Have you noticed how, before calling p1.final_price(...), we had to assign net_price to p1? There is a better way to do it. In other languages, this would be called a constructor, but in Python, it's not. It is actually an initializer, since it works on an already-created instance, and therefore it's called __init__. It's a magic method, which is run right after the object is created. Python objects also have a __new__ method, which is the actual constructor. In practice, it's not so common to have to override it though, it's a practice that is mostly used when coding metaclasses, which as we mentioned, is a fairly advanced topic that we won't explore in the book:

# oop/class.init.py
class Rectangle:
    def __init__(self, side_a, side_b):
        self.side_a = side_a
        self.side_b = side_b

    def area(self):
        return self.side_a * self.side_b

r1 = Rectangle(10, 4)
print(r1.side_a, r1.side_b)  # 10 4
print(r1.area())  # 40

r2 = Rectangle(7, 3)
print(r2.area())  # 21

Things are finally starting to take shape. When an object is created, the __init__ method is automatically run for us. In this case, I coded it so that when we create an object (by calling the class name like a function), we pass arguments to the creation call, like we would on any regular function call. The way we pass parameters follows the signature of the __init__ method, and therefore, in the two creation statements, 10 and 7 will be side_a for r1 and r2, respectively, while 4 and 3 will be side_b. You can see that the call to area() from r1 and r2 reflects that they have different instance arguments. Setting up objects in this way is much nicer and more convenient.