How to access the previous/next element in a for loop?

by

Expressed as a generator function:

def neighborhood(iterable):
    iterator = iter(iterable)
    prev_item = None
    current_item = next(iterator)  # throws StopIteration if empty.
    for next_item in iterator:
        yield (prev_item, current_item, next_item)
        prev_item = current_item
        current_item = next_item
    yield (prev_item, current_item, None)

Usage:

for prev,item,next in neighborhood(l):
    print prev, item, next

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