A simple generator that returns a incrementing string of characters a-z, aa-zz, aaa-zzz, etc.

letters.py

import itertools
import string
import math

default_alphabet = string.uppercase
 
def product_depth_first(alphabet=default_alphabet):

    alphabet_list = [alphabet]
    
    while (True):
        for result in itertools.product(*alphabet_list):
            yield ''.join(result)
        
        alphabet_list.append(alphabet)

def product_straight(cols, alphabet=default_alphabet):
    alphabet_list = [alphabet for x in range(cols)]

    for result in itertools.product(*alphabet_list):
        yield ''.join(result)

find_maxlen_straight = lambda num_items, alphabet=default_alphabet: int(math.ceil(math.log(num_items,len(alphabet))))

if __name__ == '__main__':
    
    items = range(678)
    max_len = find_maxlen_straight(len(items))
    letter_factory = product_straight(max_len)

    for i in items:
        print letter_factory.next()