__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.

Answer 2

Every method needs to accept one argument: The instance itself (or the class if it is a static method).

Read more about classes in Python.