code: def callback_url(self, request):
          
          service = self.service.split('Service')[1].lower()
          return_to = '{service}_callback'.format(service=service)
          return '%s://%s%s' % (request.scheme, request.get_host(), reverse(return_to))
docs: the url to go back after the external service call
              :param request: contains the current session
              :type request: dict
              :rtype: string