[Python-ideas] Is this PEP-able? for X in ListY while conditionZ:

[Joshua Landau]

On 1 July 2013 12:57, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:
> On 30 June 2013 00:51, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>    [x for x in iterable; break if x is None]
>>    [x for x in data if x; break if x is None]
>>
>> One nice advantage of that notation is that:
>>
>> 1. The statement after the ";" is exactly the statement that would
>> appear in the expanded loop
>> 2. It can be combined unambiguously with a filtering clause
>> 3. It clearly disallows its use with nested loops in the comprehension
>
> It has the significant disadvantage that Steven pointed out which is
> that it doesn't read very well. The most important aspect of a
> comprehension is its comprehensibility. Consider getting the prime
> numbers less than 100:
>
> primes100 = {p for p in primes(); break if p >= 100}
>
> You need to invert the if condition to understand which primes are in
> the resulting set. With for/while it reads properly and the condition
> at the right expresses a true property of the elements in the
> resulting set:
>
> primes100 = {p for p in primes() while p < 100}

If you're telling me that "{p for p in primes() while p < 100}" reads
better than "{p for p in primes(); break if p >= 100}" I have to
disagree strongly. The "break if" form looks beautiful.

I know that this involves my not-suggestion, so I might be biased, but
that's what I think.

> At the moment the obvious way to get the prime numbers less than 100
> would be to do something like:
>
<STUFF>

So you have these:

    {p for p in takewhile(lambda p: p < 100, primes())}

    set(takewhile(lambda p: p < 100, primes())

    {p for p in primes() while p < 100}

    {p for p in primes(); break if p >= 100}


I like them most bottom-to-top, but I don't think new syntax is a cost
worth having. If it is, I'm only accepting of the "break if" form,
which adds the least grammar. But not that either, because
srsly; a semicolon [youtu.be/M94ii6MVilw, contains profanity] in a
comprehension?