Is this a "gotcha" in Python?

[Chris Angelico]

On Tue, Apr 23, 2019 at 8:21 AM Skip Montanaro <skip.montanaro at gmail.com> wrote:
>
> > Interesting. So it would flag this code?
> >
> > for _ in range(5): next(f) # skip five lines
>
> As of at least recent versions (I have 2.1.1 in my Conda install at
> home) It seems to properly accept "_" as a variable name, even when
> not used. It also seems to accept any variable name as a loop index.
> Here's a silly function which demonstrates current behavior:
>
> def skipper(filename):
>     f = open(filename)
>     _ = filename.lower()
>     _up = filename.upper()
>     filename.title()
>     for i in range(5):
>         next(f)
>     return f
>
> The assignment to "_" is accepted, as is the use of "i" as the loop
> variable.

By "accepted" you mean that it isn't complaining? Good; although I'm
not sure why you'd use "_" in a simple assignment context. Clearly
it's quite happy to have *any* unused variable as a loop iterator, and
honestly, I think that's probably correct (since not everyone uses the
underscore convention).

> The assignment to _up is flagged though.

What kind of flagging? "Unused variable", or is it saying that "_up"
is an unconventional name for a local (and perhaps should have been
declared global)?

> It also doesn't
> complain about the ignored return value for filename.title(). (In all
> fairness, I don't think pyflakes was ever held up as a "deep" analyzer
> of Python source, so couldn't be expected to know that string's title
> method operates without side effect.)

Yeah, that one's a bit harder to flag. I wouldn't fault any linter for
not knowing that something's useless in that way. Would be cool if it
knew, but no big deal if it doesn't.

ChrisA