Proposal: [... for ... while cond(x)]

[Terry Reedy]

"Eighty" <eightyx at gmail.com> wrote in message 
news:1155064682.499991.246170 at h48g2000cwc.googlegroups.com...
>
> Eighty wrote:
>> I suggest a new extension of the list comprehension syntax:
>>
>> [x for x in xs while cond(x)]

This does not work.

e(x) for x in xs if cond(x)

is an abbreviation of

for x in xs:
  if cond(x)
    yield e(x)

and similarly with more for and if clauses,
whereas the analogous expansion of your proposal

for x in xs:
  while cond(x):
     yield e(x)

is an infinite loop and not at all what you mean.

>> which would be equivalent to
>>
>> list(itertools.takewhile(cond, xs))

And what would

x for x in xs while cond(x) if blah(x)
x for x in xs if blah(x) while cond(x)
x*y for x in xs while cond(x) for y in ys

mean?  The ability to mix and match clauses after the first for clause is 
an important part of comprehension syntax.

>> + "Takewhile operations" occur often, at least for me

So keep using the function provided.  I am pretty sure takewhile is rare 
for most other people.

> So does no one have a comment on this?

Ain't gonna happen.

> The one objection I can come up with
> is that this would change the set builder notation semantics too much

Yes, to the point where you are hijacking the syntax, for no useful gain, 
more than extending it ;-).

Terry Jan Reedy