该itertools例子提供了window，不只是该函数：
from itertools import islice
def window(seq, n=2):
"Returns a sliding window (of width n) over data from the iterable"
" s -> (s0,s1,...s[n-1]), (s1,s2,...,sn), ... "
it = iter(seq)
result = tuple(islice(it, n))
if len(result) == n:
yield result
for elem in it:
result = result[1:] + (elem,)
yield result
用法示例：
>>> ["".join(x) for x in window("7316717", 3)]
['731', '316', '167', '671', '717']
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