sliding window generator, should work in python>=2.6

sliding_window.py

from itertools import chain, repeat, islice
from collections import deque

def sliding_window(iterable, n, fill=False, fillvalue=None):
    it = iter(iterable)
    if fill:
        it = chain(it, repeat(fillvalue, n - 1))
    w = deque(islice(it, n - 1))
    for x in it:
        w.append(x)
        yield w
        w.popleft()


for x in sliding_window("abcdefghijkl", 4): 
    print ''.join(x)

""" outputs:
abcd
bcde
cdef
defg
efgh
fghi
ghij
hijk
"""

for x in sliding_window("abcdefghijk", 4, fill=True, fillvalue='!'):
    print ''.join(x)

""" outputs:
abcd
bcde
cdef
defg
efgh
fghi
ghij
hijk
ijk!
jk!!
k!!!
"""