fn.py

class Biz(object):
    def __init__(self, val_a: str, val_b: str, val_c: str, val_d: str) -> None:
        self._val_a = val_a
        self._val_b = val_b
        self._val_c = val_c
        # 还没有初始化_val_d，但是make_new_val引用了这个变量
        self._new_val = self.make_new_val()
        # 调用了make_new_val后才初始化了_val_d
        self._val_d = val_d

    def make_new_val(self) -> str:
        return self._val_a + self._val_b + self._val_c + self._val_d


if __name__ == '__main__':
    Biz("foo", "bar", "foo1", "bar1").make_new_val()
    # 执行时程序报错AttributeError: 'Biz' object has no attribute '_val_d'