Iterate a list as pair (current, next) in Python

Question

Here's a relevant example from the itertools module docs:

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

For Python 2, you need itertools.izip instead of zip:

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

How this works:

First, two parallel iterators, a and b are created (the tee() call), both pointing to the first element of the original iterable. The second iterator, b is moved 1 step forward (the next(b, None)) call). At this point a points to s0 and b points to s1. Both a and b can traverse the original iterator independently - the izip function takes the two iterators and makes pairs of the returned elements, advancing both iterators at the same pace.

One caveat: the tee() function produces two iterators that can advance independently of each other, but it comes at a cost. If one of the iterators advances further than the other, then tee() needs to keep the consumed elements in memory until the second iterator comsumes them too (it cannot 'rewind' the original iterator). Here it doesn't matter because one iterator is only 1 step ahead of the other, but in general it's easy to use a lot of memory this way.

And since tee() can take an n parameter, this can also be used for more than two parallel iterators:

def threes(iterator):
    "s -> (s0,s1,s2), (s1,s2,s3), (s2, s3,4), ..."
    a, b, c = itertools.tee(iterator, 3)
    next(b, None)
    next(c, None)
    next(c, None)
    return zip(a, b, c)

Roll your own!

def pairwise(iterable):
    it = iter(iterable)
    a = next(it, None)

    for b in it:
        yield (a, b)
        a = b

I’m just putting this out, I’m very surprised no one has thought of enumerate().

for (index, thing) in enumerate(the_list):
    if index < len(the_list):
        current, next_ = thing, the_list[index + 1]
        #do something

Since the_list[1:] actually creates a copy of the whole list (excluding its first element), and zip() creates a list of tuples immediately when called, in total three copies of your list are created. If your list is very large, you might prefer

from itertools import izip, islice
for current_item, next_item in izip(the_list, islice(the_list, 1, None)):
    print(current_item, next_item)

which does not copy the list at all.

Starting in Python 3.10, this is the exact role of the pairwise function:

from itertools import pairwise

list(pairwise([1, 2, 3, 4, 5]))
# [(1, 2), (2, 3), (3, 4), (4, 5)]

or simply pairwise([1, 2, 3, 4, 5]) if you don't need the result as a list.

Iterating by index can do the same thing:

#!/usr/bin/python
the_list = [1, 2, 3, 4]
for i in xrange(len(the_list) - 1):
    current_item, next_item = the_list[i], the_list[i + 1]
    print(current_item, next_item)

Output:

(1, 2)
(2, 3)
(3, 4)

This is now a simple Import As of 16th May 2020

from more_itertools import pairwise
for current, next in pairwise(your_iterable):
  print(f'Current = {current}, next = {nxt}')

Docs for more-itertools Under the hood this code is the same as that in the other answers, but I much prefer imports when available.

If you don't already have it installed then: pip install more-itertools

Example

For instance if you had the fibbonnacci sequence, you could calculate the ratios of subsequent pairs as:

from more_itertools import pairwise
fib= [1,1,2,3,5,8,13]
for current, nxt in pairwise(fib):
    ratio=current/nxt
    print(f'Curent = {current}, next = {nxt}, ratio = {ratio} ')