Top 5 Python Interview Problems on Classes and Objects

Chapter 2: Create a Student Class with Name, Age, and Grades

🧠 Objective

To learn how to:

• Define a simple class in Python
• Use instance variables and the constructor method __init__
• Create and interact with multiple object instances
• Implement class methods to encapsulate logic
• Use __str__() for a readable object representation

This is a foundational interview exercise that assesses whether a candidate understands the basics of object-oriented programming in Python.

📌 Problem Statement

Create a Python class called Student with the following requirements:

1. Each student should have a name, age, and a list of grades.
2. Add a method to calculate the average grade.
3. Add a __str__ method that prints the student in the format:
   "Student(name=John, age=20, avg_grade=88.67)"

🔧 Step-by-Step Implementation

✅ Step 1: Define the Class and Constructor

class Student:

    def __init__(self, name, age, grades):

        self.name = name

        self.age = age

        self.grades = grades

✅ Step 2: Add a Method to Calculate Average Grade

    def average_grade(self):

        if not self.grades:

            return 0

        return sum(self.grades) / len(self.grades)

• Handles empty list cases
• Returns a float representing the average

✅ Step 3: Add a Readable __str__ Method

    def __str__(self):

        avg = self.average_grade()

        return f"Student(name={self.name}, age={self.age}, avg_grade={avg:.2f})"

This improves debugging and output readability.

✅ Full Working Code

class Student:

    def __init__(self, name, age, grades):

        self.name = name

        self.age = age

        self.grades = grades

    def average_grade(self):

        if not self.grades:

            return 0

        return sum(self.grades) / len(self.grades)

    def __str__(self):

        avg = self.average_grade()

        return f"Student(name={self.name}, age={self.age}, avg_grade={avg:.2f})"

🧪 Example Usage

student1 = Student("Alice", 20, [88, 76, 92])

student2 = Student("Bob", 22, [90, 85, 87, 91])

student3 = Student("Charlie", 19, [])

print(student1)

print(student2)

print(student3)

✅ Output:

Student(name=Alice, age=20, avg_grade=85.33)

Student(name=Bob, age=22, avg_grade=88.25)

Student(name=Charlie, age=19, avg_grade=0.00)

🔍 Testing Edge Cases

📈 Extending the Model

Add Validation (Optional)

def add_grade(self, grade):

    if 0 <= grade <= 100:

        self.grades.append(grade)

    else:

        raise ValueError("Grade must be between 0 and 100")

Add Class Variable to Count Students

class Student:

    count = 0

    def __init__(self, name, age, grades):

        Student.count += 1

        ...

✅ Concept Recap Table