def Run(command):
    proc = subprocess.Popen(command, bufsize=1, stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT, universal_newlines=True)
return proc

def Trace(proc):
    while proc.poll() is None:
        line = proc.stdout.readline()
if line:
    print('Read line', line)
proc = Run([handbrakePath] + allOptions)
Python отслеживает stderr и stdout подпроцесса