that it makes the following changes:

Examples:
reversed(iterable[::-1])
set(iterable[::-1])
sorted(iterable)[::-1]

Use instead:
reversed(iterable)
set(iterable)
sorted(iterable, reverse=True)

But unless I'm doing something wrong, it doesn't:

$ cat rev.py 
def f(iterable):
    x = reversed(iterable[::-1])
    y = set(iterable[::-1])
    z = sorted(iterable)[::-1]
    return x, y, z
$ python -m ruff check --select=C415 --fix --diff rev.py 
$ python -m ruff --version
ruff 0.0.292

Notice no warnings.

The reason I was checking this in the first place was that the first example (changing reversed(iterable[::-1]) to reversed(iterable)) is obviously wrong: if Ruff did that, it would result in the eventual order of the output being the wrong way. Possibly Ruff should output just iterable in this case, though it doesn't feel like a common issue.

[charliermarsh]

Yeah, that's definitely wrong.

[charliermarsh]

Need to figure out whether it's the docs, implementation, or both. Thanks!

[JelleZijlstra]

And I'm indeed doing something wrong: if --fix is passed, Ruff doesn't print warnings.

$ python -m ruff check rev.py --select=C4
rev.py:2:9: C415 Unnecessary subscript reversal of iterable within `reversed()`
rev.py:3:9: C415 Unnecessary subscript reversal of iterable within `set()`

But that still leaves two issues:

• The doc example about reversed() is wrong
• The implementation doesn't complain about the sorted() pattern the doc mentions

[charliermarsh]

(Ruff does print warnings with --fix; I think it's --diff that suppresses the warnings, since the purpose of that flag is to show fix changes.)