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Week 6: Object-Oriented Programming (OOP)

Why Learn This?

This week introduces Object-Oriented Programming (OOP) — the foundation of most modern software. OOP helps you:

  • Organize code in a scalable, reusable way.
  • Build systems that model real-world objects and behaviors.
  • Understand the principles behind major frameworks (Flask, Django, SQLAlchemy, etc.)

Mastering OOP is essential for writing maintainable Python code and succeeding in professional software environments.

Concepts & Notes

Classes & Objects

Define a class

class Dog:
    def __init__(self, name):
        self.name = name

    def bark(self):
        print(f"{self.name} says woof!")

Create an object

fido = Dog("Fido")
fido.bark()  # Fido says woof!

Instance Variables & __init__()

  • __init__ is the constructor method.
  • self refers to the instance of the object.
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

Dunder Methods

“Dunder” = double underscore (e.g., __str__, __repr__, __len__)

class Product:
    def __init__(self, name, price):
        self.name = name
        self.price = price

    def __str__(self):
        return f"{self.name} costs ${self.price}"

Inheritance

Base and Derived Classes

class Animal:
    def speak(self):
        print("Some sound")

class Cat(Animal):
    def speak(self):
        print("Meow")

kitty = Cat()
kitty.speak()  # Meow

Method Overriding & Composition

  • Overriding: Redefine methods in a child class.
  • Composition: Use other classes as part of your class.
class Engine:
    def start(self):
        print("Engine starting...")

class Car:
    def __init__(self):
        self.engine = Engine()

    def drive(self):
        self.engine.start()
        print("Car is moving")

Quiz – Week 6

  1. What does __init__() do in a class?

  2. What keyword is used to inherit from a parent class?

  3. What does the self keyword represent?

  4. Which dunder method controls how an object is printed?

  5. True or False: You can have multiple instances of the same class.

  6. What is method overriding?

Coding Challenges – Week 6

Challenge 1: Create a Class with __init__

Instructions: Create a class called Student that stores name and grade and prints them when an instance is created.

Learning Objective: Understand constructors and instance variables.

# TODO: Create a class called `Student` that stores `name` and `grade` and prints them when an instance is created.

Challenge 2: Add a Method to the Class

Instructions: Extend your Student class with a method that increases the grade.

Learning Objective: Practice adding behavior (methods) to your class.

# TODO: Extend your `Student` class with a method that increases the grade.

Challenge 3: Use __str__ Method

Instructions: Add a __str__ method to your class so the student prints nicely.

Learning Objective: Use dunder methods to control object representation.

# TODO: Add a `__str__` method to your class so the student prints nicely.

Challenge 4: Inheritance and Method Overriding

Instructions: Create a Teacher class that inherits from Person, but overrides the introduce() method.

Learning Objective: Learn basic inheritance and method overriding.

# TODO: Create a `Teacher` class that inherits from `Person`, but overrides the `introduce()` method.

Challenge 5: Composition with Engine and Car

Instructions: Create two classes: Battery and Phone, where Phone uses a Battery.

Learning Objective: Understand class composition by combining behaviors.

# TODO: Create two classes: `Battery` and `Phone`, where `Phone` uses a `Battery`.

Challenge 6: Multiple Objects

Instructions: Create a list of Dog objects and make them bark.

Learning Objective: Use multiple instances of a class in a loop.

# TODO: Create a list of `Dog` objects and make them bark.

Week 6 Summary

  • You learned how to define and instantiate classes.
  • You wrote constructors and added methods to encapsulate object behavior.
  • You used __str__ to customize how your objects display.
  • You applied inheritance to reuse and override behavior.
  • You practiced composition to combine class functionality.

OOP gives you structure and reuse — a critical step toward building larger applications, frameworks, and systems.

For Next Week

Reading

Assignments

First:

Then, complete the following exercises -- you can, of course, do them all:

  • 02_classes-objects-methods
    • 02_02_planets.py
    • 02_03_car.py
    • 02_04_classy_shapes.py
    • 02_05_smaller-amount.py
  • 03_inheritance
    • 03_01_more_food.py
    • 03_02_inheritance.py