slice iterator?

[Ross]

I have a really long list that I would like segmented into smaller
lists. Let's say I had a list a = [1,2,3,4,5,6,7,8,9,10,11,12] and I
wanted to split it into groups of 2 or groups of 3 or 4, etc. Is there
a way to do this without explicitly defining new lists? If the above
problem were to be split into groups of 3, I've tried something like:

start = 0
stop = 3
for i in range(len(a)):
    segment = a[start:stop]
    print segment
    start += stop
    stop += stop

Unfortunately start and stop don't increment using this code. Ideally,
my outcome would be
[1,2,3]
[4,5,6]
[7,8,9]
[10,11,12]