On Thursday 23 October 2003 07:43, Guido van Rossum wrote: ...
<result> = <initializer> for <variable> in <iterable>: <result> = <expression> ... Concluding, I think the reduce() pattern is doomed -- the template is too complex to capture in special syntax.
I concur, particularly because the assignment in the pattern sketched above is too limiting. You point out that forcing augmented assignment would lose power (e.g., Horner's polynomials need bare assignment), but the inability to use it would imply inefficiencies -- e.g., flatlist = [] for sublist in listoflists: flatlist += sublist or flatlist.extend(sublist) is better than forcing a "flatlist = flatlist + sublist" as the loop body. Indeed, that's a spot where even 'sum' can be a performance trap; consider the following z.py: lol = [ [x] for x in range(1000) ] def flat1(lol=lol): return sum(lol, []) def flat2(lol=lol): result = [] for sub in lol: result.extend(sub) return result and the measurements: [alex@lancelot pop]$ timeit.py -c -s'import z' 'z.flat1()' 100 loops, best of 3: 8.5e+03 usec per loop [alex@lancelot pop]$ timeit.py -c -s'import z' 'z.flat2()' 1000 loops, best of 3: 940 usec per loop sum looks cooler, but it can be an order of magnitude slower than the humble loop of result.extend calls. We could fix this specific performance trap by specialcasing in sum those cases where the result has a += method -- hmmm... would a patch for this performance bug be accepted for 2.3.* ...? (I understand and approve that we're keen on avoiding adding functionality in any 2.3.*, but fixed-functionality performance enhancements should be just as ok as fixes to functionality bugs, right?) Anyway, there's a zillion other cases that sum doesn't cover (well, unless we extend dict to have a += synonym for update, which might be polymorphically useful:-), such as totaldict = {} for subdict in listofdicts: totaldict.update(subdict) Indeed, given the number of "modify in place and return None" methods of both built-in and user-coded types, I think the variant of "accumulation pattern" which simply calls such a method on each item of an iterator is about as prevalent as the variant with assignment "result = ..." as the loop body. Alex