Phone number -> letter permutations

phoneperms.py

from sys import argv
from string import ascii_uppercase


def map_number(number):
    """ Maps a number's digits to its possible phone letters """
    sets = []
    for n in str(number):
        assert n != '0'
        pos = (int(n) - 1) * 3
        s = ascii_uppercase[pos:pos + 3]
        sets.append(list(s))
    return sets


def update_indices(indices, lens, index=-1):
    if abs(index) > len(indices):
        return
    indices[index] = (indices[index] + 1) % lens[index]
    if indices[index] == 0:
        update_indices(indices, lens, index=index - 1)


def compute_perms(number):
    sets = map_number(number)
    lens = [len(s) for s in sets]
    permcount = reduce(lambda x, y: x * y, lens)
    indices = [0] * len(sets)
    perms = []
    for i in xrange(permcount):
        perm = ''.join([sets[n][i] for n, i in enumerate(indices)])
        perms.append(perm)
        update_indices(indices, lens)
    return perms


if __name__ == '__main__':
    perms = compute_perms(argv[1])
    print '\n'.join(perms)