From within a class method, we can refer to an instance by means of a special argument, called self by convention. self is always the first attribute of an instance method. Let's examine this behavior together with how we can share, not just attributes, but methods with all instances:

# oop/class.self.py
class Square:
    side = 8
    def area(self):  # self is a reference to an instance
        return self.side ** 2

sq = Square()
print(sq.area())  # 64 (side is found on the class)
print(Square.area(sq))  # 64 (equivalent to sq.area())

sq.side = 10
print(sq.area())  # 100 (side is found on the instance)

Note how the area method is used by sq. The two calls, Square.area(sq) and sq.area(), are equivalent, and teach us how the mechanism works. Either you pass the instance to the method call (Square.area(sq)), which within the method will take the name self, or you can use a more comfortable syntax, sq.area(), and Python will translate that for you behind the scenes.

Let's look at a better example:

# oop/class.price.py
class Price:
    def final_price(self, vat, discount=0):
        """Returns price after applying vat and fixed discount."""
        return (self.net_price * (100 + vat) / 100) - discount

p1 = Price()
p1.net_price = 100
print(Price.final_price(p1, 20, 10))  # 110 (100 * 1.2 - 10)
print(p1.final_price(20, 10))  # equivalent

The preceding code shows you that nothing prevents us from using arguments when declaring methods. We can use the exact same syntax as we used with the function, but we need to remember that the first argument will always be the instance. We don't need to necessarily call it self, but it's the convention, and this is one of the few cases where it's very important to abide by it.