from itertools import tee, chain, islice, izip
import random
import timeit
 
 
def prev_cur_next_1(sequence):
    index = 0
    while index < len(sequence):
        yield (sequence[index - 1] if index else None,
               sequence[index],
               sequence[index + 1] if index + 1 < len(sequence) else None)
        index += 1
    raise StopIteration
 
 
def prev_cur_next_2(list_arg):
    extended_list = [None]
    extended_list.extend(list_arg)
    extended_list.append(None)
    return (extended_list[i - 1:i + 2] for i in xrange(1, len(extended_list) - 1))
 
 
def prev_cur_next_3(some_iterable):
    prevs, items, nexts = tee(some_iterable, 3)
    prevs = chain([None], prevs)
    nexts = chain(islice(nexts, 1, None), [None])
    return izip(prevs, items, nexts)
 
 
def prev_cur_next_4(seq):
    return izip(chain([None], seq), seq, chain(islice(seq, 1, None), [None]))
 
 
data = [random.random() for i in xrange(1000000)]
 
samples = [prev_cur_next_1, prev_cur_next_2, prev_cur_next_3, prev_cur_next_4]
 
for sample_fx in samples:
    interval = timeit.timeit(lambda: list(sample_fx(data)), number=1)
    print '%s executed in %.3f seconds' % (sample_fx, interval)
 

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