def instance(cls, name):
        """Instantiate a model class according to import path
        args:
            name: class import path like `user.User`
        return:
            model instance
        """
        if not cls._instance.get(name):
            model_name = name.split('.')
            ins_name = '.'.join(
                ['models', model_name[0], 'model', model_name[1]])
            cls._instance[name] = cls.import_model(ins_name)()

        return cls._instance[name]
Instantiate a model class according to import path
        args:
            name: class import path like `user.User`
        return:
            model instance