__init__ and arguments in Python

Question

I want to understand arguments of the constructor __init__ in Python.

class Num:
    def __init__(self,num):
        self.n = num
    def getn(self):
        return self.n
    def getone():
        return 1
myObj = Num(3)

print myObj.getn()

RESULT: 3

I call the getone() method:

print myObj.getone()

RESULT: Error 'getone()' takes no arguments (1given).

So I replace:

def getone():
    return 1

with

def getone(self):
    return 1

RESULT:1 This is OK.

But getone() method needs no arguments.

Do I have to use meaningless argument?

Answer 1

In Python:

• Instance methods: require the self argument.
• Class methods: take the class as a first argument.
• Static methods: do not require either the instance (self) or the class (cls) argument.

__init__ is a special function and without overriding __new__ it will always be given the instance of the class as its first argument.

An example using the builtin classmethod and staticmethod decorators:

import sys

class Num:
    max = sys.maxint

    def __init__(self,num):
        self.n = num

    def getn(self):
        return self.n

    @staticmethod
    def getone():
        return 1

    @classmethod
    def getmax(cls):
        return cls.max

myObj = Num(3)
# with the appropriate decorator these should work fine
myObj.getone()
myObj.getmax()
myObj.getn()

That said, I would try to use @classmethod/@staticmethod sparingly. If you find yourself creating objects that consist of nothing but staticmethods the more pythonic thing to do would be to create a new module of related functions.