Learning Python, 3rd Edition

Chapter 28. Exception Objects

So far, I’ve been deliberately vague about what an exception actually is. Python generalizes the notion of exceptions—as mentioned in the prior chapter, they may be identified by string objects or class instance objects; class instance objects are preferred today, and will be required soon. Both approaches have merits, but classes tend to provide a better solution when it comes to maintaining exception hierarchies.

In short, class-based exceptions allow us to build exceptions that are organized into categories, have attached state information, and support inheritance. In more detail, compared to the older string exception model, class exceptions:

• Better support future changes by providing categories—adding new exceptions in the future won’t generally require changes in try statements.

• Provide a natural place for us to store context information for use in the try handler—they may have both attached state information and callable methods, accessible through instances.

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

Because of these differences, class-based exceptions support program evolution and larger systems better than string-based exceptions. String exceptions may seem easier to use at first glance, when programs are small, but they can become more difficult when programs grow large. In fact, all built-in exceptions are identified ...