Python Cookbook, 2nd Edition

Chapter 20. Descriptors, Decorators,and Metaclasses

Introduction

Credit: Raymond Hettinger

I had my power drill slung low on my toolbelt and I said, “Go ahead, honey.Break something.”

Tim Allen

on the challenges of figuring out whatto do with a new set of general-purpose tools

This chapter is last because it deals with issues that look or sound difficult, although they really aren’t. It is about Python’s power tools.

Though easy to use, the power tools can be considered advanced for several reasons. First, the need for them rarely arises in simple programs. Second, most involve introspection, wrapping, and forwarding techniques available only in a dynamic language like Python. Third, the tools seem advanced because when you learn them, you also develop a deep understanding of how Python works internally.

Last, as with the power tools in your garage, it is easy to get carried away and create a gory mess. Accordingly, to ward off small children, the tools were given scary names such as descriptors, decorators, and metaclasses (such names as pangalaticgarglebaster were considered a bit too long).

Because these tools are so general purpose, it can be a challenge to figure out what to do with them. Rather that resorting to Tim Allen’s tactics, study the recipes in this chapter: they will give you all the practice you need. And, as Tim Peters once pointed out, it can be difficult to devise new uses from scratch, but when a real problem demands a power tool, you’ll know it when you need it.

Descriptors ...