rle

def rle(word):
    out = []
    back = 0
    for front, char in enumerate(word):
        if not char.isalpha() or not char.isupper():
            raise Exception('char is not upper case letter:' + char)
        if front == len(word) - 1 or char != word[front+1]:
            out.append(word[back])
            count = front + 1 - back
            if count > 1:
                out.append(str(count))
            back = front+1
    return ''.join(out)


assert rle('AABB') == 'A2B2'
assert rle('AAAABBBCCXYZ') == 'A4B3C2XYZ'
assert rle('') == ''
# rle('AaA')