[Python-Dev] Use for enumerate() (original) (raw)

Tim Peters tim.one@comcast.net
Sat, 27 Apr 2002 01:56:56 -0400

Challenge 3: do it faster and with less code.

[Raymond Hettinger]

def getline(filename, lineno): if lineno < 1: return '' f = open(filename) i, line = zip(xrange(lineno), f)[-1] f.close() if i+1 == lineno: return line return ''

Hmm. On my box it's a little slower than Guido's getline on my standard test, here calling that function g3 (g2 and the timing driver were posted before; the input is Zope's DateTime.py, a 1657-line Python source file):

getline 4.85231314638 g2 2.8915829967 g3 5.19037613772

That's a curious result, since, as you say:

The approach is to vectorize, trading away memory allocation time and xrange time to save the overhead of the pure Python loop and test cycle.

It gets a speed boost to below 5.0 if I use range instead of xrange.

It suggests this alternative, which is a tiny bit shorter and significantly faster than Guido's:

def g4(filename, lineno): if lineno < 1: return '' f = open(filename) get = iter(f).next try: for i in range(lineno): line = get() except StopIteration: pass f.close() return line

That weighs in at 4.04 seconds on my test case.

I think the lesson to take is that building gobs of 2-tuples is more expensive than taking the same number of quick trips around the eval loop. Guido's and your function both build gobs of 2-tuples, while the zippier g4 and much zippier g2 avoid that.

... The test is saved by taking advantage of zip's feature which stops when the first iterator is exhausted.

It is clever! Too bad it's pig slow .