oop - 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

A.x is a class variable. B's self.x is an instance variable.

i.e. A's x is shared between instances.

It would be easier to demonstrate the difference with something that can be modified like a list:

#!/usr/bin/env python

class A:
    x = []
    def add(self):
        self.x.append(1)

class B:
    def __init__(self):
        self.x = []
    def add(self):
        self.x.append(1)

x = A()
y = A()
x.add()
y.add()
print("A's x:", x.x)

x = B()
y = B()
x.add()
y.add()
print("B's x:", x.x)

Output

A's x: [1, 1]
B's x: [1]