Should all member variables be initialized in __init__

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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.

instance-variables oop python
3 Answers

Here is an excerpt from sololearn.com (a free site to learn python)

"Properties provide a way of customizing access to instance attributes. They are created by putting the property decorator above a method, which means when the instance attribute with the same name as the method is accessed, the method will be called instead.

One common use of a property is to make an attribute read-only."

Example (also from sololearn.com):

class Pizza:
    def __init__(self, toppings):
    self.toppings = toppings

    @property
    def pineapple_allowed(self):
       return False

   pizza = Pizza(["cheese", "tomato"])
   print(pizza.pineapple_allowed)
   pizza.pineapple_allowed = True

Result:

  >>>
 False
 AttributeError: can't set attribute
 >>>

If var3 depends on var1 and var2 you could do 

class myClass:
    def __init__(self,var1,var2):
        self.var1=var1
        self.var2=var2
    @property
    def var3(self):
        return(self.var1+self.var2)  #var3 depends on var1 and var2
 m1=myClass(1,2)
 print(m1.var3)   # var3 is 3

var3 can also be set to whatever you want using a setter function. Note that you can avoid setting var3 to an arbitrary value by using None. 

class myClass2(object):
    def __init__(self,var1,var2):
        self.var1=var1
        self.var2=var2
        self._var3=None     # None or an initial value that makes sense
        @property
        def var3(self):
            return(self._var3)
        @var3.setter
        def var3(self,value):
            self._var3=value
   m2=myClass(1,2)
   print(m2.var3)        # var3 is none
   print(m2.var3(10))    # var3 is set to 10
user_58904600
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