Simple run length encoding problem

simple_run_length_encoding.py

def compress(s):
    """
        s -> string to be compressed
        retuns -> compressed string
    """
    if not s:
        return ""

    prev = None
    count = 0
    result = []
    
    for c in s:
        if prev and prev != c:
            result.append(prev)
            result.append(str(count))
            count = 0
        count+=1
        prev = c
    result.append(prev)
    result.append(str(count))
    return "".join(result)
    
if __name__ == '__main__':
    s = "AAABBCCDDDDEFFFFF"
    print compress(s)
    
    s = ""
    print compress(s)
    
    s = "ABCD"
    print compress(s)
    
    s = "A"
    print compress(s)