Fluent Python, 2nd Edition

Chapter 23. 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.

Raymond Hettinger, Python core developer and guru¹

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 SQLAlchemy, 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 dynamic protocol consisting of the __get__, __set__, and __delete__ methods. The property class implements the full descriptor protocol. As usual with dynamic 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. User-defined functions are descriptors. We’ll see how the descriptor protocol allows methods to operate as bound or unbound methods, depending on how they are called.

Understanding descriptors is key to Python mastery. This is what this chapter is about.

In this chapter we’ll refactor the bulk food example we first saw in “Using a Property for Attribute Validation”, replacing properties with descriptors. This will make it easier to reuse the attribute ...