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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 auth_string(self):
 """ Get the auth string. If the token is expired and auto refresh enabled, a new token will be fetched :return: the auth string :rtype: str """

        if not self._token:
            self.execute()

        if not self._token.expired:
            return 'Bearer {}'.format(self._token.access_token)

        if self.auto_refresh:
            self.execute()
            return 'Bearer {}'.format(self._token.access_token)

        raise TokenExpired()