What is the difference between a class method and an instance method in Python?

In Python, instance methods and class methods are two types of methods used inside classes, and they differ in what they operate on and how they are called. Here’s a clear, interview-ready explanation:

1. Instance Method #

• Definition: A method that operates on a specific instance (object) of a class.
• First Parameter: Always self, which refers to the instance.
• Access: Can access instance attributes and class attributes.

Example:

class Employee:
    raise_amount = 1.1  # class attribute

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def apply_raise(self):  # instance method
        self.salary = int(self.salary * self.raise_amount)
        return self.salary

emp = Employee("Aswini", 5000)
print(emp.apply_raise())  # 5500

Here, apply_raise() uses self.salary → operates on that particular object.

2. Class Method #

• Definition: A method that operates on the class itself, not on a specific object.
• First Parameter: Always cls, which refers to the class.
• Decorator: @classmethod
• Access: Can access class attributes, but not instance attributes unless passed explicitly.

Example:

class Employee:
    raise_amount = 1.1

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    @classmethod
    def set_raise_amount(cls, amount):
        cls.raise_amount = amount

Employee.set_raise_amount(1.2)  # modifies class attribute for all instances

emp1 = Employee("Aswini", 5000)
emp2 = Employee("Kumar", 6000)
print(emp1.salary * emp1.raise_amount)  # 6000
print(emp2.salary * emp2.raise_amount)  # 7200

Here, set_raise_amount() changes the class-level attribute, affecting all instances.

Key Differences #

Practical Use Case #

In a project I worked on:

• Instance methods handled individual user operations like login() or update_profile().
• Class methods handled operations affecting the class, like creating users from a CSV file or changing system-wide settings (class attributes).

This separation of responsibilities keeps the code organized and modular.