count.py

#!/usr/bin/env python

def main():
    for i in range(1, 101):
        print(i)

if __name__ == '__main__':
    main()

subprocess_sample.py

import subprocess

def main():
    p = subprocess.Popen(['./count.py'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
    while 1:
        output = p.stdout.readline()
        print(output)
        if not output:
            break
        if fizzbuzz(int(output)) != int(output):
            p.stdin.write(fizzbuzz(int(output)).encode('utf-8'))

def fizzbuzz(number):
    if number % 15 == 0:
        return 'fizzbuzz'
    elif number % 3 == 0:
        return 'fizz'
    elif number % 5 == 0:
        return 'buzz'
    else:
        return number

if __name__ == "__main__":
    main()