Advanced Python · Week 5

Objects/Classes

Step 1 of 5

Exercise 1: Introduction to Objects and Classes

Welcome to Python 2, Lesson 5! Today, we'll learn about objects and classes, fundamental concepts in object-oriented programming (OOP).

What are Objects and Classes?

A class is like a blueprint for creating objects. An object is an instance of a class, which contains its own data and behaviors.

Let's start with a simple example of a class:

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

    def bark(self):
        return f"{self.name} says Woof!"

# Create a Dog object
my_dog = Dog("Buddy", 3)
print(f"{my_dog.name} is {my_dog.age} years old.")
print(my_dog.bark())
            

Now it's your turn! Create a class called Cat with the following:

An __init__ method that takes name and color as parameters
A method called meow that returns a string like "name says Meow!"
Create a Cat object and call its meow method

Starter code

# Define your Cat class here

# Create a Cat object and call its meow method

Write your code here

Exercise 2: Creating Methods

Methods are functions that belong to a class. They can perform actions using the object's data.

Instance Methods

Instance methods are functions defined inside a class
They always take 'self' as the first parameter
They can access and modify the object's attributes

Here's an example with multiple methods:

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

    def perimeter(self):
        return 2 * (self.width + self.height)

# Using the Rectangle class
rect = Rectangle(5, 3)
print(f"Area: {rect.area()}")
print(f"Perimeter: {rect.perimeter()}")
            

Your turn! Enhance the Cat class from the previous exercise:

Add an age attribute to the __init__ method
Create a method called birthday that increases the cat's age by 1 and returns a celebratory message
Create a method called introduce that returns a string introducing the cat with its name, color, and age
Create a Cat object, call its birthday method, and then call its introduce method

Starter code

# Enhance your Cat class here

# Create a Cat object, celebrate its birthday, and introduce it

Write your code here

Exercise 3: Class and Instance Variables

In Python, we can have two types of variables in a class: class variables and instance variables.

Class vs Instance Variables

Class variables are shared by all instances of the class
Instance variables are unique to each instance
Class variables are defined outside any method, usually at the top of the class
Instance variables are typically defined in the __init__ method

Here's an example illustrating both types of variables:

class Dog:
    # Class variable
    species = "Canis familiaris"

    def __init__(self, name, age):
        # Instance variables
        self.name = name
        self.age = age

    def describe(self):
        return f"{self.name} is {self.age} years old and belongs to the {self.species} species."

dog1 = Dog("Buddy", 5)
dog2 = Dog("Max", 3)

print(dog1.describe())
print(dog2.describe())

# Changing the class variable
Dog.species = "Canis lupus familiaris"

print(dog1.describe())
print(dog2.describe())
            

Your task:

Create a class called Student with a class variable school_name set to "Python High School"
Add instance variables for name and grade
Create a method introduce that returns a string introducing the student and mentioning their school
Create two Student instances and call their introduce methods
Change the school_name class variable and call the introduce methods again to see the change

Starter code

# Create the Student class and demonstrate class and instance variables

Write your code here

Exercise 4: Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Inheritance Basics

The class being inherited from is called the parent or base class
The class that inherits is called the child or derived class
Child classes can override or extend the functionality of parent classes

Here's an example demonstrating inheritance:

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

    def speak(self):
        pass

class Dog(Animal):
    def speak(self):
        return f"{self.name} says Woof!"

class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow!"

dog = Dog("Buddy")
cat = Cat("Whiskers")

print(dog.speak())
print(cat.speak())
            

Your task:

Create a base class called Vehicle with attributes make and model
Add a method info to the Vehicle class that returns a string with the vehicle's make and model
Create two derived classes: Car and Motorcycle
Add a num_wheels attribute to each derived class (4 for Car, 2 for Motorcycle)
Override the info method in both derived classes to include the number of wheels
Create instances of Car and Motorcycle and call their info methods

Starter code

# Create the Vehicle, Car, and Motorcycle classes here

# Demonstrate inheritance

Write your code here

Weekly Project: Pet Shop Simulator

Let's put everything we've learned about objects and classes into practice by creating a simple pet shop simulator.

Your task is to create a program that:

Defines a base Animal class with common attributes and methods
Creates derived classes for different types of pets (e.g., Dog, Cat, Fish)
Implements a PetShop class that can add pets, remove pets, and list all pets
Allows users to interact with the pet shop (add pets, list pets, etc.)

Here's a starting template:

class Animal:
    def __init__(self, name, species):
        self.name = name
        self.species = species

    def make_sound(self):
        pass

class Dog(Animal):
    def make_sound(self):
        return f"{self.name} barks!"

class Cat(Animal):
    def make_sound(self):
        return f"{self.name} meows!"

class PetShop:
    def __init__(self):
        self.pets = []

    def add_pet(self, pet):
        self.pets.append(pet)
        print(f"{pet.name} has been added to the pet shop.")

    def list_pets(self):
        if not self.pets:
            print("The pet shop is empty.")
        else:
            for pet in self.pets:
                print(f"{pet.name} - {pet.species}")

# Test your Pet Shop Simulator
shop = PetShop()
dog = Dog("Buddy", "Dog")
cat = Cat("Whiskers", "Cat")

shop.add_pet(dog)
shop.add_pet(cat)
shop.list_pets()

print(dog.make_sound())
print(cat.make_sound())
            

Enhance the Pet Shop Simulator by adding the following features:

Add a Fish class that inherits from Animal
Implement a method to remove pets from the shop
Add a method to feed all the pets in the shop
Create a simple menu system for users to interact with the pet shop
Add error handling for invalid inputs or actions

This project will give you hands-on experience with classes, objects, inheritance, and basic user interaction. Have fun creating your Pet Shop Simulator!

Starter code

# Implement your enhanced Pet Shop Simulator here

Write your code here

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