Simple digest algorithm demonstration

digest.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import itertools

def simple_digest(input_string):
    chars = {}
    for c in input_string:
        chars[c] = chars.get(c, 0) + 1
    items = iter(sorted(chars.items()))
    pairs = map(lambda pair: [str(pair[1]), pair[0],], items)
    result = itertools.chain.from_iterable(pairs)
    result = ''.join(result)

    return result

if __name__ == '__main__':
    input_string = 'KNYSDFDJBYSDFDJYQ'
    result = simple_digest(input_string)
    print(result)
    # Expected output:
    assert(result == '1B4D2F2J1K1N1Q2S3Y')