Iterate over pairs in a list (circular fashion) in Python

Question

The problem is easy, I want to iterate over each element of the list and the next one in pairs (wrapping the last one with the first).

I've thought about two unpythonic ways of doing it:

def pairs(lst):     n = len(lst)     for i in range(n):         yield lst[i],lst[(i+1)%n] 

and:

def pairs(lst):     return zip(lst,lst[1:]+[lst[:1]]) 

expected output:

>>> for i in pairs(range(10)):     print i  (0, 1) (1, 2) (2, 3) (3, 4) (4, 5) (5, 6) (6, 7) (7, 8) (8, 9) (9, 0) >>>  

any suggestions about a more pythonic way of doing this? maybe there is a predefined function out there I haven't heard about?

also a more general n-fold (with triplets, quartets, etc. instead of pairs) version could be interesting.

Answer 1

def pairs(lst):     i = iter(lst)     first = prev = item = i.next()     for item in i:         yield prev, item         prev = item     yield item, first 

Works on any non-empty sequence, no indexing required.

Answer 2

I've coded myself the tuple general versions, I like the first one for it's ellegant simplicity, the more I look at it, the more Pythonic it feels to me... after all, what is more Pythonic than a one liner with zip, asterisk argument expansion, list comprehensions, list slicing, list concatenation and "range"?

def ntuples(lst, n):     return zip(*[lst[i:]+lst[:i] for i in range(n)]) 

The itertools version should be efficient enough even for large lists...

from itertools import * def ntuples(lst, n):     return izip(*[chain(islice(lst,i,None), islice(lst,None,i)) for i in range(n)]) 

And a version for non-indexable sequences:

from itertools import * def ntuples(seq, n):     iseq = iter(seq)     curr = head = tuple(islice(iseq, n))     for x in chain(iseq, head):         yield curr         curr = curr[1:] + (x,) 

Anyway, thanks everybody for your suggestions! :-)