调用子方法时如何强制执行父方法调用？

Question

我想要强制当子类从父类继承并且它覆盖父方法而不显式调用它时，会引发错误。 该错误可能在初始化错误类或调用方法时引发。

目标是确保 Mother 类的用户执行了一些体现在 Mother 方法中的操作。

例子

class Mother():
    def necessary_method(self):
         # do some necessary stuff

class GoodChild(Mother):
    def necessary_method(self):
        # necessary parent call
        super().necessary_method()

class BadChild(Mother):
    def necessary_method(self):
         # no parent call
         return

调用时：

good = GoodChild()
# works fine
bad = BadChild()
# exception could be raised here

good.necessary_method()
# works fine
bad.necessary_method()
# exception could be raised here

这真的可能吗？ 欢迎任何答案或解决方法。

Answer 1

是的，这绝对是可能的，至少在运行时是这样。你可以创建一个Metaclass 来修改类的创建方式，这个想法是通过添加一个mother_method_called 标志来更改necessary_method 签名，该标志只有在调用Mother 版本时才会为真，否则它会继续提出Value Error。比如python3:

class MotherMetaClass(type):
    def __new__(cls, name, bases, attrs):
        method_name = 'necessary_method'
        mother_class_name = 'Mother'
        original_necessary_method = attrs.get(method_name)

        def necessary_method_validated(*args, **kwargs):
            original_necessary_method(*args, **kwargs)

            if name == mother_class_name:
                cls.mother_method_called = True
            else:
                if not cls.mother_method_called:
                    raise ValueError(f'Must call "super()" when overriding "{method_name}".')
                cls.mother_method_called = False

        attrs[method_name] = necessary_method_validated

        return super().__new__(cls, name, bases, attrs)


class Mother(metaclass=MotherMetaClass):
    def necessary_method(self):
        print('Parent method called.')


class GoodChild(Mother):
    def necessary_method(self):
        super().necessary_method()
        print('Good child method called.')


class BadChild(Mother):
    def necessary_method(self):
        print("Bad child method called.")


a = GoodChild()
a.necessary_method()  # it's going to work properly


b = BadChild()
b.necessary_method()  # it's going to raise Value Error