13.1 Inheritance basics

Learning objectives

By the end of this section you should be able to

Identify is-a and has-a relationships between classes.
Differentiate between a subclass and a superclass.
Create a superclass, subclass, and instances of each.

is-a vs has-a relationships

Classes are related to each other. An is-a relationship exists between a subclass and a superclass. Ex: A daffodil is a plant. A Daffodil class inherits from a superclass, Plant.

Is-a relationships can be confused with has-a relationships. A has-a relationship exists between a class that contains another class. Ex: An employee has a company-issued laptop. Note: The laptop is not an employee.

Inheritance in Python

Inheritance uses an is-a relationship to inherit a class from a superclass. The subclass inherits all the superclass's attributes and methods, and extends the superclass's functionality.

In Python, a subclass is created by including the superclass name in parentheses at the top of the subclass's definition:


    class SuperClass:
        # SuperClass attributes and methods
    
    class SubClass(SuperClass):
        # SubClass attributes and methods

Note

Alternative inheritance terms

Python documentation for inheritance uses multiple terms to refer to the class that is inherited from and the class that inherits. This book uses superclass/subclass throughout for consistency.

**Table 13.1**
Class inherited from	Class that inherits
superclass	subclass
base class	derived class
parent class	child class

Note

Employee and Developer classes

Given the Employee class, create a Developer class that inherits from Employee. The Developer class has one method, update_codebase, which prints "Employee has updated the codebase". Then, use the Developer instance, python_dev, to call print_company and update_codebase.

class Employee():
    def print_company(self):
        print("Employee works at Stark Industries")

# Define Developer class

python_dev = Developer()
# Call Employee and Developer methods

Note

Polygon classes

Define three classes: Polygon, Rectangle, and Square:

Polygon has the method p_disp, which prints "object is a Polygon".
Rectangle inherits from Polygon and has the method r_disp, which prints "object is a Rectangle".
Square inherits from Rectangle and has the method s_disp, which prints "object is a Square".

Create an instance of each class. Then, for each instance, call all the methods the instance has access to.

# Define classes

# Create instances of Polygon, Rectangle, and Square
# Call accessible methods