Piping to FFMPEG with Python subprocess freezes

With the following code, I am able to pipe frames of a video to FFMPEG using Python, Numpy and the FFMPEG binaries:

from __future__ import print_function
import subprocess
import numpy as np
import sys

npshape = [480, 480]
cmd_out = ['ffmpeg',
           '-y', # (optional) overwrite output file if it exists
           '-f', 'rawvideo',
           '-vcodec','rawvideo',
           '-s', '%dx%d'%(npshape[1], npshape[0]), # size of one frame
           '-pix_fmt', 'rgb24',
           '-r', '24', # frames per second
           '-i', '-', # The input comes from a pipe
           '-an', # Tells FFMPEG not to expect any audio
           '-vcodec', 'mpeg4',
           'output.mp4']

fout = subprocess.Popen(cmd_out, stdin=subprocess.PIPE, stderr=subprocess.PIPE).stdin

for i in range(24*40):
    if i%(24)==0: 
        print('%d'%(i/24), end=' ')
        sys.stdout.flush()

    fout.write((np.random.random(npshape[0]*npshape[1]*3)*128).astype('uint8').tostring())

fout.close()

This works fine if I write anything less than 37 seconds worth of frames, but if I try to write anything more, the code just hangs. What is the underlying cause for this behaviour? How can I fix it?

A highly probable culprit is the disgustingly stinky subprocess.Popen line. Not only you ignore its return value - which you must never do, in order to ensure the subprocess' completion by certain point and/or check its exit code - you also make stderr a pipe but never read it - so the process must be hanging when its buffer fills.

This should fix it:

p = subprocess.Popen(cmd_out, stdin=subprocess.PIPE)
fout = p.stdin

<...>

fout.close()
p.wait()
if p.returncode !=0: raise subprocess.CalledProcessError(p.returncode,cmd_out)