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

[Cliff Wells]

On Sun, 2008-09-14 at 10:08 +0100, Arnaud Delobelle wrote:
> On 14 Sep 2008, at 09:44, Cliff Wells wrote:
> >
> > j = range(3)
> > for i in j: i  # evaluates to []
> > for i in j: continue # evaluates to []
> > for i in j: continue i # evaluates to [0,1,2]
> >

> Let's not call it continue, but YIELD for now:
> 
>      for i in J: YIELD i
> 
> Now this won't work for nested loops. E.g. in current python
> 
>      def flatten(I):
>          for J in I:
>              for j in J:
>                  yield j
> 
>      >>> '-'.join(flatten(['spam', 'eggs']))
>      's-p-a-m-e-g-g-s'
> 
> Now say you want to write that inline with a for-expression:
> 
>      '-'.join(
> 	for J in I:
>              for j in J:
>                  YIELD j
> 	)
> 
> That won't work because the j's will be accumulated in the inner loop  
> and the outer loop won't accumulate anything, therefore returning an  
> empty iterable.

How about this way instead (since for-loop is now an expression):

'-'.join(
    for j in ( for J in I: YIELD J ): YIELD j  
)

> Now compare with the current syntax:
> 
>      '-'.join(j for J in I for j in J)

Certainly more clear and concise, but since (luckily for me this time)
we're maintaining backwards-compatibility, that form would still be
available.

Cliff