CODEWITHSUNDEEP
pythonclassmethodsstatic-methodsclass-method
Theo d'Or
Theo d'Or

Reputation: 823

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

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?"

Upvotes: 14

Views: 14455

Answers (4)

information_interchange
information_interchange

Reputation: 3118

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)

Upvotes: 2

Recessive
Recessive

Reputation: 1939

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)

Upvotes: 4

jar
jar

Reputation: 2908

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)

Upvotes: 6

Daniel Roseman
Daniel Roseman

Reputation: 599610

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.

Upvotes: 15

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