Most pythonic way to get the previous element

Question

I would like an enumerate-like functional on iterators which yields the pair (previous_element, current_element). That is, given that iter

Answer 1

The best answer I have (and this requires itertools) is

def offset(iter, n=1):
    # returns tuples (None, iter0), (iter0, iter1), (iter1, iter2) ...
    previous = chain([None] * n, iter)
    return izip(previous, iter)

but I would be interested in seeing if someone has a one-liner (or a better name than offset for this function)!