[Python-Dev] for...else

[Ben Hoyt]

This is more of a python-ideas discussion, and Steven's answer is good.

I'll just add one thing. Maybe it's obvious to others, but I've liked
for...else since I found a kind of mnemonic to help me remember when the
"else" part happens: I think of it not as "for ... else" but as "break ...
else" -- saying it this way makes it clear to me that the break goes with
the else. "If this condition inside the loop is true, break. ... *else* if
we didn't break, do this other thing after the loop."

-Ben

On Mon, Jul 24, 2017 at 12:14 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> Hello Kiuhnm, and welcome.
>
> On Mon, Jul 24, 2017 at 05:35:03PM +0200, Kiuhnm via Python-Dev wrote:
> > Hello,
> >
> > I think that the expression "for...else" or "while...else" is completely
> > counter-intuitive.
>
>
> You may be right -- this has been discussed many, many times before. In
> my personal opinion, the best (and only accurate!) phrase would have
> been:
>
> for item in sequence:
>     # block
> then:
>     # block
>
> If you look at the byte-code generated by a for...else statement, you
> see that the "else" block is unconditionally executed after the for loop
> completes, unless something causes a jump outside of the entire
> statement: return, break, or raise. So it is more like:
>
> - run the loop;
> - *then* run the following block
>
> rather than:
>
> - run the loop;
> - otherwise ("else") run the following block.
>
> Others disagree and would prefer other keywords. But regardless,
> backwards compatibility means that we must keep "for...else", so I'm
> afraid that discussing alternatives is *almost certainly* a waste of
> time.
>
>
> > Wouldn't it be possible to make it clearer? Maybe
> > something like
>
> At this point, no, it is not practical to change the syntax used. Maybe
> when Python 3.0 was first introduced, but that ship has long sailed. It
> is very, very unlikely that the syntax for this will ever change, but if
> it does, it probably won't be until something in the distant future like
> Python 5.
>
> But not Python 4: Guido has already ruled that Python 4 will not include
> major backwards-incompatible changes. Going from 3 to 4 will not be as
> disruptive as going from 2 to 3.
>
> So depending on how you look at it: discussing alternative syntax to
> for...else is either ten years too late or ten years too early.
>
>
>
> --
> Steve
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