[Python-ideas] Add "while" clauses to generator expressions

Gerald Britton gerald.britton at gmail.com
Sun Jan 11 00:20:40 CET 2009

Hmmm.  I can't really see what you are saying.  Your example doesn't
quite get the intention.  The "while" clause in the example would
translate like this:

     def gen():
         for i in range(100):
             if i <= 50:
                 yield i

Also, I think that for a new python user,

(n for n in range(100) while n*n < 50)

is easier to understand and use than:

(n for n in takewhile(lambda n:n*n < 50, range(100)))

My proposed version is shorter (less chance for typos), has one less
set of parentheses (always a good thing) and reads naturally.  Also,
it is directly analogous to:

(n for n in range(100) if n*n < 50)

except that the "while" version stops when n reaches 8.  The "if"
version doesn't stop until n reaches 99.

On Sat, Jan 10, 2009 at 5:45 PM, Steven Bethard
<steven.bethard at gmail.com> wrote:
> [Fixing the top-posting. Note that only Guido is allowed to top-post
> around here. ;-)]
> On 1/10/09, Steven Bethard <steven.bethard at gmail.com> wrote:
>> I think this could end up being confusing. Current generator
>> expressions turn into an equivalent generator function by simply
>> indenting the clauses and adding a yield, for example:
>>     (i for i in range(100) if i % 2 == 0)
>> is equivalent to:
>>     def gen():
>>         for i in range(100):
>>             if i % 2 == 0:
>>                 yield i
>> Now you're proposing syntax that would no longer work like this.
>> Taking your example:
>>     (i for i in range(100) while i <= 50)
>> I would expect this to mean:
>>     def gen():
>>         for i in range(100):
>>             while i <= 50:
>>                 yield i
> On Sat, Jan 10, 2009 at 2:34 PM, Gerald Britton
> <gerald.britton at gmail.com> wrote:
>> I guess I don't think it would be confusing. On the contrary, I
>> believe that the expressions would read naturally and be a nice
>> simplification.  Of course it won't work just like "if" but that is
>> just the point!  I can (and do) accomplish the same thing with
>> "takewhile", but if the same thing can be sone with a little addition
>> to the generator expression, why not do it?
> I'm probably just repeating myself here, but the reason not to do it
> is that the current generator expressions translate almost directly
> into the corresponding generator statements. Using "while" in the way
> you've suggested breaks this symmetry, and would make Python harder to
> learn.
> Steve
> --
> I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
> tiny blip on the distant coast of sanity.
>        --- Bucky Katt, Get Fuzzy

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