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

[Eli Bendersky]

On Mon, Jan 28, 2013 at 5:33 AM, Wolfgang Maier <
wolfgang.maier at biologie.uni-freiburg.de> wrote:

> Dear all,
> I guess this is so obvious that someone must have suggested it before:
> in list comprehensions you can currently exclude items based on the if
> conditional, e.g.:
>
> [n for n in range(1,1000) if n % 4 == 0]
>
> Why not extend this filtering by allowing a while statement in addition to
> if, as in:
>
> [n for n in range(1,1000) while n < 400]
>
> Trivial effect, I agree, in this example since you could achieve the same
> by
> using range(1,400), but I hope you get the point.
> This intuitively understandable extension would provide a big speed-up for
> sorted lists where processing all the input is unnecessary.
>
> Consider this:
>
> some_names=["Adam", "Andrew", "Arthur", "Bob", "Caroline","Lancelot"]     #
> a sorted list of names
> [n for n in some_names if n.startswith("A")]
> # certainly gives a list of all names starting with A, but .
> [n for n in some_names while n.startswith("A")]
> # would have saved two comparisons
>

-1

This isn't adding a feature that the language can't currently perform. It
can, with itertools, with an explicit 'for' loop and probably other
methods. List comprehensions are a useful shortcut that should be kept as
simple as possible. The semantics of the proposed 'while' aren't
immediately obvious, which makes it out of place in list comprehensions,
IMO.

Eli
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