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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def check_output(self, **kwargs):
 """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. """

    env, kwargs = self._prepare_env(kwargs)
    return subprocess.check_output(self.cmd, env=env, **kwargs).decode('utf-8')