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OOP: Classes and Objects
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Python Classes Tutorial – Create, Use & Master Object-Oriented Programming (OOP)

Learn how to create and use classes in Python with practical examples. Master constructors, methods, attributes, and OOP principles for efficient coding.

Contents:

  1. Introduction to Classes & Objects

    • What is a class? (Blueprint for objects)
    • What is an object? (Instance of a class)

  2. How to Create a Class

    • Basic syntax: class MyClass:
    • Naming conventions (PascalCase).

  3. The __init__ Method (Constructor)

    • Purpose: Initialize object attributes.
    • Syntax: def __init__(self, ...)

  4. The self Keyword (Explain early—it’s everywhere!)

    • What is self? (Reference to the instance).
    • Why it’s needed (access attributes/methods).

  5. Instantiating Objects

    • How to create an object: obj = MyClass().
    • How __init__ is called automatically.

  6. Adding Attributes to a Class

    • Instance attributes (per-object, e.g., self.name = name).
    • Class attributes (shared, e.g., class_var = 0).
    • Class vs. Instance Variables (Compare them here).

  7. Defining Methods in a Class

    • What are methods? (Functions inside a class).
    • Example: def greet(self): print("Hello!").

  8. Passing Arguments to Methods

    • Methods with parameters: def set_age(self, age):.
    • Default arguments (e.g., def __init__(self, name="Anonymous")).

  9. Practical OOP Examples (Apply all concepts!)

    • Example 1: A Dog class with name, breed, bark().
    • Example 2: A Car class with make, model, year, and describe().
    • Example 3: A Student class with name, age, grade simple
    • Example 4: A BankAccount class with balance, deposit(), and withdraw().
    • Example 5: A Student class with name, age, grade, and methods like introduce().

their name, age, and grade.

1. Introduction to Classes & Objects

Class

  • A class is a blueprint for creating objects.
  • Defines the structure (attributes & methods) that objects will have.
  • Example: A Car class defines properties like color, model, and actions like drive().

Object

  • An object is an instance of a class.
  • Created from the class blueprint.
  • Example: A Toyota car (object) created from the Car class.

2. How to Create a Class

Syntax

class ClassName:  
    # Attributes & methods go here
  • Naming Convention: Use PascalCase (e.g., MyClass, BankAccount).

3. The __init__ Method (Constructor)

Purpose

  • Automatically called when an object is created.
  • Used to initialize attributes (set initial values).

Syntax

def __init__(self, param1, param2):  
    self.attribute1 = param1  
    self.attribute2 = param2
  • Example:
class Dog:  
    def __init__(self, name):  
        self.name = name  # Initialize 'name' attribute

4. The self Keyword

What is self?

  • Refers to the current instance of the class.
  • Used to access attributes & methods inside the class.

Why is it needed?

  • Without self, Python wouldn’t know which object’s attributes to modify.
  • Example:
class Dog:  
    def __init__(self, name):  
        self.name = name  # 'self.name' belongs to this Dog object

5. Instantiating Objects

How to Create an Object

object_name = ClassName(arguments)
  • Example:
my_dog = Dog("Buddy")  # Calls __init__ automatically

6. Adding Attributes to a Class

Instance Attributes

  • Unique to each instance (object) of a class.
  • Store data specific to that object.
  • Defined within the __init__ constructor method, using the self parameter.
class Dog:  
    def __init__(self, name):  
        self.name = name  # Instance attribute

Class Attributes

  • Shared by all objects of the class.
  • Defined outside __init__.
class Dog:  
    species = "Canine"  # Class attribute (shared)

Class vs. Instance Attributes

Feature Class Attribute Instance Attribute
Scope Shared by all objects Unique per object
Defined Outside __init__ Inside __init__

for more details, see Understanding Class and Instance Attributes in Python

7. Defining Methods in a Class

What are Methods?

  • Functions defined inside a class.
  • Must take self as the first parameter.

Example

class Dog:  
    def bark(self):  
        print("Woof!")  
 
my_dog = Dog()  
my_dog.bark()  # Output: "Woof!"

8. Passing Arguments to Methods

Methods with Parameters

class Dog:  
    def set_age(self, age):  
        self.age = age  
 
my_dog = Dog()  
my_dog.set_age(3)  # Sets age to 3

Default Arguments

class Dog:  
    def __init__(self, name="Unknown"):  
        self.name = name  
 
dog1 = Dog()          # name = "Unknown"  
dog2 = Dog("Buddy")   # name = "Buddy"

9. Practical OOP Examples

Example 1: Dog Class

class Dog:  
    species = "Canine"  # Class attribute  
 
    def __init__(self, name, breed):  
        self.name = name   # Instance attribute  
        self.breed = breed  
 
    def bark(self):  
        print(f"{self.name} says Woof!")  
 
# Create objects  
dog1 = Dog("Buddy", "Labrador")  
dog2 = Dog("Max", "Beagle")  
 
dog1.bark()  # Output: "Buddy says Woof!"

Example 2: Car Class

class Car:
    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
 
    def describe(self):
        print(f"{self.year} {self.make} {self.model}")
 
my_car = Car("Toyota", "Corolla", 2020)
my_car.describe()  # Output: 2020 Toyota Corolla

Example 3: Student Class

# Class Definition
class Student:
    # Constructor
    def __init__(self, name, age, grade): # self refers to the current object being created.
        self.name = name
        self.age = age
        self.grade = grade
    # Method
    def info(self):
        print(f"Name = {self.name} Age = {self.age} Grade = {self.grade}")
 
# Object Creation
 
student1 = Student("Hamza", 8, 3)
student2 = Student("Muhammad", 15, 10)
 
# Accessing Attributes and Methods
 
print(student1.name)
student1.info()
student2.info()

Example 4: BankAccount Class

class BankAccount:  
    def __init__(self, balance=0):  
        self.balance = balance  
 
    def deposit(self, amount):  
        self.balance += amount  
 
    def withdraw(self, amount):  
        if amount <= self.balance:  
            self.balance -= amount  
        else:  
            print("Insufficient funds!")  
 
# Usage  
account = BankAccount(100)  
account.deposit(50)  
account.withdraw(30)  
print(account.balance)  # Output: 120

Example 5: Student Class

Represents a student with their name, age, and grade.

class Student:
 
    def __init__(self, name, age, grade):
        """Initializes a Student object with the given attributes."""
        self.name = name
        self.age = age
        self.grade = grade
 
    def get_name(self):
        """Returns the student's name."""
        return self.name
 
    def get_age(self):
        """Returns the student's age."""
        return self.age
 
    def get_grade(self):
        """Returns the student's grade."""
        return self.grade
 
    def set_grade(self, new_grade):
        """Updates the student's grade."""
        self.grade = new_grade
 
    def introduce(self):
        """Prints a self-introduction message."""
        print("Hello, my name is", self.name, "and I'm in grade", self.grade)
 
# Example usage
student1 = Student("Hamza", 8, 3)
student2 = Student("Muhammad", 16, 10)
 
student1.introduce()  # Output: Hello, my name is Alice and I'm in grade 9
print(student2.get_name())  # Output: Bob
student2.set_grade(11)
print(student2.get_grade())  # Output: 11

Python Tutorial in Urdu: How to Create a Class

Python Tutorial: Python Classes - What is Class Constructor

Python Tutorial in Urdu: How to Create Classes and Instance Attributes

References and Bibliography

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