[Python-ideas] for/else statements considered harmful

Calvin Spealman ironfroggy at gmail.com
Sat Jun 16 19:05:48 CEST 2012

On Wed, Jun 6, 2012 at 7:20 PM, Alice Bevan–McGregor
<alice at gothcandy.com> wrote:
> Howdy!
> 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?")

I read it not as for/else and while/else, but break/else and this has
been a much
more natural framing for myself and those I've used the framing to explain the
behavior to.

> 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
>   else:
>       pass # we had no values to iterate
>   finally:
>       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.
> I'll isolate the for/else/finally code from my fork and post a patch this
> week-end, hopefully.
>        — Alice.
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