[Python-ideas] for/else syntax

[geremy condra]

On Wed, Sep 30, 2009 at 9:07 PM, Yuvgoog Greenle <ubershmekel at gmail.com>wrote:

> On python-dev the subject of for/else statements came up, so I had to
> mention how "ambiguous" the syntax seems to me. By "ambiguous" I meant that
> it's not obvious what should happen in the for/else and while/else
> constructs (the except/else construct is readable and great imo btw).
>
> Even though it's documented, for me it's a bad construct because it can and
> will always be misinterpreted by a non-trivial amount of people seeing it
> for the first time. It's unreadable and confusing because the "else" isn't
> referring to the "for" it's in reference to the "break". The construct is
> only useful in loops with "break" statements, so in pseudo code or human the
> "else" would have probably been translated to "if didn't break".
>
> To make things worse, it's not that newcomers will say "hmm, what does this
> for/else thing do? lets look at the manual", they will mistakenly assume
> they understand what the construct means. For evidence, see the for/else
> construct in the django templating language where "else" means the loop
> didn't run. The django interpretation IMHO is much more natural because
> "else" AFAIK means "otherwise" meaning that statements inside an "else"
> block will never be executed if the statements in the previous block were.
>
> To summarize:
>
> whatever:
> A
> else:
> B
>
> looks like it means either A happened or B happened. In python
> for/while/else constructs, this doesn't hold.
>
> --yuv
>
> Original Thread: [Python-Dev] Announcing PEP 3136
>
> ---------- Forwarded message ----------
> From: Terry Reedy <tjreedy at udel.edu>
> Date: Thu, Oct 1, 2009 at 2:47 AM
> Subject: Re: [Python-Dev] Announcing PEP 3136
> To: python-dev at python.org
>
>
> Yuvgoog Greenle wrote:
>
>> I like how python has a minimalistic and powerful syntax (-1 for the break
>> ___ PEP).
>>
>> Also, I really dislike the for/else ambiguity "butterflies".
>>
>
> Properly understood, no ambiguity.
>
> while c:
>  x()
>
> is equivalent to hypothetical
>
> label z:
> if c:
>  x()
>  goto: z
>
> So
>
> while c:
>  x()
> else:
>  y()
>
> is equivalent to
>
> label 1:
> if c:
>  x()
>  goto: 1
> else"
>  y()
>
> The else clause fires (once) if and when the if/while condition evaluates
> as false. Break and continue are restricted *unconditional* goto statements,
> and so *cannot* trigger an else clause.
>
> In for loops, the implied condition is 'there is another item in the
> collection represented by the iterable'.
>
> For any more, move to another list.
>
> Terry Jan Reedy
>
>
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>
I'm pretty sure breaking loop behavior on a point release sentences you to
the ninth ring
of software development hell. -1 from me.

Geremy Condra
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