Overriding inherited properties’ getters and setters in Python

Question

I’m currently using the @property decorator to achieve “getters and setters” in a couple of my classes. I wish to be able to inherit these @property methods in a child class.

I have some Python code (specifically, I’m working in py3k) which looks vaguely like so:

class A:
    @property
    def attr(self):
        try:
            return self._attr
        except AttributeError:
            return ''

class B(A):
    @property
    def attr(self):
        return A.attr   # The bit that doesn't work.

    @attr.setter
    def attr(self, value):
        self._attr = value

if __name__ == '__main__':
    b = B()
    print('Before set:', repr(b.attr))
    b.attr = 'abc'
    print(' After set:', repr(b.attr))

I have marked the part that doesn’t work with a comment. I want the base class’ attr getter to be returned. A.attr returns a property object (which is probably very close to what I need!).

Edit:
After receiving the answer below from Ned I thought up what I think is a more elegant solution to this problem.

class A:
    @property
    def attr(self):
        try:
            return self._attr
        except AttributeError:
            return ''

class B(A):        
    @A.attr.setter
    def attr(self, value):
        self._attr = value

if __name__ == '__main__':
    b = B()
    print('Before set:', repr(b.attr))
    b.attr = 'abc'
    print(' After set:', repr(b.attr))

The .setter decorator expects a property object which we can get using @A.attr. This means we do not have to declare the property again in the child class.

(This is the difference between working on a problem at the end of the day vs working on it at the beginning of the day!)

Asked By: casr

||

Source

Answer 1

I think you want:

class B(A):
    @property
    def attr(self):
        return super(B, self).attr

You mention wanting to return the parent class’s getter, but you need to invoke the getter, not return it.

Answered By: Ned Batchelder

Answer 2

To override a setter in python 2 I did this:

class A(object):
    def __init__(self):
        self._attr = None

    @property
    def attr(self):
        return self._attr

    @attr.setter
    def attr(self, value):
        self._attr = value


class B(A):
    @A.attr.setter
    def attr(self, value):
        # Do some crazy stuff with `value`
        value = value[0:3]
        A.attr.fset(self, value)

To understand where A.attr.fset came from see the documentation on the property class:
https://docs.python.org/2/library/functions.html#property

Answered By: jmagnusson

Overriding inherited properties’ getters and setters in Python

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