python - 类方法上的装饰器

Question

我有一些子类，它们从父类继承了一些基本功能。子类应该有一个通用构造函数 prepare_and_connect_constructor() ，它在父类的对象创建过程中发挥了一些作用。为了简单起见，魔术是由一个简单的基于函数的装饰器完成的(最终，它应该是父类的一部分)。

def decorate_with_some_magic(func):
    def prepare_and_connect(*args, **kwargs):
        print("prepare something")
        print("create an object")
        obj = func(*args, **kwargs)
        print("connect obj to something")
        return obj

    return prepare_and_connect


class Parent:

    def __init__(self, a):
        self.a = a

    def __repr__(self):
        return f"{self.a}"


class Child(Parent):

    @classmethod
    @decorate_with_some_magic
    def prepare_and_connect_constructor(cls, a, b):
        """ use the generic connection decorator right on object creation """
        obj = super().__init__(a)
        # put some more specific attributes (over the parents class)
        obj.b = b
        return obj

    def __init__(self, a, b):
        """ init without connecting """
        super().__init__(a)
        self.b = b

    def __repr__(self):
        return f"{self.a}, {self.b}"


if __name__ == '__main__':
    print(Child.prepare_and_connect_constructor("special child", "needs some help"))

使用这个代码我终于得到了

obj = super().__init__(a)
  TypeError: __init__() missing 1 required positional argument: 'a'

运行prepare_and_connect_constructor()时。

实际上，我希望 super.__init__(a) 调用应该与 Child.__init__ 中的调用相同。我猜原因与classmethod有关，但我无法弄清楚。

这个调用有什么问题吗？

更新:一般来说，错误是 __init__ 不返回对象。

由于答案中的提示和想法，我修改了代码以实现我的需要:

class Parent:

    def __init__(self, a):
        self.a = a

    @staticmethod
    def decorate_with_some_magic(func):
        def prepare_and_connect(*args, **kwargs):
            print("prepare something")
            print("create an object")
            obj = func(*args, **kwargs)
            print("connect obj to something")
            return obj

        return prepare_and_connect

    def __repr__(self):
        return f"{self.a}"


class ChildWithOneName(Parent):

    @classmethod
    @Parent.decorate_with_some_magic
    def prepare_and_connect_constructor(cls, a, b):
        """ use the generic connection decorator right on object creation """
        obj = super().__new__(cls)
        obj.__init__(a, b)
        print("Does the same as in it's __init__ method")
        return obj

    def __init__(self, a, b):
        """ init without connecting """
        super().__init__(a)
        self.b = b

    def __repr__(self):
        return f"{self.a}, {self.b}"


class GodChild(Parent):

    @classmethod
    @Parent.decorate_with_some_magic
    def prepare_and_connect_constructor(cls, a, names):
        """ use the generic connection decorator right on object creation """
        obj = super().__new__(cls)
        obj.__init__(a, names)
        # perform some more specific operations
        obj.register_all_names(names)
        print("And does some more stuff than in it's __init__ method")
        return obj

    def __init__(self, a, already_verified_names):
        """ init without connecting """
        super().__init__(a)
        self.verified_names = already_verified_names

    def register_all_names(self, names=[]):
        self.verified_names = []

        def verify(text):
            return True

        for name in names:
            if verify(name):
                self.verified_names.append(name)


    def __repr__(self):
        return f"{self.a}, {self.verified_names}"


if __name__ == '__main__':
    print(ChildWithOneName.prepare_and_connect_constructor("special child", "needs some help"), end='\n\n')
    print(GodChild.prepare_and_connect_constructor("unknown child", "needs some verification"), end='\n\n')
    print(ChildWithOneName("my child", "is clean and doesn't need extra magic"))

• decorate_with_some_magic 现在是 Parent 类的一部分(使用静态方法)，因为它是相关的通用功能
• 每个子类(为了便于说明而添加了一个)都有自己的 prepare_and_connect_constructor classmethod，它调用自己的构造函数，并可选择执行一些额外的工作

Answer 1

您对魔术方法 __init__ 和 __new__ 略有误解。 __new__ 创建一个新对象，例如返回该类的一个实例。 __init__ 只是就地修改对象。因此，解决您的问题的简单方法如下:

@classmethod
@decorate_with_some_magic
def prepare_and_connect_constructor(cls, a, b):
    """ use the generic connection decorator right on object creation """
    obj = super().__new__(cls)
    obj.__init__(a)
    # put some more specific attributes (over the parents class)
    obj.b = b
    return obj

但是我认为你不应该这样使用它。相反，您可能应该覆盖 __new__