Properly using subprocess.PIPE in python?

Question

I'm trying to use subprocess.Popen to construct a sequence to grab the duration of a video file. I've been searching for 3 days, and can't find any reason online as to why this code isn't working, but it keeps giving me a blank result:

import sys
import os
import subprocess

def main():
  the_file = "/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
  ffmpeg = subprocess.Popen(['/opt/local/bin/ffmpeg', '-i', the_file], stdout = subprocess.PIPE, )
  grep = subprocess.Popen(['grep', 'Duration'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )
  cut = subprocess.Popen(['cut', '-d', ' ', '-f', '4'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )
  sed = subprocess.Popen(['sed', 's/,//'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )

  duration = sed.communicate()
  print duration

if __name__ == '__main__':
  main()

Answer 1

As others have pointed out, you need to pass the PIPE from one process to the next. The stdout (PIPE) from one process becomes the stdin for the following task.

Something like this (starting from your example):

import sys
import os
import subprocess

def main():
  the_file = "/Volumes/Footage/Acura/MDX/
              2001/Crash Test/01 Acura MDX Front Crash.mov"
  ffmpeg = subprocess.Popen(['/opt/local/bin/ffmpeg', '-i', the_file],
                            stdout = subprocess.PIPE)
  grep = subprocess.Popen(['grep', 'Duration'], 
                          stdin = ffmpeg.stdout, stdout = subprocess.PIPE)
  cut = subprocess.Popen(['cut', '-d', ' ', '-f', '4'],
                         stdin = grep.stdout, stdout = subprocess.PIPE)
  sed = subprocess.Popen(['sed', 's/,//'],
                         stdin = cut.stdout, stdout = subprocess.PIPE)

  duration = sed.communicate()[0]
  print duration

if __name__ == '__main__':
  main()

Answer 2

stderr needs to be redirected to stdout. Also, there's no need to call other tools like cut/sed etc. do your string manipulation in Python

import subprocess
....
the_file = "/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
ffmpeg = subprocess.Popen(['/usr/bin/ffmpeg', '-i', the_file], stderr=subprocess.STDOUT,stdout = subprocess.PIPE )
out, err = ffmpeg.communicate()
if "Duration" in out:
    print out[out.index("Duration"):].split()[1]

If Python is not a must, you can use the shell directly.

the_file="/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
ffmpeg -i "$file" 2>&1 | awk '/Duration/{print $2}'