new_contents: #!/usr/bin/env python
  # Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
  # Use of this source code is governed by a BSD-style license that can be
  # found in the LICENSE file.
  #
  # Backport of subprocess.check_output from python2.7
  #  http://svn.python.org/view/*checkout*/python/tags/r271/Lib/subprocess.py
  #
  # Headers for that file are:
  # subprocess - Subprocesses with accessible I/O streams
  #
  # For more information about this module, see PEP 324.
  #
  # This module should remain compatible with Python 2.2, see PEP 291.
  #
  # Copyright (c) 2003-2005 by Peter Astrand <astrand@lysator.liu.se>
  #
  # Licensed to PSF under a Contributor Agreement.
  # See http://www.python.org/2.4/license for licensing details.
  
  import subprocess
  import unittest
  
  
  def check_output(*popenargs, **kwargs):
      r"""Run command with arguments and return its output as a byte string.
  
      If the exit code was non-zero it raises a CalledProcessError.  The
      CalledProcessError object will have the return code in the returncode
      attribute and output in the output attribute.
  
      The arguments are the same as for the Popen constructor.  Example:
  
      >>> check_output(["ls", "-l", "/dev/null"])
      'crw-rw-rw- 1 root root 1, 3 Oct 18  2007 /dev/null\n'
  
      The stdout argument is not allowed as it is used internally.
      To capture standard error in the result, use stderr=STDOUT.
  
      >>> check_output(["/bin/sh", "-c",
      ...               "ls -l non_existent_file ; exit 0"],
      ...              stderr=STDOUT)
      'ls: non_existent_file: No such file or directory\n'
      """
      if 'stdout' in kwargs:
          raise ValueError('stdout argument not allowed, it will be overridden.')
      process = subprocess.Popen(stdout=subprocess.PIPE, *popenargs, **kwargs)
      output, unused_err = process.communicate()
      retcode = process.poll()
      if retcode:
          cmd = kwargs.get("args")
          if cmd is None:
              cmd = popenargs[0]
          raise subprocess.CalledProcessError(retcode, cmd)
      return output
  
  
  class CheckOutputTester(unittest.TestCase):
    def test_check_output(self):
      self.assertRaises(subprocess.CalledProcessError,
                        check_output, '/bin/false')
  
      self.assertRaises(ValueError,
                        check_output, ['/bin/true'], stdout='/tmp/nonexistant')
  
      self.assertEqual('hello\n',
                       check_output(['/bin/echo', 'hello']))
  
      echoer = subprocess.Popen(['/bin/echo', 'hello'], stdout=subprocess.PIPE)
      self.assertEqual('hello\n',
                       check_output(['/bin/cat'], stdin=echoer.stdout))
  
  if __name__ == '__main__':
    unittest.main()
  
subject: Add subprocess.check_output