http://www.codewars.com/kata/the-observed-pin/python

get_pins.py

# coding=utf-8
from itertools import product


adjacents = ('08', '124', '1253', '236', '1457', '24568', '3569', '478', '57890', '689')
adjacents = {str(i): s for i, s in enumerate(adjacents)}


def get_pins_itertools(observed):
    for pins in product(*(adjacents[ch] for ch in observed)):
        yield "".join(pins)


def get_pins_recursive(observed):
    # split off the first character
    first, rest = observed[:1], observed[1:]
    if not first:
        return
    if not rest:
        # we are on the last digit
        for ch in adjacents[first]:
            yield ch
    for key in adjacents[first]:  # loop over our first digit
        for tail in get_pins_recursive(rest):  # add on the results for the rest
            yield key + tail