letter_counter.py

def letter_count(s):
    counter, max_index = 1, len(s) - 1
    out = []

    for index, letter in enumerate(s):
        if index < max_index:
            if letter == s[index + 1]:
                counter += 1
                continue

        out.append("{}{}".format(counter, letter))
        counter = 1

    return "".join(out)

letter_count("aaaaabbbbccccccaaaaaaa") # '5a4b6c7a'