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

[Chris Angelico]

On Tue, Jan 29, 2013 at 1:32 AM, Shane Green <shane at umbrellacode.com> wrote:
> Isn't "while" kind just the "if" of a looping construct?
>
> Would [n for n in range(1,1000) while n < 400] == [n for n in range(1,1000)
> if n < 400]?
>
> I guess your kind of looking for an "else break" feature to exit the list
> comprehension before evaluating all the input values.  Wouldn't that
> complete the "while()" functionality?

In the specific case given, they'll produce the same result, but there
are two key differences:

1) If the condition becomes true again later in the original iterable,
the 'if' will pick up those entries, but the 'while' won't; and
2) The 'while' version will not consume more than the one result that
failed to pass the condition.

I daresay it would be faster and maybe cleaner to implement this with
a language feature rather than itertools.takewhile, but list
comprehensions can get unwieldy too; is there sufficient call for this
to justify the syntax?

ChrisA