Inherit property setter in python 3.7

property_inheritance.md

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.

property_inheritance.py

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()