Python Inheritance, Access Modifiers & Method Overriding

In Plain English

Organize object hierarchies through inheritance. Master encapsulation with access modifiers (public, protected, private), extend base classes with extends, invoke parent methods with parent::, and prevent extension using the final keyword.

The Mental Model

"Access modifiers are like security clearances in a corporate headquarters: public is the open lobby, protected is the employee-only workspace shared with sister branches (subclasses), and private is the CEO personal locked vault."

Deep Dive: How It Works

Access Modifiers: public (everywhere), protected (class & subclasses only), private (declaring class only).

Inheritance: class Manager extends Employee inherits all non-private methods and properties.

Parent Invocation: parent::__construct(...) or parent::doWork() calls overridden base implementations.

Final Classes & Methods: final prevents other classes from extending or overriding critical security routines.

Syntax Blueprint

<?php

class BaseAccount {
    protected float $balance = 0;
}

class SavingsAccount extends BaseAccount {
    public function addInterest() { $this->balance *= 1.05; }
}

Use protected for shared subclass state and extends to inherit base class behavior.

Core Rules to Remember

Encapsulation: Keep internal state private or protected, exposing only intentional public API methods.

Method Overriding: Subclasses can override parent methods to specialize behavior while calling parent:: when needed.

Liskov Substitution: Subclasses must maintain compatible parameter and return types with the parent signature.

Common Beginner Traps & How to Fix Them

Mistake:Trying to access private properties from a subclass ($this->privateField).

Why it happens: private properties are strictly restricted to the declaring class.

How to fix: Change modifier to protected if subclasses legitimately need access.

Pro Tip: Mark utility classes and value objects as final class to enforce composition over fragile deep inheritance trees.

Live Interactive Example

Hit Run Code to see it live

Inheritance and Parent Constructor Chaining

PHP 8.2 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: Manager inherited getDetails() and accessed protected $salary in getTotalComp().

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Extend Base Notification Class

Create a class Notification with public function send(string $to): string returning "Notification sent to {$to}".
Create a subclass EmailNotification extends Notification that overrides send(string $to): string to return "Email dispatched to {$to}".
In main script, create $email = new EmailNotification() and print the result of $email->send("admin@zen.com").

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

Mark this lesson as completed to update your course progress.