Python abstractclass Inheritance

Question

I have two class structured as below

from abc import ABCMeta, abstractmethod


class C(metaclass=ABCMeta):
    """"""

    def __init__(self, x, y):
        self._x = x
        self._y = y

    @property
    @abstractmethod
    def x(self):
        """Get the _x"""

    @x.setter
    @abstractmethod
    def x(self, value):
        """Set the x"""

    @property
    def y(self):
        """Get the _y"""

    @y.setter
    def y(self, value):
        """Set the _y"""


class D(C):
    """"""

    def __init__(self, x, y):
        self._x = x
        self._y = y

    @property
    def x(self):
        return self._x

    @C.x.setter
    def x(self, value):
        self._x = value

    @property
    def y(self):
        return self._y

    @C.y.setter
    def y(self, value):
        self._y = value

When I initialize an instance of D. It throws a error: TypeError: Can't instantiate abstract class D with abstract methods x

When I rewrite setters decorator in D as

@x.setter
def x(self, value):
    self._x = value

it works. But in python abc document https://docs.python.org/3/library/abc.html it states: in disappreciated @abc.abstractproperty If only some components are abstract, only those components need to be updated to create a concrete property in a subclass:

class D(C):
    @C.x.setter
    def x(self, val):
        ...

I don't know why write in this way will lead to error. Please help me understand the logic here. Thank you.

user2357112 · Accepted Answer · 2021-08-24 08:48:53Z

3

When you write @C.x.setter above your setter, you're setting x to a version of C.x with the setter replaced with your new setter function. Only the setter - the getter you wrote earlier is discarded. You're still using C.x's abstract getter.

The example in the docs uses @C.x.setter because they want the behavior it provides. In the doc example, C.x has a concrete getter, and they just want to replace the setter. That's not the case in your code.

answered Aug 24, 2021 at 8:48

user2357112

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