Calling a static method inside a class in python

Question

I run into a situation, where I call a static method of a class from another static method. To be sure, that I don't ask an X-Y-question, I'm trying to give some background.

I have a class, that holds a data container and several methods to convert data inside the container. As I also want the converters to be callable from the outside without a class instance, I choose static methods:

class SomeClass(object):

  def __init__(self,some_data):
    self.data = some_data

  @staticmethod
  def convert_1(data_item):
    return 1+data_item

  @staticmethod
  def convert_2(data_item):
    return 2*data_item

Now I can do SomeClass.convert_1(data_item) without the need to create an instance of SomeClass.

Let's say, I want to have a method inside SomeClass, that does the two converts successively, and also want to have that method as a static method.

Can I do

@staticmethod
def combined_convert(data_item):
  data_item = SomeClass.convert_1(data_item)
  data_item = SomeClass.convert_2(data_item)
  return data_item

inside SomeClass? This feels wrong, as I call the class inside its own definition, but I cannot come up with another 'more pythonic' way.

The class is defined by the time you get to execute combined_convert so there is no problem. — thebjorn
– thebjorn, Commented Sep 22, 2017 at 13:22
You reference instances of the class from methods all the time (with self); why should referencing the class be a problem? It's just the top-level code within a class statement that cannot refer to the class under construction. — chepner
– chepner, Commented Sep 22, 2017 at 13:24
Have you tried using top-level functions inside modules? Looks like you are trying to use classes for namespacing — Felk
– Felk, Commented Sep 22, 2017 at 13:31
@Felk actually yes. Factoring out the static methods into top-level functions would be a solution. I feel, this is more a design choice. Or are there more caveats? — Dschoni
– Dschoni, Commented Sep 22, 2017 at 13:35
using modules for this is just idiomatic python. Using static methods in classes is usually just more typing and has no benefits, except if it's a factory method. — Felk
– Felk, Commented Sep 22, 2017 at 13:40

Amarpreet Singh · Accepted Answer · 2017-09-22 13:26:13Z

1

You can create a class method.

@classmethod
def combined_convert(cls,data_item):
    data_item = cls.convert_1(data_item)
    data_item = cls.convert_2(data_item)
    return data_item

answered Sep 22, 2017 at 13:26

Amarpreet Singh

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3 Comments

Dschoni Over a year ago

I fee, this is wrong, cause I cannot call this method from the outside withouth passing an instance, or can I?

Amarpreet Singh Over a year ago

You can call the method outside without instance

Felk Over a year ago

If you access the class itself and want to be able to refactor the code more easily, this is the way to go, because @classmethod-decorated methods are static, but automagically get the class passed as a first argument, which then can be used instead of repeating the class name. If you want a pure static method however, there's @staticmethod, which injects no arguments by itself

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