code: def auth(self, request):
          
          request_token = super(ServiceTumblr, self).auth(request)
          callback_url = self.callback_url(request)
  
          # URL to redirect user to, to authorize your app
          auth_url_str = '{auth_url}?oauth_token={token}'
          auth_url_str += '&oauth_callback={callback_url}'
          auth_url = auth_url_str.format(auth_url=self.AUTH_URL,
                                         token=request_token['oauth_token'],
                                         callback_url=callback_url)
  
          return auth_url
docs: let's auth the user to the Service
              :param request: request object
              :return: callback url
              :rtype: string that contains the url to redirect after auth