从父类覆盖类方法而不影响下游类

Question

我想覆盖一个类中的继承方法（参见下面的__init__ 方法示例），同时让其子级仍然使用父母版本。

我知道我可以通过在GrandChild 类中重新定义__init__ 方法或使用多重继承来实现所需的行为。但是，我的问题旨在通过仅更改 Child 类及其 __init__ 实现来实现相同目的。

（实际用例是明显更复杂的“遗留代码”，每个级别都有几个类。因此，这个问题的动机是在各个类中实现所需的行为，而不必触及其他类的实现或继承结构）

如果这是不可能的，我也希望能对此做出解释。

class Parent:
    def __init__(self, a,b):
        self.a = a 
        self.b = b

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

class Child(Parent):
    # I would like to overwrite this method only for the Child Class and none of its children / downstream inhertiances
    def __init__(self, a):
        super().__init__(a, None)

class GrandChild(Child):
    # This Class should use the __init__ method of class Parent
    pass


parent = Parent("a","b")
child = Child("c")
# This throws a Type error right now since it calls the init method of class Child
grandchild = GrandChild("d", "e")

编辑：

如上所述，我知道我可以通过不同的方式实现所需的行为，例如更改类结构（如下所示）。然而，问题实际上更多是关于 python 是否允许仅对 Child 类进行更改。如果这在 python 中实际上是不可能的（不仅仅是不受欢迎的），那么解释为什么比提供替代实现来改变任何超出 Child 类的实现的替代实现来回答我的问题。

class ChildCommonFunctionality(Parent):
    # Use this class for all common functionality originally provided by Child Class
    pass


class Child(ChildCommonFunctionality):
    # Use this class to override the init method
    def __init__(self, a):
        super().__init__(a, None)

class GrandChild(ChildCommonFunctionality):
    # This Class should use the __init__ method of class Parent
    pass

Answer 1

我找到了一种使用 _init_subclass 的方法来确保 Child 的所有子类都使用 Parent 的构造函数，而不是受 this post 启发的 Child 中定义的构造函数：

class Parent:
    def __init__(self, a,b):
        self.a = a 
        self.b = b

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

class Child(Parent):
    # I would like to overwrite this method only for the Child Class and none of its children / downstream inhertiances
    def __init__(self, a):
        super().__init__(a, None)

    def __init_subclass__(cls):
        cls.__init__ = super().__init__

class GrandChild(Child):
    # This Class should use the __init__ method of class Parent
    pass

尽管这有点 hacky，但它提供了实际绕过 Childs 初始化方法的所需功能

Answer 2

你可以这样做：

class Parent:
    def __init__(self, a, b = None):
        self.a = a 
        self.b = b

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

class Child(Parent):
    # I would like to overwrite this method only for the Child Class and none of its children / downstream inhertiances
    def __init__(self, a, b = None):
        super().__init__(a, b) # Or None instead of b... but that's not good when called by GrandChild

class GrandChild(Child):
    # This Class should use the __init__ method of class Parent
    pass

parent = Parent("a","b")
child = Child("c")
grandchild = GrandChild("d", "e")

编辑：您还可以用 GrandChild 中的强制参数替换可选参数：

class GrandChild(Child):
    def __init__(self, a, b):
        super().__init__(a, b)

Answer 3

这段代码可以解决问题，在@dspr 的建议中添加几行：

class Parent:
    def __init__(self, a, b = None):
        self.a = a
        self.b = b

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

class Child(Parent):
    # I would like to overwrite this method only for the Child Class and none of its children / downstream inhertiances
    def __init__(self, a, b = None):
        if type(self) == Child:
            if b is not None:
                raise ValueError(
                  "Second argument is not allowed for direct use in Child class")
            super().__init__(a, None) #Or (a, b) if you trust b to be None as it is here
        else:
            super().__init__(a, b)

class GrandChild(Child):
    # This Class should use the __init__ method of class Parent
    pass

parent = Parent("a","b")
child = Child("c")
print(child.b)      # None
grandchild = GrandChild("d", "e")
print(grandchild.b) # e
child = Child("f", "g")
print(child.b)      # ValueError