Why use a classmethod over an Instance method in python

Question

I have been trying to get my head around classmethods for a while now. I know how they work but I don't understand why use them or not use them.

For example.

I know i can use an instance method like this:

class MyClass():
    def __init__(self):
        self.name = 'Chris'
        self.age = 27

    def who_are_you(self):
        print('Hello {}, you are {} years old'.format(self.name, self.age)

c = MyClass()
c.who_are_you()

I also know that by using the classmethod I can call the who_are_you() without creating an instance of my class:

    class MyClass():
        name = 'Chris'
        age = 27

        @classmethod
        def who_are_you(cls):
            print('Hello {}, you are {} years old'.format(cls.name, cls.age)

MyClass.who_are_you()

I dont get why you would pick one method over the other

Answer 1

In your second example, you've hard-coded the name and age into the class. If name and age are indeed properties of the class and not a specific instance of the class, than using a class method makes sense. However, if your class was something like Human of which there are many instances with different names and ages, then it wouldn't be possible to create a class method to access the unique names and ages of the specific instance. In that case, you would want to use an instance method.

In general:

• If you want to access a property of a class as a whole, and not the property of a specific instance of that class, use a class method.
• If you want to access/modify a property associated with a specific instance of the class, then you will want to use an instance method.