The itertools version is quite lovely, but if you must do an iterative version please at least make it idiomatic. If you really needed an index, you could at least use enumerate() ;-P

mystr = "ABBBCCDDZ"

def get_iterparts(mystr):
    buf = ""
    for char in mystr:
        if not buf or buf.endswith(char):
            buf += char
        else:
            yield buf
            buf = char
    if buf:
        yield buf
print list(get_iterparts(mystr))

def get_parts(mystr):
    parts = []
    buf = ""
    for char in mystr:
        if not buf or buf.endswith(char):
            buf += char
        else:
            parts.append(buf)
            buf = char
    if buf:
        parts.append(buf)
    return parts
print get_parts(mystr)
[download]