Declaring new variables inside class methods

Question

I just read about class and method variables in Python, and I am wondering if there is a difference between these two examples:

class Example(object):
    def __init__(self, nr1, nr2):
        self.a = nr1
        self.b = nr2

    def Add(self):
        c = self.a + self.b
        return c

class Example2(object):
    def __init__(self, nr1, nr2):
        self.a = nr1
        self.b = nr2

    def Add(self):
        self.c = self.a + self.b
        return self.c

Basically if I do:

print Example(3,4).Add()
print Example2(3,4).Add()

I get the same result:

7
7

So my questions are:

1. What is the difference between self.c = self.a + self.b and c = self.a + self.b?
2. Should all new variables declared inside classes be declared with the self statement?

Thanks for any help!

Answer 1

The difference is that on the second example you store the result of the addition to an object variable c. In the first example, c is inside a method and thus can not be a class variable since it is instantiated when the Add method is called and not when the class is defined.

Variables inside a method should be written with self just if you want to store them. A local, unimportant variable can be written inside a method just like you did on example with c.