def IncrementString(str_in, inc=1):
    # The last character of str_in must be in [A-Za-z0-9]
    # 'Z' wraps to 'A' with a carry to next-to-last character
    # 'z' wraps to 'z' with a carry to next-to-last character
    # '9' wraps to '0' with a carry to next-to-last character
    # So "aaaa" is legal, and "/foo/bar/aaa" is legal, but not "/foo/"
    # or "/foo/zzz" (which carries) because we can't increment the "/".
    chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

    if str_in == "":
        raise ValueError("String is not incrmentable: [%s]" % str_in)

    ch_last = str_in[-1]
    if ch_last not in chars:
        raise ValueError("String is not incrmentable: [%s]" % str_in)

    if ch_last == "z":
        ret = IncrementString(str_in[:-1]) + 'a'
    elif ch_last == "Z":
        ret = IncrementString(str_in[:-1]) + 'A'
    elif ch_last == "9":
        ret = IncrementString(str_in[:-1]) + '0'
    else:
        ret = str_in[:-1] + chr(ord(ch_last) + 1)
    return ret
# IncrementString