code: def check_output(self, **kwargs):
      
      env, kwargs = self._prepare_env(kwargs)
      return subprocess.check_output(self.cmd, env=env, **kwargs).decode('utf-8')
docs: Runs this command returning its captured stdout.
  
      :param kwargs: Any extra keyword arguments to pass along to `subprocess.Popen`.
      :returns: The captured standard output stream of the command.
      :rtype: string
      :raises: :class:`subprocess.CalledProcessError` if the command fails.