[Python-ideas] for/else statements considered harmful

Bruce Leban bruce at leapyear.org
Thu Jun 7 01:58:50 CEST 2012

If we could go back in time I would completely agree. But since we can't,
flipping meaning of else would be too error inducing and therefore not at
all likely.

So at risk of bike shedding I would suggest

for ...
[ else not: ]
else [ finally ] :

If a context-sensitive keyword would work I'd go for something more like

for ...
[ else empty: ]
else [ no match ] :

This would not introduce any incompatibilities.

--- Bruce
(from my phone)
On Jun 6, 2012 4:31 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 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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