Basic OOP - Part 01

#programming #oop #interview #python

Everything in Python is Object-Oriented

Example:

a = 2  # 'a' is an object of integer type

Similarly:

roll = [1, 2, 3, 4]
roll.append(5)  # list method
roll.pop()      # remove last element

Here, roll is an object of the list class.

🧱 What is a Class?

A class is a blueprint. It has:

Data / Properties → Variables
Functions / Behaviors → Methods

Example (Structure only):

class Human:
    name
    age
    phone_no

    def demo():
        pass

Class names should be in PascalCase → ThisIsClass
Variable and method names should be in snake_case → this_is_variable

💡 OOP Benefits

✅ Reusable code
✅ No global variables required
✅ Organized and modular code
✅ Easier debugging
✅ Data protection via encapsulation

🔁 Procedural vs Object-Oriented (OOP)

🧨 Non-OOP Example (Risky!):

balance = 100

def withdraw(amount):
    global balance
    balance -= amount

✅ OOP Version (Cleaner and Safer):

class BankAccount:
    def __init__(self, balance):
        self.balance = balance

    def withdraw(self, amount):
        self.balance -= amount

# Usage
account1 = BankAccount(100)
account2 = BankAccount(200)

account1.withdraw(50)  # Only affects account1

🔧 Class with Constructor (`init()`)

A special method called when the object is created.

Example:

class Atm:
    def __init__(self):
        print("ATM object created")

Note:

It's spelled __init__, not __int__
Called automatically when an object is instantiated

🎓 `init()` is a Magic Method

Magic methods in Python have double underscores: __method__.
You cannot skip __init__() if you want to initialize object attributes.

🍽 OOP Analogy — Like a Recipe!

OOP Concept	Cooking Analogy
Class	Recipe
Object	Cooked Dish
Instantiation	Cooking
Attributes	Spice Level, Garnish
Methods	Cooking Instructions

🍛 Custom Class Example: Dish

Define:

class Dish:
    def __init__(self, name, spice_level, garnish):
        self.name = name
        self.spice_level = spice_level
        self.garnish = garnish

    def serve(self):
        print(f"Serving {self.name} with {self.spice_level} spice and {self.garnish} garnish.")

Create objects:

dish1 = Dish("Curry", "Medium", "Cilantro")
dish2 = Dish("Curry", "Spicy", "Mint Leaves")

dish1.serve()
dish2.serve()

Output:

Serving Curry with Medium spice and Cilantro garnish.
Serving Curry with Spicy spice and Mint Leaves garnish.

🙋 What is `self`?

Refers to the current object instance.
Always required as the first parameter in instance methods.

Example:

self.name = name  # means "this object’s name = the given name"

🧬 Inheritance (Optional but Powerful)

Allows a class to inherit properties and methods from another.

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

class SweetDish(Dish):
    def serve(self):
        print(f"Serving sweet {self.name}")

cake = SweetDish("Cake")
cake.serve()  # Output: Serving sweet Cake