Local variable is assigned but never used (F841) cannot be ignored with dummy-variable-rgx in except block

[phillipuniverse]

Example:

try:
    1 + 1
except Exception as _err:
    print("Problem")

Ruff always triggers F841 on the except Exception as _err line:

F841 [*] Local variable `_` is assigned to but never used

Ruff version 0.0.282

[charliermarsh]

Interestingly, Flake8 doesn't respect the dummy-variable-rgx here either. What's the use-case? Did this occur in a real example, where you want to assign the exception but not use it?

[phillipuniverse]

Flake8 doesn't respect the dummy-variable-rgx here either.

Hm, that is interesting.

Originally I had written this in a project:

try:
    1 + 1
except:
    logger.exception("Cannot add numbers :(")

And then I got feedback that it would be nice to be able to capture the caught exception in a variable for easy access in pdb and other debuggers. So I thought it would be simple to rewrite as:

try:
    1 + 1
except Exception as _err:
    logger.exception("Cannot add numbers :(")

and was surprised when as _err got rewritten by the autofixer. What I ended up shipping was this:

try:
    1 + 1
except Exception as err:
    logger.exception("Cannot add numbers :(", exc_info=err)

Maybe there is a slight benefit in this final solution, as logger.exception(msg) is equivalent to logger.error(msg, exc_info=True) which under the covers gets the original exception with sys.exc_info().

So it's an easy workaround and maybe worthwhile to keep the exact same behavior as flake8 in this scenario.

[tylerlaprade]

it would be nice to be able to capture the caught exception in a variable for easy access in pdb and other debuggers

This is the tail wagging the dog, IMHO. My team complained about RET504 removing the variable they introduced for the sole purpose of debugging. I held firm, as I don't believe we should allow the code to become messier for the sake of debugging practices.

[charliermarsh]

Yeah, hmm... IMO it's strange to use that as _err pattern, but it's also surprising not to respect the dummy-variable-rgx here.

[phillipuniverse]

removing the variable they introduced for the sole purpose of debugging

@tylerlaprade if it makes you feel any better for holding firm, there are other ways to expose that return value.

In Pycharm you can check the 'Show Return Values' option

In pdb, you can use __return__, detailed tip from SO

[houmankamali-Zipline]

I have a similar use case by for walrus assignment:

    while _has_not_timed_out := (last_try_epoch_sec - start_epoch_sec) < timeout_sec:
        if _reached_retry_interval := (time() - last_try_epoch_sec) >= retry_interval_sec: