[Python-Dev] PEP 3142: Add a "while" clause to generator expressions

[rdmurray at bitdance.com]

On Wed, 21 Jan 2009 at 21:46, Steven D'Aprano wrote:
> On Wed, 21 Jan 2009 03:10:24 pm Terry Reedy wrote:
>> It is a carefully designed 1 to
>> 1 transformation between multiple nested statements and a single
>> expression.
>
> I'm sure that correspondence is obvious to some, but it wasn't obvious
> to me, and I don't suppose I'm the only one. That's not a criticism of
> the current syntax. Far from it -- the current syntax is excellent,
> regardless of whether or not you notice that it corresponds to a
> if-loop nested inside a for-loop with the contents rotated outside.

It wasn't obvious to me until I read this thread, but now that I know
about it I feel a huge sense of relief.  I was never comfortable with
extending (or reading an extension of) a list comprehension beyond the
obvious yield/for/if pattern before.  Now I have a reliable tool to
understand any complex list comprehension.  I would not want to lose that!

>> But this proposal ignores and breaks that.  Using 'while
>> x' to mean 'if x: break' *is*, to me, 'ad hoc'.
>
> But it doesn't mean that. The proposed "while x" has very similar
> semantics to the "while x" in a while-loop: break when *not* x.

Half right.  'while x' in the proposed syntax is equivalent to 'if not x:
break',  But IMO it goes too far to say it has similar semantics to 'while
x' in a while loop.  Neither

     while x*x<4:
         for x in range(10):
             yield x*x

nor

     for x in range(10):
         while x*x<4:
             yield x*x

are the same as

     for x in range(10):
         if not x*x<4: break
         yield x*x

I understand that you are saying that 'while x' is used in the same
logical sense ("take a different action when x is no longer true"),
but that I don't feel that that is enough to say that it has similar
semantics.  Or, perhaps more accurately, it is just similar enough to be
very confusing because it is also different enough to be very surprising.
The semantics of 'while' in python includes the bit about creating a
loop, and does _not_ include executing a 'break' in the surrounding loop.
To give 'while' this new meaning would be, IMO, un-pythonic.  (If python
had a 'for/while' construct, it would be a different story...and then
it would probably already be part of the list comprehension syntax.)

>> So I detest the proposed change.  I find it ugly and anti-Pythonic.

I'd say +1 except that I don't find it ugly, just un-Pythonic :)

--RDM