Top 5 Python Interview Problems on Classes and Objects

Chapter 2: Track the Number of Instances Created Using a Class Variable

🧠 Objective

To master:

• The use of class variables (shared by all instances)
• Tracking the total number of instances of a class
• The difference between instance and class variables
• Using class methods to access and return shared data

This type of question is often asked in interviews to test your understanding of object identity, scope, and lifecycle in Python classes.

📌 Problem Statement

Create a class called Person that:

1. Has instance attributes: name, email
2. Tracks the total number of instances created using a class variable
3. Includes a class method to return the current instance count
4. Uses a readable __str__() method to display the object

🔧 Step-by-Step Implementation

✅ Step 1: Define the Class and Class Variable

class Person:

    total_instances = 0  # class variable

This variable is shared across all instances and will increment every time a new object is created.

✅ Step 2: Initialize and Track Count in __init__

    def __init__(self, name, email):

        self.name = name

        self.email = email

        Person.total_instances += 1

✅ Use the class name Person.total_instances to access and increment the class-level count.

✅ Step 3: Add a Class Method to Retrieve Count

    @classmethod

    def get_instance_count(cls):

        return cls.total_instances

✅ This method is bound to the class, not the instance — note the use of cls instead of self.

✅ Step 4: Add __str__ for Readability

    def __str__(self):

        return f"Person(name={self.name}, email={self.email})"

✅ Full Working Code

class Person:

    total_instances = 0  # shared across all objects

    def __init__(self, name, email):

        self.name = name

        self.email = email

        Person.total_instances += 1

    @classmethod

    def get_instance_count(cls):

        return cls.total_instances

    def __str__(self):

        return f"Person(name={self.name}, email={self.email})"

🧪 Example Usage

p1 = Person("Alice", "alice@example.com")

p2 = Person("Bob", "bob@example.com")

p3 = Person("Charlie", "charlie@example.com")

print(p1)

print(p2)

print(p3)

print("Total instances created:", Person.get_instance_count())

✅ Output:

Person(name=Alice, email=alice@example.com)

Person(name=Bob, email=bob@example.com)

Person(name=Charlie, email=charlie@example.com)

Total instances created: 3

🔁 Class vs Instance Variable Comparison Table

🧠 Adding More Features

🔹 Count Active vs Deleted Instances

def __del__(self):

    Person.total_instances -= 1

Use this to reduce the count when an object is deleted.
⚠️ Caution: __del__() is not always reliable in production (GC timing varies).

🔹 Count Instances Per Subclass (Advanced)

Use cls.__name__ inside the class method to track instances of subclasses individually.

✅ Summary Table