Python 3.x：应该如何覆盖从父类继承的属性？

Question

一个简单的例子可能会显示更多：

class NaturalNumber():
    def __init__(self, val):
        self._value = val

    def _get_value(self):
        return self._value

    def _set_value(self, val):
        if val < 0:
            raise ValueError(
                f"Cannot set value to {val}: Natural numbers are not negative."
            )
        self._value = val

    value = property(_get_value, _set_value, None, None)


class EvenNaturalNumber(NaturalNumber):
    # This seems superfluous but is required to define the property.
    def _get_value(self):
        return super()._get_value()

    def _set_value(self, val):
        if val % 2:
            raise ValueError(
                f"Cannot set value to {val}: Even numbers are divisible by 2."
            )
        super()._set_value(val)

    # This seems superfluous but parent property defined with parent setter.
    value = property(_get_value, _set_value, None, None)

这是来自真实用例的简化示例，我想在其中注入代码以对生产类进行测试。原理是一样的，这里我在继承于

value

类的

EvenNaturalNumber

类中对

NaturalNumber

进行额外的验证。我的老板不喜欢我摆脱他的所有装饰器，所以理想情况下，无论底层类是如何编写的，解决方案都应该有效。

看起来很自然的是：

class NaturalNumber():
    def __init__(self, val):
        self._value = val
    
    @property
    def value(self):
        return self._value
    
    @value.setter
    def value(self, val):
        if val < 0:
            raise ValueError(
                f"Cannot set value to {val}: Natural numbers are not negative."
            )
        self._value = val


class EvenNaturalNumber(NaturalNumber):
    @property
    def value(self):
        return super().value
    
    @value.setter
    def value(self, val):
        if val % 2:
            raise ValueError(
                f"Cannot set value to {val}: Even numbers are divisible by 2."
            )
        super().value = val

但是这个错误与 EvenNaturalNumber.value 上的有效集（例如，= 2）有关。与

AttributeError: 'super' object has no attribute 'value'

。

就我个人而言，我会说这是 python 语言中的一个错误！但我怀疑我错过了什么。

---

我找到了使用装饰器的解决方案，但这似乎相当复杂：

class NaturalNumber():
    def __init__(self, val):
        self._value = val

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, val):
        if val < 0:
            raise ValueError(
                f"Cannot set value to {val}: Natural numbers are not negative."
            )
        self._value = val


class EvenNaturalNumber(NaturalNumber):
    @property
    def value(self):
        return super().value
    
    @value.setter
    def value(self, val):
        if val % 2:
            raise ValueError(
                f"Cannot set value to {val}: Even numbers are divisible by 2."
            )
        super(type(self), type(self)).value.fset(self, val)

另一种方法是：

class NaturalNumber():
    def __init__(self, val):
        self._value = val
    
    @property
    def value(self):
        return self._value
    
    @value.setter
    def value(self, val):
        if val < 0:
            raise ValueError(
                f"Cannot set value to {val}: Natural numbers are not negative."
            )
        self._value = val


class EvenNaturalNumber(NaturalNumber):
    # This seems superfluous but is required to define the property.
    def _set_value(self, val):
        if val % 2:
            raise ValueError(
                f"Cannot set value to {val}: Even numbers are divisible by 2."
            )
        NaturalNumber.value.fset(self, val)

    # This seems superfluous as parent property defined with parent setter.
    value = NaturalNumber.value.setter(_set_value)

但是第二个解决方案似乎相当不令人满意，因为它利用了 value 属性在

NaturalNumber

类中定义的知识。我看不到任何方法来迭代

EvenNaturalNumber.__mro__

，除非我在

EvenNaturalNumber._set_value(self, val)

中执行此操作，但这就是

super()

的工作，不是吗？

如有任何改进或建议，我们将不胜感激。否则，我的老板就只能和

super(type(self), type(self)).value.fset(self, val)

一起生活了！

Answer 1

我不确定为什么要在重写的属性中调用父属性。您能解释一下为什么要这样做吗？

我的意思是，这样做的正常我将是一个可以轻松覆盖和调用的辅助方法：

class NaturalNumber:
    def __init__(self, val):
        self._value = val

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, val):
        self._validate(val)
        self._value = val

    def _validate(self, val: int | float) -> None:
        if val < 0:
            raise ValueError(
                f"Cannot set value to {val}: Natural numbers are not negative."
            )


class EvenNaturalNumber(NaturalNumber):
    def _validate(self, val: int | float) -> None:
        super()._validate(val)
        if val % 2:
            raise ValueError(
                f"Cannot set value to {val}: Even numbers are divisible by 2."
            )