python3：覆盖属性的setter

Question

class Base:
    def __init__(self):
        self._prop = None
        pass

    def get_prop(self):
        print('base get prop')
        return self._prop

    def set_prop(self, prop):
        print('base set prop')
        self._prop = prop

    prop = property(get_prop, set_prop)
    pass


class Derived(Base):
    def set_prop(self, prop):
        print('Derived set prop')
        self._prop = prop
    pass


base = Base()
base.prop = 1
print(base.prop)

derive = Derived()
derive.prop = 1
print(derive.prop)

result:

base set prop
base get prop
1
base set prop
base get prop
1

Expecting to get Derived set prop since I override it in the derived class.

When I googled it, I saw annotating with @Base.prop.setter . I tried that as well, but that didn't work. I think it's because how they create the Base 's prop is different.

But I cannot easily update the Base class in this situation.

One way is to completely re-implement prop in the Derive class, but I know it's not the right way. Is there another way?

Answer 1

This is because when you assign to Base.prop with property(get_prop, set_prop) , this property object is instantiated with the set_prop function object as defined in the Base class. The fact that the Derived class inherits the Base class does not magically update the reference to the Base.set_prop method within the property object.

You can either define another prop inside the Derived class:

class Derived(Base):
    def set_prop(self, prop):
        print('Derived set prop')
        self._prop = prop

    prop = property(Base.get_prop, set_prop)

or you can simply use property as a decorator:

class Base:
    def __init__(self):
        self._prop = None
        pass

    @property
    def prop(self):
        print('base get prop')
        return self._prop

    @prop.setter
    def prop(self, prop):
        print('base set prop')
        self._prop = prop

class Derived(Base):
    @Base.prop.setter
    def prop(self, prop):
        print('Derived set prop')
        self._prop = prop