[Python-ideas] Variations on a loop

[Arnaud Delobelle]

On 29 Aug 2008, at 13:49, Bruce Frederiksen wrote:

> Bruce Leban wrote:
>>
>> For example:
>>
>>    result = ""
>>    for x in items:
>>        result += str(x)
>>    interstitially:
>>        result += ", "
>>    contrariwise:
>>        result = "no data"
> We already have statements that only apply within loops (break and  
> continue), how about some expressions that only apply in for loops:  
> 'last' and 'empty':
>
> for x in items:
>   result += str(x)
>   if not last:
>       result += ', '
> else:
>   if empty:
>       result = "no data"
>
> -bruce (not to be confused with Bruce :-)

note that this can be achieved without 'empty'

for x in items:
     result += str(x)
     if last:
         break
     else:
         result += ', '
else:
     result = "no data"

Something like this can be implemented in Python with the following  
helper function:

def flaglast(iterable):
     it = iter(iterable)
     for prev in it:
         break
     else:
         return
     for item in it:
         yield prev, False
         prev = item
     yield prev, True

E.g.

 >>> list(flaglast([1,2,3]))
[(1, False), (2, False), (3, True)]

Then you can write your code snippet as:

for x, last in flaglast(items):
     result += str(x)
     if last:
         break
     else:
         result += ', '
else:
     result = "no data"

-- 
Arnaud