Inserting Code to Run on Attribute Access

A better solution would allow you to run code automatically on attribute access, if needed. At various points in this book, we’ve met Python tools that allow our scripts to dynamically compute attribute values when fetching them and validate or change attribute values when storing them. In this chapter, were going to expand on the tools already introduced, explore other available tools, and study some larger use-case examples in this domain. Specifically, this chapter presents:

 

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  • The __getattr__ and __setattr__ methods, for routing undefined attribute fetches and all attribute assignments to generic handler methods.
  • The __getattribute__ method, for routing all attribute fetches to a generic handler method in new-style classes in 2.6 and all classes in 3.0.
  • The property built-in, for routing specific attribute access to get and set handler functions, known as properties.
  • The descriptor protocol, for routing specific attribute accesses to instances of classes with arbitrary get and set handler methods.

The first and third of these were briefly introduced in Part VI; the others are new topics introduced and covered here.

As we’ll see, all four techniques share goals to some degree, and it’s usually possible to code a given problem using any one of them. They do differ in some important ways, though. For example, the last two techniques listed here apply to specific attributes, whereas the first two are generic enough to be used by delegation-based classes that must route arbitrary attributes to wrapped objects. As we’ll see, all four schemes also differ in both complexity and aesthetics, in ways you must see in action to judge for yourself.

Besides studying the specifics behind the four attribute interception techniques listed in this section, this chapter also presents an opportunity to explore larger programs than we’ve seen elsewhere in this book. The CardHolder case study at the end, for example, should serve as a self-study example of larger classes in action. We’ll also be using some of the techniques outlined here in the next chapter to code decorators, so be sure you have at least a general understanding of these topics before you move on.