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

Question

Is there a way to access a list's (or tuple's, or other iterable's) next or previous element while looping through it with a for loop?

l = [1, 2, 3]
for item in l:
    if item == 2:
        get_previous(l, item)

Answer 1

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

Answer 2

l = [1, 2, 3]

for i, j in zip(l, l[1:]):
    print(i, j)

Answer 3

l = [1, 2, 3]
for i, item in enumerate(l):
    if item == 2:
        previous = l[i - 1]
        print(previous)

Output:

1

This will wrap around and return the last item in the list if the item you're looking for is the first item in list. In other words changing the third line to if item == 1: in the above code will cause it to print 3.