On Sun, Mar 30, 2014 at 4:54 PM, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote: > On Sun, 30 Mar 2014 13:15:18 +1100, Chris Angelico wrote: > >> Chained comparisons where you're checking a single variable against two >> constants make perfect sense: >> >> 2 < x < 5 >> >> Chained comparisons where you check a single constant against two >> variables don't, so much: >> >> x < 2 < y >> >> What exactly does that mean, and why is it written that way? > > It checks that 2 is strictly bounded between x on the left and y on the > right, i.e. that 2 is inside the open interval x...y. I don't know why > you think that's unclear. But then I do have a maths background and I'm > used to chaining comparisons. > > Write it like this: > > low = x > high = y > a = 2 > > low < a < high > > Does that make more sense? Well-chosen names are good. The fact that a is > a constant rather than a variable is no big deal: > > low < 2 < high
The problem isn't that I can't see what the comparisons are. It makes very good sense to bound a variable within constants; but you already know exactly where 2 is on the number line, so asking "Is 2 between these two variables" seems a bit odd. Maybe it's less so with the strong mathematical background, but it seems odd to me. >> It'd be more useful but less clear if one of the conditions points the >> other way: >> >> x < 2 > y >> >> which checks that they're both less than two, > > which is quite different from what you wrote the first time. > > >> but IMO in a less-than-clear way. > > That's an understatement. If I saw code chaining comparisons in that > fashion, I would assume the second operator > was a typo. > > Chaining less-than and greater than operators should, for clarity, always > be written in a single order. E.g. a <= b < c < d, not a <= b < d > c. > > (The second contains a subtle bug too.) Agreed. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
