False positive: B031

[AA-Turner]

Ruff 0.0.260

Reproducer:

(sphinx) PS I:\Development\sphinx> ruff -V                                                
ruff 0.0.260
(sphinx) PS I:\Development\sphinx> type bug_b031.py
from itertools import groupby


def f():
    for w, g in groupby('string', len):
        if w == 1:
            return (''.join(g)).split(" ")
        else:
            return list(g)  # Ruff indicates B031 here


def g():
    for w, g in groupby('string', len):
        tmp = [*g]  # by introducing an interstitial variable, we avoid B031
        if w == 1:
            return (''.join(tmp)).split(" ")
        else:
            return list(tmp)
(sphinx) PS I:\Development\sphinx> ruff --isolated --show-source --select B031 bug_b031.py
bug_b031.py:9:25: B031 Using the generator returned from `itertools.groupby()` more than once will do nothing on the second usage
  |
9 |             return list(g)  # Ruff indicates B031 here
  |                         ^ B031
  |

Found 1 error.
(sphinx) PS I:\Development\sphinx> 

bug_b031.py

from itertools import groupby


def f():
    for w, g in groupby('string', len):
        if w == 1:
            return (''.join(g)).split(" ")
        else:
            return list(g)  # Ruff indicates B031 here


def g():
    for w, g in groupby('string', len):
        tmp = [*g]  # by introducing an interstitial variable, we avoid B031
        if w == 1:
            return (''.join(tmp)).split(" ")
        else:
            return list(tmp)

Sorry for the rather contrived example, this was reduced from some code in Sphinx. Broadly I think that the error comes from the heuristic checking any re-use, whereas here as the symbol g is used twice, but only once per branch of an if/elif/else chain.

Please would it be possible to add some heuristic for tracking usages like this, in if/elif/else chains?

Thanks,
Adam

[charliermarsh]

👍 I think this is the same as #3801. flake8-bugbear also has this problem. I think we should be able to avoid it though.

[AA-Turner]

Ahh, sorry--should have had a quick look before submitting this!

Thanks,
Adam

[charliermarsh]

All good, it's useful to know that others have hit this too and to see another example!

[dhruvmanila]

I'll try to look into it over the weekend but I guess it would be better to track it in one issue. We could move the example in the other one.

[charliermarsh]

@dhruvmanila - Sounds good, I'm going to mark as a duplicate.