Python Classes

Learning Objectives

By the end of this lesson, you will understand how object-oriented programming begins with classes and objects.

What Is Object-Oriented Programming?

Object-Oriented Programming (OOP) is a programming approach where programs are designed around objects.

An object can represent something from the real world, such as a student, customer, product, employee, car, bank account, or order.

Objects can contain both data and behavior.

What Is a Class?

A class is a blueprint or template used to create objects.

Instead of separately defining the structure and behavior of every student, for example, we can define a Student class and create many student objects from it.

What Is an Object?

An object is an actual instance created from a class.

If Student is the class, then Ali and Sara can be individual objects created from that class.

class Student:
    pass

student1 = Student()
student2 = Student()

Here, Student is the class and student1 and student2 are objects.

Class Syntax

Python uses the class keyword to define a class.

class ClassName:
    # class body
    pass

Python conventionally uses PascalCase for class names.

class Student:
    pass

class Product:
    pass

class BankAccount:
    pass

Creating a Simple Class

A class can initially contain nothing more than the pass statement.

class Student:
    pass

student1 = Student()

print(student1)

Creating Objects

To create an object, write the class name followed by parentheses.

class Student:
    pass

student1 = Student()
student2 = Student()

print(type(student1))
print(type(student2))

<class '__main__.Student'>
<class '__main__.Student'>

Attributes

Attributes are values or pieces of data associated with an object.

For example, a student might have a name, age, and course.

class Student:
    pass

student1 = Student()

student1.name = "Ali"
student1.age = 22
student1.course = "Python"

print(student1.name)
print(student1.age)
print(student1.course)

Ali
22
Python

Methods

A method is a function defined inside a class.

Methods usually describe something an object can do.

class Student:

    def introduce(self):
        print("Hello, I am a student.")

student1 = Student()

student1.introduce()

Hello, I am a student.

Understanding self

One of the most important concepts when learning Python classes is self.

self refers to the current object on which the method is being called.

class Student:

    def introduce(self):
        print("Hello from the student object.")

student1 = Student()

student1.introduce()

When Python sees:

student1.introduce()

Python effectively passes student1 as the first argument to the method.

Student.introduce(student1)

Instance Attributes

Instance attributes belong to a particular object.

Different objects can have different values for the same attribute.

class Student:

    def introduce(self):
        print("My name is", self.name)


student1 = Student()
student2 = Student()

student1.name = "Ali"
student2.name = "Sara"

student1.introduce()
student2.introduce()

My name is Ali
My name is Sara

Notice that the same method works with both objects, because self refers to whichever object called the method.

Instance Methods

Instance methods can access and modify attributes of the current object through self.

class Product:

    def show_details(self):
        print("Product:", self.name)
        print("Price:", self.price)


product = Product()

product.name = "Laptop"
product.price = 85000

product.show_details()

Product: Laptop
Price: 85000

Working with Multiple Objects

One of the major benefits of classes is that we can create many objects using the same blueprint.

class Product:

    def show_details(self):
        print(self.name, "-", self.price)


product1 = Product()
product2 = Product()
product3 = Product()

product1.name = "Laptop"
product1.price = 85000

product2.name = "Phone"
product2.price = 50000

product3.name = "Headphones"
product3.price = 8000

product1.show_details()
product2.show_details()
product3.show_details()

Laptop - 85000
Phone - 50000
Headphones - 8000

Class Attributes

A class attribute is defined directly inside the class and is shared by objects unless an instance provides its own value.

class Student:

    school = "PyLearn Academy"


student1 = Student()
student2 = Student()

print(student1.school)
print(student2.school)

PyLearn Academy
PyLearn Academy

Both objects can access the shared class attribute.

Instance vs Class Attributes

Understanding the difference between these two types of attributes is very important in object-oriented programming.

class Student:

    school = "PyLearn Academy"


student1 = Student()
student2 = Student()

student1.name = "Ali"
student2.name = "Sara"

print(student1.name)
print(student2.name)

print(student1.school)
print(student2.school)

Ali
Sara
PyLearn Academy
PyLearn Academy

Updating Attributes

Object attributes can be changed after an object has been created.

class Product:
    pass


product = Product()

product.name = "Laptop"
product.price = 85000

print(product.price)

product.price = 80000

print(product.price)

85000
80000

Deleting Attributes & Objects

Python provides the del statement for deleting attributes or references to objects.

class Student:
    pass


student = Student()

student.name = "Ali"
student.age = 22

del student.age

print(student.name)

Ali

You can also delete an object reference:

student = Student()

del student

Real-World Examples

Classes become especially useful when a program contains many entities with similar structure and behavior.

Example 1 — Bank Account

class BankAccount:

    def show_balance(self):
        print("Balance:", self.balance)


account = BankAccount()

account.owner = "Ali"
account.balance = 50000

account.show_balance()

Example 2 — Car

class Car:

    def start(self):
        print(self.brand, "is starting.")


car = Car()

car.brand = "Toyota"

car.start()

Example 3 — Employee

class Employee:

    def show_details(self):
        print("Name:", self.name)
        print("Department:", self.department)


employee = Employee()

employee.name = "Sara"
employee.department = "Marketing"

employee.show_details()

Common Mistakes

1. Forgetting the colon

# Wrong
class Student

# Correct
class Student:
    pass

2. Forgetting indentation

# Wrong
class Student:
print("Hello")

# Correct
class Student:
    print("Hello")

3. Forgetting self

# Wrong
class Student:
    def introduce():
        print("Hello")

# Correct
class Student:
    def introduce(self):
        print("Hello")

4. Confusing class and object

5. Using an attribute that does not exist

class Student:
    pass

student = Student()

print(student.name)

This produces an AttributeError because name has not been assigned to the object.

Practice Exercises

Mini Challenge — Product Inventory System

Build a small inventory system using Python classes.

Requirements

• Create a Product class.
• Create at least three product objects.
• Each product should have a name.
• Each product should have a price.
• Each product should have a stock quantity.
• Create a method called show_details().
• Display the details of every product.

Starter Structure

class Product:

    def show_details(self):
        print("Name:", self.name)
        print("Price:", self.price)
        print("Stock:", self.stock)


product1 = Product()

product1.name = "Laptop"
product1.price = 85000
product1.stock = 10

product1.show_details()

Once this works, create product2 and product3 and give them different values.

Quick Knowledge Check

Test your understanding before moving to the next lesson.

Lesson Summary

Python classes are the foundation of object-oriented programming.