unused variables in list comprehension not reported

[lilydjwg]

Test code:

def t(items):
  return ['{x}' for x in items]

There is a missing f prefix which makes the local x variable unused, but pyflakes doesn't notice this.

$ pyflakes --version
2.2.0 Python 3.8.6 on Linux

[asottile]

I agree this particular example should probably produce an error, but pyflakes is a bit timid about assignments in unpackings

for example, should this produce a warning (pyflakes currently assumes any sort of unpacking can introduce variables that don't need to be referenced)?

return [f'{x} for x, y in items]

[lilydjwg]

Aren't unused names expected to be prefixed with _? If not, maybe pyflakes can detect missing f-prefixes in another way?

This kind of errors is not easy to notice otherwise: the types check with any issue; the {x} syntax inside normal strings is also special (for .format) so it's highlighted anyway.

[asottile]

that's not something that's errorable either

consider this valid snippet:

FMT_S = 'foo {bar}'

def make_thing():
    return FMT_S.format(bar=...)

(of course this is a silly example that could be inlined, but it is still a case that comes up). this would introduce a high-false-positive rule which isn't really appropriate for pyflakes and is better left to a flake8 plugin or pylint