code: def unauthenticated_view(self):
          
          # Prepare Flash message
          url = request.url
          flash(_("You must be signed in to access '%(url)s'.", url=url), 'error')
  
          # Redirect to USER_UNAUTHENTICATED_ENDPOINT
          safe_next_url = self.make_safe_url(url)
          return redirect(self._endpoint_url(self.USER_UNAUTHENTICATED_ENDPOINT)+'?next='+quote(safe_next_url))
docs: Prepare a Flash message and redirect to USER_UNAUTHENTICATED_ENDPOINT