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

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