def render(self, context): instance = self.instance.resolve(context) app_label = instance._meta.app_label model = instance._meta.object_name.lower() url = reverse('image_preview', args=(app_label, model, instance.id, self.size)) if not url_exists(url): url = None if self.context: context[self.context] = url else: context['image_preview'] = url return ''