[Python-ideas] Statements vs Expressions... why?

[Arnaud Delobelle]

On 14 Sep 2008, at 07:36, Cliff Wells wrote:

> On Sun, 2008-09-14 at 07:23 +0100, Arnaud Delobelle wrote:
>> On 13 Sep 2008, at 23:17, Cliff Wells wrote:
>>
>>> On Sat, 2008-09-13 at 19:01 +0100, Arnaud Delobelle wrote:
>>>>
>>>> So what does:
>>>>
>>>> a = (if False: 1)
>>>>
>>>> evaluate to?
>>>
>>> That's a good question.  This is one of those areas where a  
>>> definition
>>> would need to be created.  My inclination is to say None (much  
>>> like a
>>> function with no return statement).
>>>
>>
>> Assuming the return value of "None", I go back to an example I gave
>> earlier:
>>
>>    factors = for x in range(2, n):
>>        if n % x == 0:
>>            x
>>
>> This doesn't work as intended (filtering out the non-factors).  How  
>> to
>> make it work?  The only way I can think of is to make (if 0: 1)  
>> return
>> a special "non-value" which loops will then filter out.  But then we
>> all know what happens to non-values.
>>
>> So how would you solve this problem?
>
> By writing it properly ;-)
>
> factors = for x in range ( 2, n ):
>    if n % x == 0:
>        yield x
>
> As I mentioned previously, in order to merge the concept of generator
> with a for-expression would require bringing in the yield keyword,  
> just
> as it does now for generator functions.
>
> The example you gave would evaluate to None (or perhaps an empty  
> list or
> generator - that's a detail that would take more consideration before
> defining it).
>

OK, but this seems to me incompatible with current Python:

def chain(I, J):
     for i in I: yield i
     for j in J: yield j

Currently

 >>> '-'.join(chain('spam', 'eggs'))
's-p-a-m-e-g-g-s'

With your proposal, the first *expression* (for i in I: yield i) will  
evaluate to something like iter(I) and then be discarded.  Then the  
second *expression* (for j in J: yield j) will evaluate to something  
like iter(J) which will be discarded.  So chain('spam', 'eggs') will  
return None.

>
> Regards,
> Cliff
>
>

-- 
Arnaud