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from itertools import zip, tee, chain, islice

def prev_curr_next(iterable):
    prev, curr, nxt = tee(iterable, 3)
    prev = chain([None], prev)
    nxt = chain(islice(nxt, 1, None), [None])
    return zip(prev, curr, nxt)

"""
Usage:

for prev, curr, nxt in prev_curr_next('abcde'):
    print prev, curr, nxt

Output: 

prev None curr a next b
prev a curr b next c
prev b curr c next d
prev c curr d next e
prev d curr e next None
"""