Python - why use “self” in a class?

Question

How do these 2 classes differ?

class A():
    x=3

class B():
    def __init__(self):
        self.x=3

Is there any significant difference?

Answer 1

I've just started learning Python and this confused me as well for some time. Trying to figure out how it all works in general I came up with this very simple piece of code:

# Create a class with a variable inside and an instance of that class
class One:
    color = 'green'

obj2 = One()


# Here we create a global variable(outside a class suite).
color = 'blue'         

# Create a second class and a local variable inside this class.       
class Two:             
    color = "red"

    # Define 3 methods. The only difference between them is the "color" part.
    def out(self):     
        print(self.color + '!')

    def out2(self):
        print(color + '!')

    def out3(self):
        print(obj2.color + '!')

# Create an object of the class One
obj = Two()

When we call out() we get:

>>> obj.out()

red!

When we call out2():

>>> obj.out2()

blue!

When we call out3():

>>> obj.out3()

green!

So, in the first method self specifies that Python should use the variable(attribute), that "belongs" to the class object we created, not a global one(outside the class). So it uses color = "red". In the method Python implicitly substitutes self for the name of an object we created(obj). self.color means "I am getting color="red" from the obj"

In the second method there is no self to specify the object where the color should be taken from, so it gets the global one color = 'blue'.

In the third method instead of self we used obj2 - a name of another object to get color from. It gets color = 'green'.