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Learning Python, 4th Edition by Mark Lutz

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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 ...

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