如何在保存一次后将模型实例设为只读？

class MyModel:
    def __init__(self):
        self.is_readonly = False

    def save(self):
        if self.is_readonly:
            raise ReadOnlyError("This model instance is read-only.")
        # 保存模型实例的代码

    def set_readonly(self):
        self.is_readonly = True

my_model = MyModel()
my_model.set_readonly()