FFMPEG and Pythons subprocess

I've often noticed problems reading standard output (or even standard error!) with subprocess, due to buffering issues that are hard to defeat. My favorite solution, when I do need to read such stdout/stderr from the subprocess, is to switch to using, instead of subprocess, pexpect (or, on Windows, wexpect).

Simply add ,universal_newlines=True to your subprocess.Popen line.

cmd="ffmpeg -i in.mp4 -y out.avi"
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,universal_newlines=True)
for line in process.stdout:
    print(line)

For now you got line in cycle like:

frame= 1900 fps=453 q=18.6 Lsize=    3473kB time=00:01:16.08 bitrate= 373.9kbits/s

Use the time= value to determine progress in percentage.