So I’ve been using this code in a few programs at work:

p = subprocess.Popen(...)
for line in p.stdout.readline():
    ...
    print(line)

It turns out there’s a bunch of output buffering going on here. You could put a sys.stdout.flush() after that print, but it won’t help.

The iterator buffers. Do this:

p = subprocess.Popen(...)
while True:
  line = p.stdout.readline()
  if not line:
      break
    ---
    print(line)

Et violà! No buffering.