13.2 Attribute access

Learning objectives

By the end of this section you should be able to

Implement a subclass that accesses inherited attributes from the superclass.
Write a subclass's __init__ that inherits superclass instance attributes and creates new instance attributes.

Creating a simple subclass

Subclasses have access to the attributes inherited from the superclass. When the subclass's __init__ isn't explicitly defined, the superclass's __init__ method is called. Accessing both types of attributes uses the same syntax.

Using init to create and inherit instance attributes

A programmer often wants a subclass to have new instance attributes as well as those inherited from the superclass. Explicitly defining a subclass's __init__ involves defining instance attributes and assigning instance attributes inherited from the superclass.

Concepts in Practice

Accessing a subclass's attributes

Consider the Employee and Developer example code:

class Employee:
  count = 0
  def __init__(self):
    Employee.count += 1
    self.e_id = Employee.count
    self.hire_year = 2023

  def emp_display(self):
    print(f"Employee {self.e_id} hired in {self.hire_year}")

class Developer(Employee):
  def __init__(self):
    Employee.count += 1
    self.e_id = Employee.count
    self.hire_year = 2023
    self.lang_xp = ["Python", "C++", "Java"]

  def dev_display(self):
    print(f"Proficient in {self.lang_xp}")

emp_1 = Employee()
dev_1 = Developer()

Try It

Creating a subclass with an instance attribute

Given a class Dessert, create a class, Cupcake, inherited from Dessert. Cupcake class methods:

__init__(self): initializes inherited instance attribute ingredients with ["butter", "sugar", "eggs", "flour"], and initializes instance attribute frosting with "buttercream"
display(self): prints a cupcake's ingredients and frosting

Then call the display method on a new Cupcake object. The output should match:


    Made with ["butter", "sugar", "eggs", "flour"] and topped with buttercream frosting

class Dessert:
    def __init__(self):
        self.ingredients = ["butter", "sugar", "eggs", "flour"]

# Define Child class

bday_cake = Cupcake()
bday_cake.display()

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.