On Thu, Aug 10, 2017 at 12:54:57AM +1000, Nick Coghlan wrote:

Guido wrote:
> > I haven't come around to this yet. It looks like it will make explaining
> > comprehensions more complex, since the translation of "while X" into "if not
> > X: break" feels less direct than the translations of "for x in xs" or "if
> > pred(x)". (In particular, your proposal seems to require more experience
> > with mentally translating loops and conditions into jumps -- most regulars
> > of this forum do that for a living, but I doubt it's second nature for the
> > OP.)
> Yeah, if we ever did add something like this, I suspect a translation
> using takewhile would potentially be easier for at least some users to
> understand than the one to a break condition:

"Some users"? Sure, why not? There's probably somebody out there who 
understands takewhile, but if so, I don't know who they are :-)

I always have to look at the docs for takewhile to remind myself whether 
it drops items ("takes them away") while the condition is true, or 
yields items ("gives items") while the condition is true.

>     {x for x in itertools.count(0) if 1000 <= x while x < 1000000}
>     <=>
>     x = set()
>     for x in itertools.count(0):
>         if 1000 <= x:
>             set.add(x)
>         # If you've never used the loop-and-a-half idiom, it's
>         # not obvious why "while <expr>" means "if not <expr>: break"
>         if not x < 1000000:
>             break

I'd like to take issue with that "not obvious" comment. I think that 
anyone who knows while loops knows that the loop exits when the 
condition becomes false. That's exactly the behaviour we get for the 
(hypothetical) [expr for x in seq while condition] syntax: when the 
condition is false, the loop and hence the comprehension, exits.

For such simple cases, there's no need to think about "loop and a half". 
The obvious explanation is that the loop exits when the while condition 

Based on my experience with beginners on the tutor mailing list, and 
elsewhere, I think there's a definite learning "hump" to get over before 
people grok even the trivial case of

    [expression for x in sequence]

but once they do, then adding an "if" clause is obvious, and I expect 
that the same will apply to "when".

Once you move beyond the simple case of a single for and no more than a 
single if (or while), I don't think there's *anything* obvious about 
comprehension syntax at all, while clause or no while clause. Holding 
the while clause to a standard that comprehensions already fail (in my 
opinion) is unfair:

[expression for x in seq1 for y in seq2 if pred1 for z in seq3 
    if pred2 if pred3 if pred4 for w in seq4 while condition for v in seq5]

I don't think it's the "while" that tips that over the edge, 
readability-wise :-)

In any case, I think we're all guessing whether or not people will 
understand the "while condition" syntax. So I've done an informal survey 
on the Python-Ideas list, and once folks have had a day or so to answer 
I'll report what they say. It's not a truly scientific UI test, but it's 
the best I can do.

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