Question

Which of these would be considered better/clearer/faster/more ‘Pythonic’? I don’t care about the content of the list L, just how long it is.

a = [f(n) for n, _ in enumerate(L)]

or

a = [f(n) for n in range(len(L))]

If it makes any difference, the function f makes use of len(list) as well.

Answer 1

Some quick timing runs seem to give the 2nd option using range() a slight edge over enumerate():

timeit a = [f(n) for n, _ in enumerate(mlist)]
10000 loops, best of 3: 118 us per loop

timeit a = [f(n) for n in range(len(mlist))]
10000 loops, best of 3: 102 us per loop

and just for fun using xrange() (Python v2.7.2)

timeit a = [f(n) for n in xrange(len(mlist))]
10000 loops, best of 3: 99 us per loop

I would favor readable code first, then using xrange() if available (i.e., Pre-Python v 3.x), followed by range() and enumerate().