Timeit's, split numbers into threes

gistfile1.txt

In [1]: a = '12345678'

In [2]: %timeit [a[max(i-3,0):i] for i in range(len(a), 0, -3)][::-1]
100000 loops, best of 3: 2.19 us per loop

In [3]: %timeit format(int(a), ',').split(',')
100000 loops, best of 3: 2.04 us per loop

In [4]: %timeit [a[max(0,i-2+(len(a)-1)%3):i+1+(len(a)-1)%3] for i in range(0, len(a), 3)]
100000 loops, best of 3: 2.91 us per loop