gistfile1.py

def num_to_base(num, base='ab'):
    if num == 0:
        return base[0]
    res = []
    while num:
        num, m = divmod(num, len(base))
        res.append(base[m])
    return ''.join(res)

for i in xrange(5):
    print num_to_base(i),
    # a b ab bb aab

print

for i in xrange(4, 10):
    print num_to_base(i),
    # aab bab abb bbb aaab baab