类方法上的装饰器

Question

我有从父类继承一些基本功能的子类。 子类应该有一个通用构造函数prepare_and_connect_constructor()，它围绕父类的对象创建做了一些魔术。 为简单起见，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"))

使用这段代码我终于得到了

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__ 中的相同。 我猜原因与classmethod有关，但我想不通。

这个电话有什么问题？

更新：总的来说，问题在于__init__ 没有返回对象。

由于答案中的提示和想法，我修改了我的代码以实现我所需要的：

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），因为它是一个相关的通用功能
• 每个子类（为了说明而添加了一个）都有自己的prepare_and_connect_constructor classmethod，它调用自己的构造函数并选择性地执行一些额外的工作

Answer 1

你对__init__和__new__的魔术方法有一点误解。 __new__ 创建一个新对象，例如返回该类的一个实例。 __init__ 只是在原地修改对象。因此，解决您的问题的一个简单方法是：

@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，我认为您不应该这样使用它。相反，您可能应该覆盖__new__

Answer 2

装饰器处理可调用对象。由于调用函数和初始化类没有区别，你可以直接在类上使用你的装饰器：

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:

    @classmethod
    def prepare_and_connect_constructor(cls, a, b):
        return decorate_with_some_magic(cls)(a, b)

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

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

class Child(Parent):

    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__':
    normal_child = Child("normal child", "no help needed")
    print(normal_child)
    special_child = Child.prepare_and_connect_constructor("special child", "needs some help")
    print(special_child)

输出：

normal child, no help needed
prepare something
create an object
connect obj to something
special child, needs some help