decoder.py

#!/usr/bin/env python
from string import lowercase
from collections import defaultdict
from pprint import pprint

def coded_vals():
    for i, l in enumerate(lowercase):
        yield l, i+1


def make_map(code_map):
    d = defaultdict(int)
    for k,v in code_map:
        d[k] += v
    return d

def alpha_to_num(letters, mapping):
    return [mapping[l] for l in letters]

def num_to_alpha(nums, mapping):
    contains = []
    for i in xrange(len(nums)):
        contains.append(nums[i])
        if i+1 < len(nums):
            compound = ''.join(map(str, [nums[i], nums[i+1]]))
            print compound
            if int(compound) <= 26:
                contains.append(compound)
            #contains.append(''.join((map(str, [nums[i],nums[i+1]]))))
    return contains

if __name__ == '__main__':
    letters = 'hello'
    mapping = make_map(coded_vals())
    nums = alpha_to_num(letters,mapping)
    print 'original text: {}\n'.format(letters)
    print 'coded to numbers: {}\n'.format(''.join(map(str, nums)))
    #print 'coding back to letters: {}\n'.format(num_to_alpha(nums, mapping))
    alpha = num_to_alpha(nums, mapping)
    print alpha