[Python-ideas] for/else statements considered harmful
steve at pearwood.info
Thu Jun 7 01:45:36 CEST 2012
Alice Bevan–McGregor wrote:
> Was teaching a new user to Python the ropes a short while ago and ran
> into an interesting headspace problem: the for/else syntax fails the
> obviousness and consistency tests. When used in an if/else block the
> conditional code is executed if the conditional passes, and the else
> block is executed if the conditional fails. Compared to for loops where
> the for code is repeated and the else code executed if we "naturally
> fall off the loop". (The new user's reaction was "why the hoek would I
> ever use for/else?")
Yes, I love for/else and while/else but regret the name. The else is
conceptually unlike the else in if/else, and leads to the common confusion
that the else suite if the iterable is empty.
> I forked Python 3.3 to experiment with an alternate implementation that
> follows the logic of pass/fail implied by if/else: (and to refactor the
> stdlib, but that's a different issue ;)
> for x in range(20):
> if x > 10: break
> pass # we had no values to iterate
> pass # we naturally fell off the loop
> It abuses finally (to avoid tying up a potentially common word as a
> reserved word like "done") but makes possible an important distinction
> without having to perform potentially expensive length calculations
> (which may not even be possible!) on the value being iterated: that is,
> handling the case where there were no values in the collection or
> returned by the generator.
> Templating engines generally implement this type of structure. Of
> course this type of breaking change in semantics puts this idea firmly
> into Python 4 land.
Sadly, yes. Where were you when Python 3.0 was still being planned? :)
> I'll isolate the for/else/finally code from my fork and post a patch
> this week-end, hopefully.
> — Alice.
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