Getting realtime output from ffmpeg to be used in progress bar (PyQt4, stdout)

Question

I've looked at a number of questions but still can't quite figure this out. I'm using PyQt, and am hoping to run ffmpeg -i file.mp4 file.avi and get the output as it streams so I can create a progress bar.

I've looked at these questions: Can ffmpeg show a progress bar? catching stdout in realtime from subprocess

I'm able to see the output of a rsync command, using this code:

import subprocess, time, os, sys

cmd = "rsync -vaz -P source/ dest/"
p, line = True, 'start'


p = subprocess.Popen(cmd,
                     shell=True,
                     bufsize=64,
                     stdin=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                     stdout=subprocess.PIPE)

for line in p.stdout:
    print("OUTPUT>>> " + str(line.rstrip()))
    p.stdout.flush()

But when I change the command to ffmpeg -i file.mp4 file.avi I receive no output. I'm guessing this has something to do with stdout / output buffering, but I'm stuck as to how to read the line that looks like

frame=   51 fps= 27 q=31.0 Lsize=     769kB time=2.04 bitrate=3092.8kbits/s

Which I could use to figure out progress.

Can someone show me an example of how to get this info from ffmpeg into python, with or without the use of PyQt (if possible)

---

EDIT: I ended up going with jlp's solution, my code looked like this:

#!/usr/bin/python
import pexpect

cmd = 'ffmpeg -i file.MTS file.avi'
thread = pexpect.spawn(cmd)
print "started %s" % cmd
cpl = thread.compile_pattern_list([
    pexpect.EOF,
    "frame= *\d+",
    '(.+)'
])
while True:
    i = thread.expect_list(cpl, timeout=None)
    if i == 0: # EOF
        print "the sub process exited"
        break
    elif i == 1:
        frame_number = thread.match.group(0)
        print frame_number
        thread.close
    elif i == 2:
        #unknown_line = thread.match.group(0)
        #print unknown_line
        pass

Which gives this output:

started ffmpeg -i file.MTS file.avi
frame=   13
frame=   31
frame=   48
frame=   64
frame=   80
frame=   97
frame=  115
frame=  133
frame=  152
frame=  170
frame=  188
frame=  205
frame=  220
frame=  226
the sub process exited

Perfect!

Answer 1

In this specific case for capturing ffmpeg's status output (which goes to STDERR), this SO question solved it for me: FFMPEG and Pythons subprocess

The trick is to add universal_newlines=True to the subprocess.Popen() call, because ffmpeg's output is in fact unbuffered but comes with newline-characters.

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)

Also note that in this code sample the STDERR status output is directly redirected to subprocess.STDOUT