range(len(list)) or enumerate(list)? [duplicate]

Question

Possible Duplicate:
Only index needed: enumerate or (x)range?

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().

Answer 2

The (x)range solution is faster, because it has less overhead, so I'd use that.

In Python 2.x, use xrange instead of range, because xrange uses less memory, because it doesn't create a temporary list. In Python 3.x, there is only range, which is the less-memory version.

Thus, in Python 2.x, iterating over a range(n) uses O(n) memory temporarily, and iterating over an xrange(n) uses O(1) memory temporarily. Runtime is O(n) for both.