OOP Part 2: Inheritance and Polymorphism

OOP Part 2: Inheritance and Polymorphism

Master advanced OOP concepts including inheritance, method overriding, polymorphism, and abstract classes. Model vehicle hierarchies reflecting Indian transportation systems.

Inheritance: Code Reuse and Hierarchy

Inheritance allows creating new classes based on existing ones, inheriting attributes and methods.

# Parent class (base class)
class Vehicle: def __init__(self, brand, model, year): self.brand = brand self.model = model self.year = year def display_info(self): print(f'{self.year} {self.brand} {self.model}') def start_engine(self): print('Engine started')

# Child class (derived class)
class Car(Vehicle): def __init__(self, brand, model, year, num_doors): super().__init__(brand, model, year)  # Call parent constructor self.num_doors = num_doors def display_info(self): super().display_info()  # Call parent method print(f'Doors: {self.num_doors}')

class Truck(Vehicle): def __init__(self, brand, model, year, cargo_capacity): super().__init__(brand, model, year) self.cargo_capacity = cargo_capacity  # in tons def display_info(self): super().display_info() print(f'Cargo capacity: {self.cargo_capacity} tons')

# Creating objects
car = Car('Maruti', 'Swift', 2023, 4)
truck = Truck('Tata', 'Truck', 2023, 10)

car.display_info()
# Output:
# 2023 Maruti Swift
# Doors: 4

truck.start_engine()  # Inherited method
# Output: Engine started 

Method Overriding: Customizing Behavior

Child classes can override parent methods to provide custom implementations.

class Animal: def __init__(self, name): self.name = name def sound(self): return 'Some generic sound' def display(self): print(f'Animal: {self.name}')

class Dog(Animal): def sound(self): return 'Woof! Woof!'

class Cat(Animal): def sound(self): return 'Meow!'

class Cow(Animal): def sound(self): return 'Moo!'

# Polymorphism in action
animals = [Dog('Buddy'), Cat('Whiskers'), Cow('Bessie')]

for animal in animals: animal.display() print(f'Sound: {animal.sound()}') print()

# Output:
# Animal: Buddy
# Sound: Woof! Woof!
# 
# Animal: Whiskers
# Sound: Meow!
# 
# Animal: Bessie
# Sound: Moo! 

Indian Vehicle Hierarchy

Model the Indian transportation system with various vehicle types.

class IndianVehicle: def __init__(self, registration, owner, model): self.registration = registration  # License plate self.owner = owner self.model = model def get_road_tax(self): '''Base method for road tax calculation''' raise NotImplementedError('Subclass must implement get_road_tax()') def display_info(self): print(f'Registration: {self.registration}') print(f'Owner: {self.owner}') print(f'Model: {self.model}') print(f'Road Tax: ${self.get_road_tax():.2f}')

class Auto(IndianVehicle): '''3-wheeler auto rickshaw common in India''' def __init__(self, registration, owner, model, seating_capacity=4): super().__init__(registration, owner, model) self.seating_capacity = seating_capacity def get_road_tax(self): return 1500  # Annual road tax for auto def display_info(self): super().display_info() print(f'Seating: {self.seating_capacity}')

class Bus(IndianVehicle): '''Public transport bus''' def __init__(self, registration, owner, model, capacity, route): super().__init__(registration, owner, model) self.capacity = capacity self.route = route def get_road_tax(self): return 5000  # Higher tax for commercial bus def display_info(self): super().display_info() print(f'Capacity: {self.capacity} passengers') print(f'Route: {self.route}')

class Train(IndianVehicle): '''Indian Railway train''' def __init__(self, registration, owner, model, coaches, passengers_per_coach): super().__init__(registration, owner, model) self.coaches = coaches self.passengers_per_coach = passengers_per_coach def total_capacity(self): return self.coaches * self.passengers_per_coach def get_road_tax(self): # Tax based on capacity return self.total_capacity() * 10 def display_info(self): super().display_info() print(f'Coaches: {self.coaches}') print(f'Total Capacity: {self.total_capacity()} passengers')

# Using the vehicle hierarchy
vehicles = [ Auto('DL01AA1234', 'Raj Kumar', 'Bajaj Auto'), Bus('HR26BL5678', 'City Transport', 'Tata Bus', 50, 'Delhi-Gurugram'), Train('IR001', 'Indian Railways', 'LHB Train', 24, 100)
]

for vehicle in vehicles: vehicle.display_info() print('-' * 40) 

Multiple Inheritance

A class can inherit from multiple parent classes.

class Drawable: def draw(self): print('Drawing...')

class Resizable: def resize(self, percentage): print(f'Resizing to {percentage}% of original')

class Movable: def move(self, x, y): print(f'Moving to ({x}, {y})')

class Shape(Drawable, Resizable, Movable): def __init__(self, name): self.name = name def describe(self): print(f'Shape: {self.name}')

class Circle(Shape): def __init__(self, name, radius): super().__init__(name) self.radius = radius def area(self): return 3.14 * self.radius ** 2 def describe(self): super().describe() print(f'Radius: {self.radius}') print(f'Area: {self.area():.2f}')

# Using multiple inheritance
circle = Circle('Circle', 5)
circle.describe()
circle.draw()
circle.resize(150)
circle.move(10, 20)

# Method Resolution Order (MRO)
print('MRO:', Circle.__mro__) 

Abstract Classes and Interfaces

Abstract classes define interfaces that subclasses must implement.

from abc import ABC, abstractmethod

class PaymentMethod(ABC): '''Abstract base class for payment methods''' @abstractmethod def process_payment(self, amount): '''Process payment - must be implemented by subclasses''' pass @abstractmethod def refund(self, amount): pass def validate_amount(self, amount): '''Concrete method available to all subclasses''' if amount <= 0: raise ValueError('Amount must be positive') return True

class CreditCard(PaymentMethod): def __init__(self, card_number, holder): self.card_number = card_number self.holder = holder def process_payment(self, amount): self.validate_amount(amount) print(f'Processing credit card payment of ${amount} for {self.holder}') return True def refund(self, amount): self.validate_amount(amount) print(f'Refunding ${amount} to credit card') return True

class UPI(PaymentMethod): def __init__(self, upi_id): self.upi_id = upi_id def process_payment(self, amount): self.validate_amount(amount) print(f'Processing UPI payment of ${amount} from {self.upi_id}') return True def refund(self, amount): self.validate_amount(amount) print(f'Refunding ${amount} to {self.upi_id}') return True

class MobileWallet(PaymentMethod): def __init__(self, phone_number): self.phone_number = phone_number def process_payment(self, amount): self.validate_amount(amount) print(f'Processing wallet payment of ${amount} from {self.phone_number}') return True def refund(self, amount): self.validate_amount(amount) print(f'Refunding ${amount} to {self.phone_number}') return True

# Using abstract classes
payments = [ CreditCard('1234-5678-9012-3456', 'Aditya Singh'), UPI('aditya@paytm'), MobileWallet('9876543210')
]

for payment in payments: payment.process_payment(500) print()

# Cannot instantiate abstract class
# p = PaymentMethod()  # TypeError: Can't instantiate abstract class 

Comparison: Composition vs Inheritance

Choose between inheritance and composition based on the relationship.

# Inheritance: "is-a" relationship
class Animal: pass

class Dog(Animal): pass

# Composition: "has-a" relationship
class Engine: def start(self): print('Engine started')

class Car: def __init__(self): self.engine = Engine() def start(self): self.engine.start()

# Better example: Composition for flexibility
class Person: def __init__(self, name, job): self.name = name self.job = job def work(self): self.job.perform_work()

class SoftwareEngineer: def perform_work(self): print('Writing code')

class Teacher: def perform_work(self): print('Teaching students')

class Doctor: def perform_work(self): print('Treating patients')

# Using composition
people = [ Person('Aditya', SoftwareEngineer()), Person('Priya', Teacher()), Person('Rajesh', Doctor())
]

for person in people: print(f'{person.name} is working:') person.work() 

Practice Problems

1. Create an inheritance hierarchy for Indian educational institutions (School, College, University)
2. Create an abstract Shape class with subclasses for different geometric shapes
3. Implement a payment system with multiple payment methods using inheritance or composition
4. Create a multi-level inheritance hierarchy (grandparent, parent, child classes)
5. Implement an abstract Employee class with specific employee types (Manager, Developer, Analyst)

Key Takeaways

• Inheritance enables code reuse by creating hierarchies of related classes
• Method overriding allows subclasses to customize parent behavior
• Polymorphism enables treating different objects uniformly through their common interface
• Abstract classes enforce implementation contracts for subclasses
• Choose inheritance for "is-a" relationships and composition for "has-a" relationships

From Concept to Reality: OOP Part 2: Inheritance and Polymorphism

In the professional world, the difference between a good engineer and a great one often comes down to understanding fundamentals deeply. Anyone can copy code from Stack Overflow. But when that code breaks at 2 AM and your application is down — affecting millions of users — only someone who truly understands the underlying concepts can diagnose and fix the problem.

OOP Part 2: Inheritance and Polymorphism is one of those fundamentals. Whether you end up working at Google, building your own startup, or applying CS to solve problems in agriculture, healthcare, or education, these concepts will be the foundation everything else is built on. Indian engineers are known globally for their strong fundamentals — this is why companies worldwide recruit from IITs, NITs, IIIT Hyderabad, and BITS Pilani. Let us make sure you have that same strong foundation.

Object-Oriented Programming: Modelling the Real World

OOP lets you model real-world entities as code "objects." Each object has properties (data) and methods (behaviour). Here is a practical example:

class BankAccount: """A simple bank account — like what SBI or HDFC uses internally""" def __init__(self, holder_name, initial_balance=0): self.holder = holder_name self.balance = initial_balance # Private in practice self.transactions = [] # History log def deposit(self, amount): if amount <= 0: raise ValueError("Deposit must be positive") self.balance += amount self.transactions.append(f"+₹{amount}") return self.balance def withdraw(self, amount): if amount > self.balance: raise ValueError("Insufficient funds!") self.balance -= amount self.transactions.append(f"-₹{amount}") return self.balance def statement(self): print(f"
--- Account Statement: {self.holder} ---") for t in self.transactions: print(f"  {t}") print(f"  Balance: ₹{self.balance}")

# Usage
acc = BankAccount("Rahul Sharma", 5000)
acc.deposit(15000) # Salary credited
acc.withdraw(2000) # UPI payment to Swiggy
acc.withdraw(500) # Metro card recharge
acc.statement()

This is encapsulation — bundling data and behaviour together. The user of BankAccount does not need to know HOW deposit works internally; they just call it. Inheritance lets you extend this: a SavingsAccount could inherit from BankAccount and add interest calculation. Polymorphism means different account types can respond to the same .withdraw() method differently (savings accounts might check minimum balance, current accounts might allow overdraft).

Real-World System Design: Swiggy's Architecture

When you order food on Swiggy, here is what happens behind the scenes in about 2 seconds: your location is geocoded (algorithms), nearby restaurants are queried from a spatial index (data structures), menu prices are pulled from a database (SQL), delivery time is estimated using ML models trained on historical data (AI), the order is placed in a distributed message queue (Kafka), a delivery partner is assigned using a matching algorithm (optimization), and real-time tracking begins using WebSocket connections (networking). EVERY concept in your CS curriculum is being used simultaneously to deliver your biryani.

How the Web Request Cycle Works

Every time you visit a website, a precise sequence of events occurs. Here is the flow:

 You (Browser) DNS Server Web Server | | | |---[1] bharath.ai --->| | | | | |<--[2] IP: 76.76.21.9| | | | | |---[3] GET /index.html ----------------->  | | | | | | [4] Server finds file, | | runs server code, | | prepares response | | | |<---[5] HTTP 200 OK + HTML + CSS + JS --- | | | | [6] Browser parses HTML | Loads CSS (styling) | Executes JS (interactivity) | Renders final page |

Step 1-2 is DNS resolution — converting a human-readable domain name to a machine-readable IP address. Step 3 is the HTTP request. Step 4 is server-side processing (this is where frameworks like Node.js, Django, or Flask operate). Step 5 is the HTTP response. Step 6 is client-side rendering (this is where React, Angular, or Vue operate).

In a real-world scenario, this cycle also involves CDNs (Content Delivery Networks), load balancers, caching layers, and potentially microservices. Indian companies like Jio use this exact architecture to serve 400+ million subscribers.

Production Engineering: OOP Part 2: Inheritance and Polymorphism at Scale

Understanding oop part 2: inheritance and polymorphism at an academic level is necessary but not sufficient. Let us examine how these concepts manifest in production environments where failure has real consequences.

Consider India's UPI system processing 10+ billion transactions monthly. The architecture must guarantee: atomicity (a transfer either completes fully or not at all — no half-transfers), consistency (balances always add up correctly across all banks), isolation (concurrent transactions on the same account do not interfere), and durability (once confirmed, a transaction survives any failure). These are the ACID properties, and violating any one of them in a payment system would cause financial chaos for millions of people.

At scale, you also face the thundering herd problem: what happens when a million users check their exam results at the same time? (CBSE result day, anyone?) Without rate limiting, connection pooling, caching, and graceful degradation, the system crashes. Good engineering means designing for the worst case while optimising for the common case. Companies like NPCI (the organisation behind UPI) invest heavily in load testing — simulating peak traffic to identify bottlenecks before they affect real users.

Monitoring and observability become critical at scale. You need metrics (how many requests per second? what is the 99th percentile latency?), logs (what happened when something went wrong?), and traces (how did a single request flow through 15 different microservices?). Tools like Prometheus, Grafana, ELK Stack, and Jaeger are standard in Indian tech companies. When Hotstar streams IPL to 50 million concurrent users, their engineering team watches these dashboards in real-time, ready to intervene if any metric goes anomalous.

The career implications are clear: engineers who understand both the theory (from chapters like this one) AND the practice (from building real systems) command the highest salaries and most interesting roles. India's top engineering talent earns ₹50-100+ LPA at companies like Google, Microsoft, and Goldman Sachs, or builds their own startups. The foundation starts here.

Key Vocabulary

Here are important terms from this chapter that you should know:

💡 Interview-Style Problem

Here is a problem that frequently appears in technical interviews at companies like Google, Amazon, and Flipkart: "Design a URL shortener like bit.ly. How would you generate unique short codes? How would you handle millions of redirects per second? What database would you use and why? How would you track click analytics?"

Think about: hash functions for generating short codes, read-heavy workload (99% redirects, 1% creates) suggesting caching, database choice (Redis for cache, PostgreSQL for persistence), and horizontal scaling with consistent hashing. Try sketching the system architecture on paper before looking up solutions. The ability to think through system design problems is the single most valuable skill for senior engineering roles.

Where This Takes You

The knowledge you have gained about oop part 2: inheritance and polymorphism is directly applicable to: competitive programming (Codeforces, CodeChef — India has the 2nd largest competitive programming community globally), open-source contribution (India is the 2nd largest contributor on GitHub), placement preparation (these concepts form 60% of technical interview questions), and building real products (every startup needs engineers who understand these fundamentals).

India's tech ecosystem offers incredible opportunities. Freshers at top companies earn ₹15-50 LPA; experienced engineers at FAANG companies in India earn ₹50-1 Cr+. But more importantly, the problems being solved in India — digital payments for 1.4 billion people, healthcare AI for rural areas, agricultural tech for 150 million farmers — are some of the most impactful engineering challenges in the world. The fundamentals you are building will be the tools you use to tackle them.

Crafted for Class 7–9 • Python Mastery • Aligned with NEP 2020 & CBSE Curriculum