aaabbbccc -> a3b3c3

compress.py

def compress(input):
    if len(input) == 0:
        return ""
    output = []
    count = 1
    prev = input[0]
    for c in input[1:-1]:
        if c == prev:
            count += 1
        else:
            output.append(prev)
            output.append(str(count))
            count = 1
        prev = c
    
    if input[-1] == input[-2]:
        count += 1
    else:
        output.append(input[-2])
        output.append(str(count))
        count = 1
    output.append(input[-2])
    output.append(str(count))
    
    return ''.join(output)