Fluent Python

Chapter 20. Attribute Descriptors

Learning about descriptors not only provides access to a larger toolset, it creates a deeper understanding of how Python works and an appreciation for the elegance of its design.¹

Raymond Hettinger, Python core developer and guru

Descriptors are a way of reusing the same access logic in multiple attributes. For example, field types in ORMs such as the Django ORM and SQL Alchemy are descriptors, managing the flow of data from the fields in a database record to Python object attributes and vice versa.

A descriptor is a class that implements a protocol consisting of the __get__, __set__, and __delete__ methods. The property class implements the full descriptor protocol. As usual with protocols, partial implementations are OK. In fact, most descriptors we see in real code implement only __get__ and __set__, and many implement only one of these methods.

Descriptors are a distinguishing feature of Python, deployed not only at the application level but also in the language infrastructure. Besides properties, other Python features that leverage descriptors are methods and the classmethod and staticmethod decorators. Understanding descriptors is key to Python mastery. This is what this chapter is about.

Descriptor Example: Attribute Validation

As we saw in “Coding a Property Factory”, a property factory is a way to avoid repetitive coding of getters and setters by applying functional programming patterns. A property factory is a higher-order function ...