Python Inheritance: Extend an Existing Class

Extend base-class behavior by defining a subclass.

How It Works

Inheritance lets a child class reuse or override behavior from a parent class. The child inherits available attributes and methods; use composition instead when the relationship is not truly an is-a relationship.

issubclass() can help inspect class relationships.

Step-by-Step Example

class Notification:
    def message(self):
        return "General notification"

class EmailNotification(Notification):
    def message(self):
        return "Email notification"

print(EmailNotification().message())

Understanding the Example

The class groups related state and methods so each instance can manage its own values.
The function definition gives the operation a reusable name and makes its inputs explicit.

A successful run of this example prints the following output:

Email notification

Where This Fits in a Real Program

A class is useful when data and the operations that preserve its rules belong together. Distinguish class-level attributes from per-instance state, and make invalid operations fail clearly. Inheritance is not always needed: simpler composition or a standalone function may be easier to maintain.

Practice: Try It Yourself

Add an SMSNotification subclass with a specialized message.

For this exercise, predict the result before running the changed code. Then explain the difference between the original and modified cases using the actual values, not just whether the program completed.

Common Beginner Mistakes

Assuming child classes automatically override methods without defining the same name.
Using inheritance only for code sharing when the domain relationship is not meaningful.

Key Takeaways

Extend base-class behavior by defining a subclass.
Inheritance lets a child class reuse or override behavior from a parent class.
issubclass() can help inspect class relationships.