test.py

import subprocess
import os

r,w = os.pipe()
proc = subprocess.Popen(
    "echo 123456123123123123123123123123 >&" + str(w) + " && (sleep 2 && echo bar >&" + str(w) + ") && sleep 4 && exec " + str(w) +"<&- && sleep 6 && echo foo >&" + str(w),
    shell = True,
    executable = '/bin/bash', # use bash so we can make use of process subsitution
    pass_fds = (w,)
)
os.close(w)

has_whole_video_been_processed = False
while True:
    if not has_whole_video_been_processed:
        in_bytes = os.read(r, 1)
    if not in_bytes:
        print("done processing")
        has_whole_video_been_processed = True
        break;
    else:
        print('read: ' + in_bytes.decode("utf-8"))

print("done")