gistfile1.py

## This is on python 2.7:

In [1]: a = '12345678'

In [2]: %timeit [a[max(i-3,0):i] for i in range(len(a), 0, -3)][::-1]
1000000 loops, best of 3: 1.91 us per loop

In [3]: %timeit format(int(a), ',').split(',')
1000000 loops, best of 3: 1.62 us per loop

In [4]: %timeit [a[max(0,i):i+3] for i in range((len(a)-1)%3-2, len(a), 3)]
1000000 loops, best of 3: 1.67 us per loop

## And python 3.2:

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: 2.58 us per loop

In [3]: %timeit format(int(a), ',').split(',')
1000000 loops, best of 3: 1.98 us per loop

In [4]: %timeit [a[max(0,i):i+3] for i in range((len(a)-1)%3-2, len(a), 3)]
100000 loops, best of 3: 2.45 us per loop