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How do you add a setter to a property so assignment (obj.attr = value) triggers custom logic, like validation? How would you convert an existing plain attribute into a property later, without breaking any existing code that reads or writes it directly? What is a descriptor, and which dunder methods define the descriptor protocol? What is the difference between a data descriptor and a non-data descriptor, and how does this affect attribute lookup priority? How would you implement a lazy, cached property using a custom descriptor (or functools.cached_property) that only computes its value once? How do you write a descriptor that behaves differently when accessed on the CLASS itself versus on an INSTANCE? How would you implement a type-validating descriptor that enforces a specific type constraint, reusable across multiple attributes and classes? How does Python's own implementation of instance methods use the descriptor protocol to achieve automatic 'self' binding? How do you implement a descriptor-based ORM-style field that lazily loads related data only when actually accessed? How would you write a descriptor supporting attribute DELETION via __delete__, and what real use case does that enable?
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How does __set_name__ let a descriptor automatically learn the attribute name it's being assigned to on the owning class?

Intermediate

Answer

__set_name__(self, owner, name) is automatically called by Python ONCE, at CLASS CREATION time, passing the owning class and the exact attribute name the descriptor was assigned to — letting a reusable descriptor derive a private storage attribute name (like '_price') without the user having to pass it explicitly.
class Validated:
    def __set_name__(self, owner, name):
        self.private_name = f"_{name}"  # automatically learns its own name
    def __get__(self, instance, owner):
        return getattr(instance, self.private_name)
    def __set__(self, instance, value):
        setattr(instance, self.private_name, value)

class Product:
    price = Validated()  # __set_name__ automatically learns 'price' -> '_price'
Real-world example Building a reusable descriptor class that automatically derives its own storage attribute name without manual configuration.

Common follow-ups: Before __set_name__ was added (Python 3.6+), how did descriptors typically need the attribute name passed to them?

Magic Methods & Operator Overloading
All 15 Descriptors & Properties questions