Popen.communicate() returns only err when running ffmpeg

Question

I'm trying to run ffmpeg from python and get some reasonable output. Doesn't matter what ffmpeg's log level is set, the only output I get from Popen.communicate() is err one. The content isn't an error however. I tried the same code to run other commands (ls) and it seems to be Ok with both out and err outputs.

I checked here and Google and unfortunately found nothing. Most of the examples use os or commands modules, not subprocess.

This is my test code:

command = [
        'ffmpeg',
        '-v', 'debug',
        '-i', '1.mov',
        '-vcodec', 'libx264',
        '-profile:v', 'high',
        '-preset', 'slower',
        '-b:v', '1000k',
        '-vf', 'scale=-1:720',
        '-threads', '0', 
        '-acodec', 'libfdk_aac',
        '-b:a', '192k',
        '-y',   
        '2.mp4',
        ]
p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
out, err =  p.communicate()
f = open('out.log', 'w')
f.write(out)
f.close()
f = open('error.log', 'w')
f.write(err)
f.close()

Have you ever see something like this? Am I doing any mistakes or is there an bug in ffmpeg?

Answer 1

I seemed to misinterpreted stdout and stderr in case of ffmpeg. All the "console output" is normally directed to stderr. Redirecting stderr to stdout shall solve the issue. This is how:

p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

Sorry for this ... I was too concentrated to Python part that I completely overlooked quite obvious thing. :-)