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Inherit property setter in python 3.7

Python @property inheritance the right way

Given a Parent class with value property, Child can inherit and overload the property while accessing Parent property getter and setter.

Although we could just reimplement the Child.value property logic completely without using Parent.value whatsover, this would violate the DRY principle and, more important, it wouldn't allow for proper multiple inheritance (as show in the example property_inheritance.py bellow).

Two options:

  • Child redefines value property completely, both getter and setter.

  • Child uses Parent.value property, and only overloads setter. Parent class must be referenced explicitly.

In either pattern, using Parent.value.getter is as simple as

return super().value`

However, accessing Parent.property.setter is more verbose:

super(Child, type(self)).value.fset(self, new_value)

It would be nice to just super().value = new_value but this won't work due to super current implementation (as Python 3.7). There is a Python Issue regarding this rather unintuitive behaviour.

class Number:
"""
>>> n = Number(-4.1)
>>> print(n.value)
-4.1
"""
def __init__(self, value):
"""Uses property setter."""
self.value = value
@property
def value(self):
return self._value
@value.setter
def value(self, new_value):
self._value = new_value
class Integer(Number):
"""
>>> i = Integer(-4.1)
>>> print(i.value)
-4
"""
@property
def value(self):
return super().value
@value.setter
def value(self, new_value):
_value = int(new_value)
super(Integer, type(self)).value.fset(self, _value)
class Positive(Number):
"""
>>> p = Positive(-4.1)
>>> print(p.value)
4.1
"""
@Number.value.setter
def value(self, new_value):
_value = abs(new_value)
super(Positive, type(self)).value.fset(self, _value)
class Natural(Integer, Positive):
""" With multiple inheritance, it works as expected.
>>> n = Natural(-4.1)
>>> print(n.value)
4
"""
if __name__ == '__main__':
from doctest import testmod
testmod()
@gitPrinz

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@gitPrinz gitPrinz commented Sep 21, 2020

PyCharm will tell me "Cannot find reference 'fset' in 'None | str'", but it works!
Thank you very much 👍

@TogzhanK

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@TogzhanK TogzhanK commented Jan 9, 2021

Thank you so much! Usually, during tutorials only super() implemented on int and str are shown which doesn't explain syntax for other methods. This really helped me!

@willrazen

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@willrazen willrazen commented May 21, 2021

super(type(self), type(self)).setter.fset(self, value) doesn't follow mro adequately (because of the second type(self)).
Check my solution: https://gist.github.com/willrazen/bef3fcb26a83dffb6692e5e10d3e67ac
Usage: duper(super()).setter = value

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