Why should I use a classmethod in python? [duplicate]

Question

I am writing a function in some class in python, and people suggested to me to add to this function a @classmethod decorator.

My code:

import random


class Randomize:
    RANDOM_CHOICE = 'abcdefg'

    def __init__(self, chars_num):
        self.chars_num = chars_num

    def _randomize(self, random_chars=3):
        return ''.join(random.choice(self.RANDOM_CHOICE)
                       for _ in range(random_chars))

The suggested change:

    @classmethod
    def _randomize(cls, random_chars=3):
        return ''.join(random.choice(cls.RANDOM_CHOICE)
                       for _ in range(random_chars))

I'm almost always using only the _randomize function.

My question is: What is the benefits from adding to a function the classmethod decorator?

Ranged

It makes it clearer that the method doesn't use any state from the instance, usually named self. Also it means you can test it on the class without creating an instance. – Haiduk

Also, if this class doesn't contain much else, it's probably better to use a function and functools.partial(). Maybe. – Piperine

Answer 1

If you see _randomize method, you are not using any instance variable (declared in init) in it but it is using a class var i.e. RANDOM_CHOICE = 'abcdefg'.

import random

class Randomize:
    RANDOM_CHOICE = 'abcdefg'

    def __init__(self, chars_num):
        self.chars_num = chars_num

    def _randomize(self, random_chars=3):
        return ''.join(random.choice(self.RANDOM_CHOICE)
                       for _ in range(random_chars))

It means, your method can exist without being an instance method and you can call it directly on class.

  Randomize._randomize()

Now, the question comes does it have any advantages?

• I guess yes, you don't have to go through creating an instance to use this method which will have an overhead.

  ran = Randomize() // Extra steps
  ran._randomize()   
  

You can read more about class and instance variable here.

Hydrostatic

I still don't see the point I guess. If you were to have use cases where you want to call that method and don't require an instance of the class, why not have that method live outside that class? – Cullie

@Cullie why not have that method live outside of that class? well, that's 'cus the method is well suited in that class but you can also use it in other stuff. Why a classmethod instead of a method somewhere else? Because it needs parameters from the class, furthermore it doesn't need parameters from the instance. It's useful when you need a resource from a module, but don't want to deal with the "wasted memory(generalization)" that comes with instanciating a class. It's like why you need staticmethod? You don't need it per se, but it's tiddier to have related stuff in one place. – Audet