string = 'ABCDXY'
[string[i:i+2] for i in xrange(0, len(string), 2)]
>>> s = "abcde"
>>> k = 2
>>> [s[i:i+k].ljust(k, "_") for i in range(0, len(s), k)]
['ab', 'cd', 'e_']
s = 'abcdef'
L = zip(s[::2], s[1::2])
# -> [('a', 'b'), ('c', 'd'), ('e', 'f')]