Counting letters in a string

counting.py

# From https://www.linkedin.com/groups/25827/25827-6166706414627627011
# "Today i did a simple test that you give a string to a function, and then the 
# return output should count the letters sequencially; as following example:
# in: "aaaaabbbbccccccaaaaaaa" => out: "5a4b6c7a"
#

def func(input_string):
    if not input_string:
        return ""
    if len(input_string) == 1:
        return "1%s" % input_string
    change_points = []
    start = 0
    for x in range(0, len(input_string) - 1):
        if input_string[x] != input_string[x+1]:
            change_points.append((start, x+1))
            start = x + 1
    change_points.append((start, len(input_string)))
    return "".join(["%s%s" % (end - start, input_string[start]) for start, end in change_points])