Python Classes and Objects
A class is a blueprint, and an object is a real thing made from that blueprint.
Why Classes?
Imagine you are writing software for an institute. Every student has a name, a roll number and a course, and every student can pay fees or take a test. A class lets you describe all of this once. Then you create as many objects (students) as you need.
This style is called object-oriented programming (OOP).
Creating a Class and an Object
Example
class Student:
institute = "Workownix"
s1 = Student()
print(s1.institute)Workownix
Student is the class. s1 is an object (also called an instance) of that class.
By convention, class names start with a capital letter and use CapitalCase, like BankAccount.
The __init__ Method
__init__ runs automatically every time you create a new object. Use it to give each object its own data.
Example
class Student:
def __init__(self, name, course):
self.name = name
self.course = course
s1 = Student("Ayesha", "Python")
s2 = Student("Bilal", "Web Development")
print(s1.name, "-", s1.course)
print(s2.name, "-", s2.course)Ayesha - Python Bilal - Web Development
self means "this object". self.name = name stores the name inside the object. The values stored this way are called attributes.
Methods
A method is a function inside a class. Its first parameter is always self.
Example
class BankAccount:
def __init__(self, owner, balance=0):
self.owner = owner
self.balance = balance
def deposit(self, amount):
self.balance += amount
def withdraw(self, amount):
if amount > self.balance:
print("Not enough balance")
else:
self.balance -= amount
def show(self):
print(f"{self.owner}: Rs {self.balance}")
acc = BankAccount("Hamza", 5000)
acc.deposit(2000)
acc.withdraw(10000)
acc.withdraw(1500)
acc.show()Not enough balance Hamza: Rs 5500
Changing and Deleting Attributes
Example
acc = BankAccount("Sara", 1000)
acc.balance = 3000
acc.show()
del acc.balanceSara: Rs 3000
You can change an attribute directly. del removes it from the object.
The __str__ Method
__str__ decides what print() shows for an object:
Example
class Product:
def __init__(self, name, price):
self.name = name
self.price = price
def __str__(self):
return f"{self.name} (Rs {self.price})"
p = Product("Headphones", 2500)
print(p)Headphones (Rs 2500)
Inheritance
A class can inherit everything from another class and then add or change things. The original is the parent class, and the new one is the child class.
Example
class Person:
def __init__(self, name):
self.name = name
def introduce(self):
return f"I am {self.name}"
class Teacher(Person):
def __init__(self, name, subject):
super().__init__(name)
self.subject = subject
def introduce(self):
return f"I am {self.name} and I teach {self.subject}"
p = Person("Ali")
t = Teacher("Ms. Fatima", "Python")
print(p.introduce())
print(t.introduce())I am Ali I am Ms. Fatima and I teach Python
super().__init__(name) calls the parent's __init__, so you do not repeat code. The child's introduce() replaces the parent's version. This is called overriding.
Key Words
| Word | Meaning |
|---|---|
| Class | The blueprint |
| Object / instance | One real item made from the class |
| Attribute | Data stored in an object |
| Method | A function that belongs to a class |
| Inheritance | A child class reusing a parent class |
Exercise
Exercise
What is the purpose of the __init__ method?