Fluent Python

Chapter 21. Class Metaprogramming

[Metaclasses] are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don’t (the people who actually need them know with certainty that they need them, and don’t need an explanation about why).¹

Tim Peters, Inventor of the timsort algorithm and prolific Python contributor

Class metaprogramming is the art of creating or customizing classes at runtime. Classes are first-class objects in Python, so a function can be used to create a new class at any time, without using the class keyword. Class decorators are also functions, but capable of inspecting, changing, and even replacing the decorated class with another class. Finally, metaclasses are the most advanced tool for class metaprogramming: they let you create whole new categories of classes with special traits, such as the abstract base classes we’ve already seen.

Metaclasses are powerful, but hard to get right. Class decorators solve many of the same problems more simply. In fact, metaclasses are now so hard to justify in real code that my favorite motivating example lost much of its appeal with the introduction of class decorators in Python 2.6.

Also covered here is the distinction between import time and runtime: a crucial pre-requisite for effective Python metaprogramming.

Warning

This is an exciting topic, and it’s easy to get carried away. So I must start this chapter with the following admonition:

If you are not authoring a framework, you should ...