Why is a method of a Python class declared without "self" and without decorators not raising an exception?

Question

I thought that the following code would result in an error because as far as I have read, a method in a Python class must either have "self" (or any other label, but "self" by convention) as its first argument, or "cls" or similar if the @classmethod decorator is used, or none if the @staticmethod decorator is used.

How come I get no error running this with Python 3.5 in the Terminal, even though test_method does not meet these requirements? It seems to work fine as a static method, but without the decorator.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import sys

class MyClass:

    def test_method(args):
        print(args[1])

    @staticmethod
    def static_method():
        print("static_method")

    @classmethod
    def class_method(cls):
        print("class_method")


def main(args):
    MyClass.test_method(args)


if __name__ == '__main__':
    sys.exit(main(sys.argv))

Output:

$ python3 testscript.py "testing"
$ testing

EDIT:

My question could also be phrased differently, drawing attention away from self and to @staticmethod: "How come I'm getting a seemingly working static method without the @staticmethod decorator?"

Wisconsin

Short answer is: it's not a method until you create an instance of the class. It's simply a function. Try MyClass().text_method('test') and you will get an error. – Cuttler

@Cuttler If it is not a method but simply a function, how is it different from a static method declared with the decorator? – Sell

Perhaps this question answers , your query – Fernandefernandel

A method will be accesible from an instance of the class, @Theod'Or. In other words, this cannot be used from an instance, only from the class itself. – Cuttler

Answer 1

In Python 2, functions defined in a class body are automatically converted to "unbound methods", and cannot be called directly without a staticmethod decorator. In Python 3, this concept was removed; MyClass.text_method is a simple function that lives inside the MyClass namespace, and can be called directly.

The main reason to still use staticmethod in Python 3 is if you also want to call the method on an instance. If you don't use the decorator, the method will always be passed the instance as the first parameter, causing a TypeError.

Faceoff

I've accepted this answer because it makes the distinction between Python 2 and 3 clear. It's really what was throwing me, I did not know this change had taken place (not been using Python for a while). Thank you! – Sell

What do you mean by "can be called directly"? That I don't have to create an object of type MyClass first on which I will call text_method()? – Picky

Yes, call with MyClass.text_method(...). – Berseem

Answer 2

There is nothing special about this. In python 3 there is no difference between a function defined inside a class or a function defined outside a class. Both of them are normal functions.

The self that you are talking about here or maybe cls comes into picture only when you access the function through an instance. Hence here you didn't get any error.

However if you modify your code just a little bit to look like the following, then you'd get an error that you expected.

def main(args):
    MyClass().test_method(args)
    # Should throw an error

EDIT:

• @staticmethod will work on both class instances like MyClass().test_method(args)and just a regular direct call like MyClass.test_method(args)
• However a regular method(without self in it) can't be called on a class instance. So you will always have to call it as MyClass.test_method(args)

Argot

How is a normal function declared in a class different from a static method? – Sell

@Theod'Or It is identical to staticmethods, the only difference being, you can't call it on class instances. – Argot

@Argot do you happen to know why Python provides these two types of static methods? (with and without decorators), e.g. what would be a good reason for restricting instance calls? – Clint

Answer 3

self isn't necessarily required. However, if you want to reference any variable or value that is associated with the object(instantiation of the class) (E.g. for a class about cars, it's speed, self.speed) you'll need to have self as a parameter in the function. For this reason, it's common practice to always have self as an argument, otherwise you aren't really using the class for the right reason.

EDIT: This will actually throw an error if you do the following:

class a():
    def __init__(self, x):
        self.asd = x
    def hello(x):
        print(x)

>>> g = a(4)
>>> g.hello(5)

as when calling "hello", both "self" and "4" will be passed as parameters. It would work in the following instance, which is what I was saying above:

>>> g = a
>>> g.hello(4)

or

>>> a.hello(4)

Onions

This isn't really true. self is required for an instance method, even if you don't use it, because it will be passed anyway; if the method doesn't accept it, you will get a TypeError. – Faceoff

Can you give me an example of some code that throws this error? Because the following: class a(): def hello(x): print(x) works fine – Onions

a.hello() would work, but then you're not calling it on an instance so you wouldn't expect to have self anyway. a().hello() would give that error. – Faceoff

You're right, I just tested it. That's really interesting, learn something new every day (: – Onions

Answer 4

To add on to the existing answers here and provide a code example:

class MyClass:
    def __init__(self):
        pass

    def myStaticMethod():
        print("a static method")

    @staticmethod
    def myStaticMethodWithArg(my_arg):
        print(my_arg)
        print("a static method")


MyClass.myStaticMethod()
MyClass.myStaticMethodWithArg("skhsdkj")
abc = MyClass()
abc.myStaticMethodWithArg("avc")

Try removing the @staticmethod decorator and rerunning the code and see what happens! (The very last call will fail since the method is passed in both self and the string input. By adding the decorator, we can guide the interpreter to perform our desired action)

Calli