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

[David Mertz]

I'm not quite certain if this is what the OP is proposing, but I think that
extending comprehensions to allow a 'while' clause would be intuitive and
somewhat useful.  It's true that itertools.takewhile() basically gets us
the same thing, but actual syntax would be nice, and also more
straightforward for comprehensions other than generator comprehensions.
E.g.

  attendees = {guest:guest.plus_N for guest in waiting_list while not
room_full()}

This would actually produce the same result as:

  attendees = {guest:guest.plus_N for guest in waiting_list if not
room_full()}

But it would save the extra looping over a bunch of final False values of
'room_full()'.

On Tue, Jun 25, 2013 at 6:35 AM, <jimjhb at aol.com> wrote:

> Syntax:
>
>  for X in ListY while conditionZ:
>
>  The 'for' loop would proceed as long as conditionZ remains true.
>
>  The motivation is to be able to make use of all the great aspects of the
> python 'for' (no indexing or explicit
> end condition check, etc.) and at the same time avoiding a 'break' from
> the 'for'.
>
>  (NOTE:  Many people are being taught to avoid 'break' and 'continue' at
> all costs, so they instead convert
> the clean 'for' into a less-clean 'while'.  Or they just let the 'for' run
> out.  You can argue against this teaching
> practice (at least for Python) but that doesn't mean it's not prevalent
> and prevailing.)
>
>  [People who avoid the 'break' by functionalizing an inner portion of the
> loop are just kidding themselves and making their own code worse, IMO.
> Takewhile from itertools also works, but that's clumsy and wordy as well.]
>
>  I'm not super familiar with CPython, but I'm pretty sure I could get
> this up and working without too much effort.
>
>  Please note that I don't feel the answer to this is 'just use break'.
>  Programmers are now being taught to avoid 'break' and 'continue' as if
> they were 'goto's.  The result (now) is that people are avoiding the 'for'
> (with its GREAT properties) because they can't break out of it.
>
>  Comments and Questions welcome.
>
>  Thanks.
>
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>


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