Class Methods vs Instance Methods in Python: Why, When, and How to Use Them

In Python, the object-oriented programming (OOP) paradigm encourages developers to structure their code around objects, which are instances of classes. While writing Python lessons, you will encounter a variety of methods, two of which are the most common class methods And non-class methods (which are often called instance methods). Understanding the difference between them, as well as when and why to use them, can lead to cleaner, more efficient, and more maintainable code.

In this post, we’ll explore the core concepts of class methods versus non-class methods (instance methods), their differences, and provide guidance on when to use each, with practical examples.

What are instance methods?

Instance methods are the most common type of method in Python. They operate on the instances (objects) of the class. This means they take self as the first parameter, which points to the instance that called the method. Instance methods can access and modify object-specific data, that is, the attributes associated with that specific instance.

How instance methods work

Consider a simple one Car class:

class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model

    def display_info(self):
        return f"Car Brand: {self.brand}, Model: {self.model}"

Here, display_info is an instance method because it works on the self parameter, referring to the specific Car predisposition.

You can make several Car objects, and the display_info method shows the corresponding information for each:

car1 = Car('Toyota', 'Corolla')
car2 = Car('Honda', 'Civic')

print(car1.display_info())  # Car Brand: Toyota, Model: Corolla
print(car2.display_info())  # Car Brand: Honda, Model: Civic

Why use instance methods?

Instance methods are ideal when you:

• Open or change the attributes of the instance.
• Encapsulate behavior that is specific to an instance.
• Ensure a clear relationship between methods and object status.

In general, use instance methods for most of your class methods unless there is a specific need for another type of method.

What are class methods?

Class methods differ from instance methods in that they work with the class itself, rather than any instance of the class. They take cls as their first parameter, which refers to the class, not the instance. You define class methods using the @classmethod decorator.

How class methods work

Class methods access class-level data (attributes shared by all instances of the class), but they do not interact with instance-specific data unless explicitly passed. Here is a simple example of a class method:

class Car:
    cars_created = 0  # Class-level attribute shared by all instances

    def __init__(self, brand, model):
        self.brand = brand
        self.model = model
        Car.cars_created += 1

    @classmethod
    def total_cars_created(cls):
        return f"Total Cars Created: {cls.cars_created}"

In this example the total_cars_created method is a class method because it involves interaction cls.cars_createda class-level attribute.

car1 = Car('Toyota', 'Corolla')
car2 = Car('Honda', 'Civic')

print(Car.total_cars_created())  # Total Cars Created: 2

Why use class methods?

Class methods are useful when you:

• Work with class-level data.
• Provide functionality that relates to the class rather than a specific instance.
• Create alternate constructors (that is, methods that create instances in different ways).

A classic example is using a class method as an alternative constructor:

class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model

    @classmethod
    def from_string(cls, car_string):
        brand, model = car_string.split("-")
        return cls(brand, model)

Here, from_string is a class method that allows you to create a Car instance from a string:

car = Car.from_string("Toyota-Corolla")
print(car.brand)  # Toyota
print(car.model)  # Corolla

This is a common pattern when you want to give users another way to instantiate objects.

Key differences between class methods and instance methods

When to use class methods vs instance methods?

When to use instance methods

• Access or modify instance data: When you need to work with specific object data. Instance methods are your default choice when writing methods for your class.

Example:

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

    def greet(self):
        print(f"Hello, my name is {self.name}.")

When to use class methods?

• Working with class-level data: When you are working with data that is relevant to the class rather than to a particular instance.

• Alternative constructors: When you need to create objects in a way that differs from the default constructor.

Example:

class Employee:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def from_birth_year(cls, name, birth_year):
        age = 2024 - birth_year
        return cls(name, age)

Conclusion

The choice between class methods and instance methods largely depends on the type of data you need to access (class level or instance specific) and the scope of the behavior you want to define. Instance methods work on individual objects and are your go-to for most class methods. On the other hand, class methods are ideal for working with class-level attributes, alternative constructors, or for providing functionality related to the class as a whole.

By understanding the role each type of method plays in Python OOP, you can design classes that are clean, intuitive, and maintainable, using the right tools for the right task.