code: def auth_string(self):
          
          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()
docs: 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