Fluent Python, 2nd Edition

Chapter 22. Dynamic Attributes and Properties

The crucial importance of properties is that their existence makes it perfectly safe and indeed advisable for you to expose public data attributes as part of your class’s public interface.

Martelli, Ravenscroft, and Holden, “Why properties are important”¹

Data attributes and methods are collectively known as attributes in Python. A method is an attribute that is callable. Dynamic attributes present the same interface as data attributes—i.e., obj.attr—but are computed on demand. This follows Bertrand Meyer’s Uniform Access Principle:

All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation.²

There are several ways to implement dynamic attributes in Python. This chapter covers the simplest ways: the @property decorator and the __getattr__ special method.

A user-defined class implementing __getattr__ can implement a variation of dynamic attributes that I call virtual attributes: attributes that are not explicitly declared anywhere in the source code of the class, and are not present in the instance __dict__, but may be retrieved elsewhere or computed on the fly whenever a user tries to read a nonexistent attribute like obj.no_such_attr.

Coding dynamic and virtual attributes is the kind of metaprogramming that framework authors do. However, in Python the basic techniques are straightforward, so we can use them in everyday ...