Two-finger iterator (Python recipe) by Karl Dickman

ActiveState Code (http://code.activestate.com/recipes/576997/)

What about:

seq = [1, 3, 9, 5, 22, 11]
for item, next in zip(seq, seq[1:]):
  print item, next

Don't use my way if iterating has side effects however

For longer seq i'd rather do that: (in Python 2.x)

from itertools import izip, islice
for item, next_item in izip(seq, islice(seq, 1, None)):
    print item, next_item  # next() is a builtin function (since Python 2.6)

But we still iterate over it twice.

So (in case of long seqences) the recipe by Karl is better.

You can also express the same idea in such a way:

iterator = iter(seq)
item = next(iterator)
for next_item in iterator:
    print item, next_item  # or anything else...
    item = next_item

I like Jan's final suggestion. I've modified my own copy to:

def two_finger(iterable):
    iterator = iter(iterable)
    item = next(iterator)
    for next_item in iterator:
        yield item, next_item
        item = next_item

The other problem with the code in comment #1 is that it really only works for lists. You can't use it with, say, xrange. Jan's first alternative works for sequences, but not for generators. If you use generators, you have to use islice(seq, 0, None)--but then it won't work with indexable sequences.

Jan's first alternative works for sequences, but not for generators. If you use generators, you have to use islice(seq, 0, None)

Neither islice(seq, 1, None) nor islice(seq, 0, None) do the task correctly using generators (and generally iterators). Of course, not because of using islice (it works with generators very well), but because the same iterator object is used twice by izip.

To have a universal solution this way, we'd use itertools.tee to split iterator into two, e.g.:

from itertools import izip, islice, tee
it1, it2 = tee(seq)
for item, next_item in izip(it1, islice(it2, 1, None)):
    print item, next_item

Though I'd preffer the solution with one iterator and item = next_item.

From the documentation of itertools ("Recipes" section, which is a very fine read):

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

Leave it to the people who wrote the language to come up with the best version. :)