[Python-ideas] while conditional in list comprehension ??

[Ian Cordasco]

On Mon, Jan 28, 2013 at 8:34 PM, Oscar Benjamin
<oscar.j.benjamin at gmail.com> wrote:
> I was referring to the case of constructing an object that does not
> preserve order by iterating over an object that does. Clearly a while
> clause would be a lot less useful if you were iterating over an object
> whose order was arbitrary: so don't use it in that case.
>

Yeah, I'm not sure how well telling someone to use a construct of the
language will go over.

> A (contrived) example - caching Fibonacci numbers:
>
> # Fibonacci number generator
> def fib():
>     a = b = 1
>     while True:
>         yield a
>         a, b = b, a+b
>
> # Cache the first N fibonacci numbers
> fib_cache = {n: x for n, x in zip(range(N), fib())}
> # Alternative
> fib_cache = {n: x for n, x in enumerate(fib()) while n < N}
>
> # Cache the Fibonacci numbers less than X
> fib_cache = {}
> for n, x in enumerate(fib()):
>     if x > X:
>         break
>     fib_cache[n] = x
> # Alternative 1
> fib_cache = {n: x for n, x in enumerate(takewhile(lambda x: x < X, fib()))}
> # Alternative 2
> fib_cache = {n: x for n, x in enumerate(fib()) while x < X}
>

As contrived as it may be, it is a good example. Still, I dislike the
use of `while` and would rather Steven's suggestion of `until` were
this to be included. This would make `until` a super special case, but
then again, this construct seems special enough that only a few
examples of its usefulness can be constructed. I guess I'm more -0
with `until` than -1.

Thanks for the extra example Oscar. It was helpful.

Cheers,
Ian