Top 5 Python Interview Problems on Classes and Objects

Chapter 6: Design a Shape Class Hierarchy Demonstrating Polymorphism

🧠 Objective

This chapter teaches you how to:

• Use polymorphism in Python OOP
• Create a base class with common methods
• Override methods in subclasses to provide specialized behavior
• Use abstract base classes (optional, advanced)
• Handle different object types with a single interface

This problem is a common interview favorite to test how well you understand method overriding, dynamic dispatch, and interface consistency.

📌 Problem Statement

Create a class hierarchy for different 2D shapes using polymorphism:

1. Define a base class Shape with a method area().
2. Create the following subclasses:
• Circle (with radius)
• Rectangle (with width and height)
• Triangle (with base and height)
3. Override the area() method in each subclass to return the correct area.
4. Loop over a list of shapes and print their area using a common interface.

🔧 Step-by-Step Implementation

✅ Step 1: Define the Base Class

class Shape:

    def area(self):

        raise NotImplementedError("Subclasses must implement this method")

✅ This ensures all shapes must define their own area() method.

✅ Step 2: Subclass Circle

import math

class Circle(Shape):

    def __init__(self, radius):

        self.radius = radius

    def area(self):

        return math.pi * self.radius ** 2

✅ Step 3: Subclass Rectangle

class Rectangle(Shape):

    def __init__(self, width, height):

        self.width = width

        self.height = height

    def area(self):

        return self.width * self.height

✅ Step 4: Subclass Triangle

class Triangle(Shape):

    def __init__(self, base, height):

        self.base = base

        self.height = height

    def area(self):

        return 0.5 * self.base * self.height

🧪 Example Usage with Polymorphism

shapes = [

    Circle(5),

    Rectangle(4, 6),

    Triangle(3, 7)

]

for shape in shapes:

    print(f"{shape.__class__.__name__} Area: {shape.area():.2f}")

✅ Output:

Circle Area: 78.54

Rectangle Area: 24.00

Triangle Area: 10.50

✅ Polymorphism in Action

✅ All objects use the same method name (area)
✅ But each class provides its own implementation

🔄 Optional: Use Abstract Base Classes (abc module)

from abc import ABC, abstractmethod

class Shape(ABC):

    @abstractmethod

    def area(self):

        pass

✅ Enforces that Shape cannot be instantiated and requires subclasses to implement area().

✅ Summary Table

✅ Real-World Uses of Polymorphism