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What does the @property decorator do, and how does it let a method be accessed like an attribute? 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 the difference between a data descriptor and a non-data descriptor, and how does this affect attribute lookup priority? How does __set_name__ let a descriptor automatically learn the attribute name it's being assigned to on the owning class? 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?
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What is a descriptor, and which dunder methods define the descriptor protocol?

Intermediate

Answer

A descriptor is any object implementing __get__, __set__, and/or __delete__, placed as a CLASS attribute — when accessed through an instance, Python automatically calls these methods instead of doing a plain attribute lookup, giving you full control over get/set/delete behavior; @property is actually implemented USING descriptors under the hood.
class PositiveNumber:
    def __set_name__(self, owner, name):
        self.name = f"_{name}"
    def __get__(self, instance, owner):
        return getattr(instance, self.name)
    def __set__(self, instance, value):
        if value < 0:
            raise ValueError("Must be positive")
        setattr(instance, self.name, value)

class Product:
    price = PositiveNumber()  # a reusable descriptor
Real-world example Building a REUSABLE validation rule (like PositiveNumber) that can be applied to multiple attributes across different classes.

Common follow-ups: What's the practical advantage of a reusable descriptor class over writing a separate @property for each validated attribute?

Magic Methods & Operator Overloading
All 15 Descriptors & Properties questions