decoder.py

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

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 [num for num in (''.join(map(str, [mapping[l] for l in letters])))]

def gen_pairs(numbers, mapping):
    pairs = []
    for i, l in enumerate(numbers):
        if i == (len(numbers) - 1):
            pairs.append(l)
            break
        else:
            pairs.append((numbers[i], numbers[i+1]))
    return [''.join(p) for p in pairs]


def check_pairs(numbers, mapping, pairs):
    for pair in pairs:
        pair = int(pair)
        if pair < 27:
            for k,v in mapping.items():
                if pair == v:
                    #print k
                    yield k
        elif pair >= 27:
           pair = [num for num in map(str, [pair])]  
           for i, n in enumerate(pair):
               for k,v in mapping.items():
                   if pair[i] == v:
                       yield k
            

if __name__ == '__main__':
    letters = 'hello'
    mapping = make_map(coded_vals())
    numbers = alpha_to_num(letters,mapping)
    trans = gen_pairs(numbers, mapping)
    checker = ''.join([n for n in check_pairs(numbers, mapping, trans)])
    print 'Original letters: {}\nCoded: {}\n'.format(letters, numbers)
    print 'Coded numbers translated to alpha: {}\n'.format(checker)