title: why is subprocess.popen returning an empty string
body: <p>I'm using python's subprocess.popen to fetch info of a video file.</p>
  
  <pre><code>output = Popen('ffmpeg -i "'+avifile+'" -dframes 0 -vframes 0', 
      executable="/bin/bash", stdout=PIPE, stderr=STDOUT,
      shell=True).communicate()[0]
  </code></pre>
  
  <p>The thing is whenever I run it the output variable is an empty string when I know there should be something. I can manually run ffmpeg fine.</p>
  
  <p>I'm thinking maybe its a problem with the pipes and redirecting I do. Was wondering if someone could fix this up.</p>