Classes are blueprints for creating objects in Python. Here’s how to define and use them.

Basic Class Definition

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def bark(self):
        return f"{self.name} says woof!"

# Create an instance
my_dog = Dog("Buddy", 3)
print(my_dog.name)   # Buddy
print(my_dog.bark()) # Buddy says woof!

The init Method

The constructor, called when creating a new instance:

class Person:
    def __init__(self, name, age):
        self.name = name    # Instance attribute
        self.age = age
        self.is_adult = age >= 18  # Computed attribute

person = Person("Alice", 25)
print(person.is_adult)  # True

Instance vs Class Attributes

class Counter:
    count = 0  # Class attribute (shared by all instances)
    
    def __init__(self, name):
        self.name = name    # Instance attribute
        Counter.count += 1  # Increment class attribute

c1 = Counter("first")
c2 = Counter("second")
print(Counter.count)  # 2

Methods

Different types of methods:

class MyClass:
    class_var = "I'm a class variable"
    
    def __init__(self, value):
        self.value = value
    
    # Instance method
    def instance_method(self):
        return f"Value is {self.value}"
    
    # Class method
    @classmethod
    def class_method(cls):
        return f"Class var: {cls.class_var}"
    
    # Static method
    @staticmethod
    def static_method(x, y):
        return x + y

obj = MyClass(10)
print(obj.instance_method())       # Value is 10
print(MyClass.class_method())      # Class var: I'm a class variable
print(MyClass.static_method(3, 4)) # 7

Inheritance

Create a class based on another:

class Animal:
    def __init__(self, name):
        self.name = name
    
    def speak(self):
        return "Some sound"

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())  # Buddy says woof!
print(cat.speak())  # Whiskers says meow!

Using super()

Call parent class methods:

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

class Dog(Animal):
    def __init__(self, name, breed):
        super().__init__(name)  # Call parent __init__
        self.breed = breed

dog = Dog("Buddy", "Golden Retriever")
print(dog.name, dog.breed)  # Buddy Golden Retriever

Special Methods (Dunder Methods)

class Book:
    def __init__(self, title, pages):
        self.title = title
        self.pages = pages
    
    def __str__(self):
        return f"{self.title}"
    
    def __len__(self):
        return self.pages
    
    def __eq__(self, other):
        return self.title == other.title

book = Book("Python Guide", 300)
print(str(book))    # Python Guide
print(len(book))    # 300

Summary

Use class keyword to define a class
Use __init__ for initialization
Use self to reference instance attributes
Use inheritance with class Child(Parent)
Use super() to call parent methods

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