Should all member variables be initialized in __init__

Question

Maybe this is more of a style question than a technical one but I have a class with several member variables and I want to have it work so that some of the member variables are initialized when the user first creates an instance of the class (i.e. in the __init__ function) and I want the other member variables to be defined from arguments of member functions that will be called later on. So my question is should I initialize all member variables in the __init__ function (and set the ones that will be defined later on to dummy values) or initialize some in the __init__ function and some in later functions. I realize this might be difficult to understand so here are a couple of examples.

This example has var3 set to 0 initially in the __init__ function, then set to the desired value later on in the my_funct function.

class myClass(object):
   def __init__(self,var1,var2):
        self.var1=var1
        self.var2=var2
        self.var3=0

  def my_funct(self,var3):
       self.var3=var3

and in this example, var3 is not defined at all in the __init__ function

class myClass(object):
   def __init__(self,var1,var2):
        self.var1=var1
        self.var2=var2

  def my_funct(self,var3):
       self.var3=var3

I don't think either way would make a big difference (maybe a slight difference in memory usage). But I was wondering if one of these is preferred over the other for some reason.

Answer 1

In object-oriented programming it's up to the developer to ensure an object is always in a consistent state after instantiation and after a method finishes. Other than that you're free to develop the class as you wish (keeping in mind certain principles with subclassing / overriding and so on).

A tool such as Pylint will warn when you're setting instance variables outside __init__. It can be argued that setting all instance variables in the __init__ is cleaner but it's not a rule that must be abided by at all times.