how can this iterator be optimized?

[Gabriel Genellina]

En Wed, 11 Feb 2009 23:22:26 -0200, Basilisk96 <basilisk96 at gmail.com>  
escribió:

> def MakePairs(path):
>     import os
>     import operator
>     join = os.path.join
>     rawPairs = (
>         (join(path, s), func(s))
>         for s in os.listdir(path)
>         if func(s) is not None and func(s).endswith("some criterion")
>     )
>     #Use the second item in the pair as the sort criterion
>     result = sorted(rawPairs, key=operator.itemgetter(1))
>     return result
>
> where "func" is a single-argument function that returns either a
> string or None, but is an expensive call.
> I am pretty sure that the sorted() construct cannot be improved much
> further, but...
> ...does anyone have ideas on improving the "rawPairs" iterator so that
> it calls "func(s)" only once per iteration?  Perhaps a lambda
> construct, but I am not sure how to go about it...?

Don't use a generator then. If you're going to finally return a list (and  
sorted does exactly that), build a list right from the start:

def MakePairs(path):
     join = os.path.join
     result = []
     append = result.append
     for s in os.listdir(path):
         key = func(s)
         if key is not None and key.endswith("some criterion"):
             append((join(path, s), key))
     #Use the second item in the pair as the sort criterion
     result.sort(key=operator.itemgetter(1))
     return result

-- 
Gabriel Genellina