Learning Python, 5th Edition

Chapter 35. Exception Objects

So far, I’ve been deliberately vague about what an exception actually is. As suggested in the prior chapter, as of Python 2.6 and 3.0 both built-in and user-defined exceptions are identified by class instance objects. This is what is raised and propagated along by exception processing, and the source of the class matched against exceptions named in try statements.

Although this means you must use object-oriented programming to define new exceptions in your programs—and introduces a knowledge dependency that deferred full exception coverage to this part of the book—basing exceptions on classes and OOP offers a number of benefits. Among them, class-based exceptions:

• Can be organized into categories. Exceptions coded as classes support future changes by providing categories—adding new exceptions in the future won’t generally require changes in try statements.

• Have state information and behavior. Exception classes provide a natural place for us to store context information and tools for use in the try handler—instances have access to both attached state information and callable methods.

• Support inheritance. Class-based exceptions can participate in inheritance hierarchies to obtain and customize common behavior—inherited display methods, for example, can provide a common look and feel for error messages.

Because of these advantages, class-based exceptions support program evolution and larger systems well. As we’ll find, all built-in exceptions are identified ...