p=Popen(cmd, stdin=PIPE, stdout=PIPE)
for ....: # a few million
    p.stdin.write(...)
p.stdin.close()
out = p.stdout.read()
p.stdout.close()
exitcode = p.wait()

l = list()
for line in sys.stdin:
   l.append(line)
sys.stdout.write(str(len(l)))