Extending a Property in a Subclass

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

Within a subclass, you want to extend the functionality of a property defined in a parent class.

Solution

Consider the following code, which defines a property:

class Person(object):
    def __init__(self, name):
        self.name = name

    # Getter function
    @property
    def name(self):
        return self._name

    # Setter function
    @name.setter
    def name(self, value):
        if not isinstance(value, str):
            raise TypeError('Expected a string')
        self._name = value

    # Deleter function
    @name.deleter
    def name(self):
        raise AttributeError("Can't delete attribute")

Here is an example of a class that inherits from Person and extends the name property with new functionality:

class SubPerson(Person):
    @property
    def name(self):
        print('Getting name')
        return super().name

    @name.setter
    def name(self, value):
        print('Setting name to', value)
        super(SubPerson, SubPerson).name.__set__(self, value)

    @name.deleter
    def name(self):
        print('Deleting name')
        super(SubPerson, SubPerson).name.__delete__(self)

Here is an example of the new class in use:

>>> s = SubPerson('Guido')
Setting name to Guido
>>> s.name
Getting name
'Guido'
>>> s.name = 'Larry'
Setting name to Larry
>>> s.name = 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "example.py", line 16, in name
       raise TypeError('Expected ...