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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 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 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? How do properties and descriptors interact with __slots__, and what special consideration is needed? How would you use a WeakKeyDictionary inside a descriptor to store per-instance data without relying on the instance's own __dict__ or requiring a dedicated __slots__ entry?
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How do you write a descriptor that behaves differently when accessed on the CLASS itself versus on an INSTANCE?

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Answer

__get__(self, instance, owner) receives 'instance' as None when accessed via the CLASS (e.g., MyClass.attr) rather than an instance (e.g., my_obj.attr) — checking `if instance is None` inside __get__ lets you return something different (often the descriptor itself, matching @property's own convention) for class-level access.
class Descriptor:
    def __get__(self, instance, owner):
        if instance is None:
            return self  # accessed via the class itself: return the descriptor
        return f"Value for {instance}"

class Example:
    attr = Descriptor()

print(Example.attr)          # <Descriptor object> -- instance is None here
print(Example().attr)        # "Value for <Example object>" -- instance is set
Real-world example Building a descriptor (like an ORM field definition) that exposes metadata when accessed via the class, but returns actual data via an instance.

Common follow-ups: How does Django's ORM use exactly this class-vs-instance distinction to let Model.field_name work for QUERYING?

OOP
All 15 Descriptors & Properties questions