title: Python subprocess package does not report errors from subprocess?
body: <p>On some machine, for the following piece of code</p>
  
  <pre><code>p = subprocess.Popen(cmd, stdout = subprocess.PIPE, stderr = subprocess.STDOUT);
  out, err = p.communicate()
  </code></pre>
  
  <p>This script would just hang at <code>p.communicate()</code> and does not return. </p>
  
  <p>After I manually run the command, I finally see the error messages. </p>
  
  <p>Why is this, and how should I solve it? </p>
  
  <p>Thanks. </p>