On 05/20/2016 04:55 AM, Jon Ribbens wrote:
On 2016-05-20, Steven D'Aprano wrote:
On Fri, 20 May 2016 03:55 am, Jon Ribbens wrote:
I guess we should thank our lucky stars that you don't have a time
machine then, since that change would very much be one for the worse
in my opinion. for...else is perfectly straightforward and clearly
the right keywords to use. for...then would be entirely wrong.
"Entirely" wrong? "Clearly" the right keyword? "Perfectly" straightforward?
They are extremely strong words given the posts where *even the defenders*
of "else" have admitted that it is a hard keyword to understand. But that's
okay. Maybe you've thought of something the rest of us haven't, and have an
entire consistent mental model of for...else that is easy to understand and
makes it "perfectly straightforward and clearly the right keyword".
Can you explain your model which makes "else" appropriate?
Certainly. "else:" is (almost?) invariably used in the situation where
you are iterating through something in order to find a value which
matches a certain condition. So the "for:" block means "find this
value" and the "else:" means "else do this".
I'm happy that you have a working mental model for for/else (seriously,
I am) but please don't discount the confusion and consternation for the
many folks who don't start with that mental model.
The number of times I have /wanted/ to use the for/else structure for
searching is small (and I remember them both ;) -- every other time what
I wanted was an _else_ that ran iff the iterable was already empty when
the _for_ encountered it.
because like many people, my mental model was "the for block runs, OR ELSE
the else block runs". This *incorrect* model seems like it works: if you
set seq=[], say, it prints "seq is empty" as expected.
The problem there is that the mental model is *completely* wrong.
D'oh. Completely wrong, but easy to guess because of the similarity
with if/else.
I never would have thought of that model if it had been called "then":
for x in seq:
pass
then:
print("executes after the for block completes")
I would find that very confusing. "then:" makes it sound like
executing that block is the usual case, when in practice it is
usually the exception - the fallback code if the expected value
was not found.
If you don't take the extra step of _break_ it is the usual case. Most
of my for loops always run all the way through, so why have an else?
Two possible reasons:
- the loop didn't run at all
- the loop didn't run all the way
Guido picked the one that was obvious to him (him being Dutch and all ;) .
I just read Nick's post about it, and while it helps, I think the
syntactic similarity between try/else and for/else (did the block run
without error) is dwarfed by the semantic difference: a try/else else
block runs if nothing /bad/ happened whereas a for/else else block runs
if something bad /did/ happen; to wit, the thing you were looking for
was not found (or your loop didn't run at all ;) .
But as others have said, this isn't going to change now, and I'm okay
with that. But, please, be a bit more understanding of those who don't
immediately grok the for/else and while/else loops.
--
~Ethan~
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