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

What about the simple (obvious) solution?

def offset(iterable):
    prev = None
    for elem in iterable:
        yield prev, elem
        prev = elem

Answer 2

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)!

Answer 3

def pairwise(iterable):
    """s -> (s0,s1), (s1,s2), (s2, s3), ...
    see http://docs.python.org/library/itertools.html
    """
    a, b = itertools.tee(iterable)
    b.next()
    return itertools.izip(a, b)

EDIT moved doc string into the function

Answer 4

def offset(iter, n=1, pad=None):
    i1, i2 = itertools.tee(iter)
    i1_padded = itertools.chain(itertools.repeat(pad, n), i1)
    return itertools.izip(i1_padded, i2)

@bpgergo + @user792036 = this. Best of two worlds :).

Answer 5

To put more itertools on the table:

from itertools import tee, izip, chain

def tee_zip(iterable):
   a, b = tee(iterable)
   return izip(chain([None], a), b)