[Python-Dev] for...else

[Steven D'Aprano]

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