Python Class Methods

Summary: in this tutorial, you’ll learn about Python class methods and when to use them appropriately.

Introduction to Python class methods

So far, you learned about instance methods that are bound to a specific instance of a class.

Instance methods can access instance variables within the same class. To invoke instance methods, you need to create an instance of the class first.

The following defines the Person class:

class Person:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def get_full_name(self):
        return f"{self.first_name} {self.last_name}"

    def introduce(self):
        return f"Hi. I'm {self.first_name} {self.last_name}. I'm {self.age} years old."Code language: Python (python)

The Person class has three instance methods including __init__(), get_full_name(), and introduce().

Suppose that you want to add a method that creates an anonymous person to the Person class.

In order to do so, you would come up with the following code:

class Person:
    # ... other methods
    def create_anonymous(self):
        return Person('John', 'Doe', 25)
Code language: Python (python)

The create_anonymous() is an instance method that returns an anonymous person.

However, to invoke the create_anonymous() method, you need to create an instance, which doesn’t make sense in this case.

This is why Python class methods come into play.

A class method isn’t bound to any specific instance. It’s bound to the class only.

To define a class method:

• First place the @classmethod decorator above the method definition. For now, you just need to understand that the @classmethod decorator will change an instance method to a class method.
• Second, rename the self parameter to cls. The cls means class. However, class is a keyword so you cannot use it as a parameter.

The following shows the new version of the Person class:

class Person:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def get_full_name(self):
        return f"{self.first_name} {self.last_name}"

    def introduce(self):
        return f"Hi. I'm {self.first_name} {self.last_name}. I'm {self.age} years old."

    @classmethod
    def create_anonymous(cls):
        return Person('John', 'Doe', 25)
Code language: Python (python)

The create_anonymous() method cannot access instance attributes. But it can access class attributes via the cls variable.

Calling Python class methods

To call a class method, you use the class name, followed by a dot, and then the method name like this:

ClassName.method_name()Code language: Python (python)

The following example shows how to call the create_anonymous() class method of the Person class:

anonymous = Person.create_anonymous()
print(anonymous.introduce())
Code language: Python (python)

Output:

Hi. I'm John Doe. I'm 25 years old.Code language: Python (python)

Class methods vs. instance methods

The following table illustrates the differences between class methods and instance methods:

When to use Python class methods

You can use class methods for any methods that are not bound to a specific instance but the class. In practice, you often use class methods for methods that create an instance of the class.

When a method creates an instance of the class and returns it, the method is called a factory method. For example, the create_anonymous() is a factory method because it returns a new instance of the Person class.

Summary

• Python class methods aren’t bound to any specific instance, but classes.
• Use @classmethod decorator to change an instance method to a class method. Also, pass the cls as the first parameter to the class method.
• Use class methods for factory methods.