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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:

 

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.