Chapter 34. Exception Objects

So far, I’ve been deliberately vague about what an exception actually is. As suggested in the prior chapter, in Python 2.6 and 3.0 both built-in and user-defined exceptions are identified by class instance objects. Although this means you must use object-oriented programming to define new exceptions in your programs, classes and OOP in general offer a number of benefits.

Here are some of the advantages of class-based exceptions:

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  • They can be organized into categories. Exception classes support future changes by providing categories—adding new exceptions in the future won’t generally require changes in try statements.
  • They have attached state information. Exception classes 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.
  • They 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. In fact, all built-in exceptions are identified by classes and are organized into an inheritance tree, for the reasons just listed. You can do the same with user-defined exceptions of your own.

In Python 3.0, user-defined exceptions inherit from built-in exception superclasses. As we’ll see here, because these superclasses provide useful defaults for printing and state retention, the task of coding user-defined exceptions also involves understanding the roles of these built-ins.

Note

Version skew note: Python 2.6 and 3.0 both require exceptions to be defined by classes. In addition, 3.0 requires exception classes to be derived from the BaseException built-in exception superclass, either directly or indirectly. As we’ll see, most programs inherit from this class’s Exception subclass, to support catchall handlers for normal exception types—naming it in a handler will catch everything most programs should. Python 2.6 allows standalone classic classes to serve as exceptions, too, but it requires new-style classes to be derived from built-in exception classes, the same as 3.0.