Behavior of the for-else construct

[Chris Angelico]

On Fri, 4 Mar 2022 at 00:25, computermaster360
<computermaster360 at gmail.com> wrote:
>
> I want to make a little survey here.
>
> Do you find the for-else construct useful? Have you used it in
> practice? Do you even know how it works, or that there is such a thing
> in Python?

Yes, yes, and yes-yes. It's extremely useful.

> I have used it maybe once. My issue with this construct is that
> calling the second block `else` doesn't make sense; a much more
> sensible name would be `then`.

Ehh, I think "finally" would be a better keyword, but that has very
close connections with exception handling.

> Now, imagine a parallel universe, where the for-else construct would
> have a different behavior:
>
>     for elem in iterable:
>         process(elem)
>     else:
>         # executed only when the iterable was initially empty
>         print('Nothing to process')
>
> Wouldn't this be more natural? I think so. Also, I face this case much
> more often than having detect whether I broke out of a loop early
> (which is what the current for-else construct is for).

This also has value, but not as much.

> Now someone may argue that it's easy to check whether the iterable is
> empty beforehand. But is it really? What if it's an iterator?
> Then one would have to resort to using a flag variable and set it in
> each iteration of the loop. An ugly alternative would be trying to
> retrieve
> the first element of the iterable separately, in a try block before
> the for-loop, to find out whether the iterable is empty. This would of
> course
> require making an iterator of the iterable first (since we can't be
> sure it is already an iterator), and then -- if there are any elements
> -- processing
> the first element separately before the for-loop, which means
> duplicating the loop body. You can see the whole thing gets really
> ugly really quickly...
>
> What are your thoughts? Do you agree? Or am I just not Dutch enough...?

Both forms have value, and only one of them can be called for-else.
Maybe if the current one had been called for-finally, then what you're
proposing could have been for-else, and we could theoretically have
had for-else-finally (where it goes into the for block once for each
element, but if there aren't any, it goes into the else block instead,
and either way, if you never break, it goes into the finally before
moving on). That ship has sailed, though, and given that people would
be confused very greatly by for-finally, I'm not overly sorry with the
current state of affairs.

ChrisA