Why is __set__ called on assignment?

Question

Python

There is the following code:

class NonNegative:
    def __init__(self, name):
        self.name = name

    def __get__(self, instance, owner):
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Cannot be negative.')
        instance.__dict__[self.name] = value

class Order:
    price = NonNegative('price')

    def __init__(self, name, price, quantity):
        self._name = name
        self.price = price

apple_order = Order('apple', 1, 10)
apple_order.price = -10

-I look through the dabagger, when the line " self.price = price " is executed, there is a transition to " __set__ " of the " NonNegative " class . Why is this happening? We don't use the " price " instance of " NonNegative " in any way. As far as I understand " self.price " in self.price = price is just an instance field of the " Order " class.

Answer 1

Those. apparently, when "self.price" is executed, Python first looks (1) if the instance has such a field, if not, then (2) looks for this field in the class (and we have this field and is a descriptor), if it is not there either then (3) it is simply created. In my case, the stop occurs at the second point.
The whole point is that through self we can access the fields of the class.
javedimka : "In the __ini__ method, self.price is already a descriptor, because self.price is defined at the class level, you don't write 1 there, you call the descriptor's __set__ method during initialization. Try passing -10 during initialization, everything will become clear right away "

Answer 2

Fluent python, chapter on descriptors.

Answer 3

We do not use the "price" instance in any way.

I don't understand why you think so? We use it, it is enough to do print(type(self.price)) and see what is what.