from subprocess import Popen, STDOUT, PIPE
from time import sleep

x = Popen('iosnoop -A -f ' + FileName, shell=True, stderr=STDOUT, stdout=PIPE, stdin=PIPE)
while x.poll() == None:
    output = x.stdout.readline()
    print(output)
    sleep(0.5)

print('IOsnoop has ended for some reason:')
print(x.stdout.read())