Chapter Summary

In this chapter, we explored coding user-defined exceptions. As we learned, exceptions are implemented as class instance objects in Python 2.6 and 3.0 (an earlier string-based exception model alternative was available in earlier releases but has now been deprecated). Exception classes support the concept of exception hierarchies that ease maintenance, allow data and behavior to be attached to exceptions as instance attributes and methods, and allow exceptions to inherit data and behavior from superclasses.

We saw that in a try statement, catching a superclass catches that class as well as all subclasses below it in the class tree—superclasses become exception category names, and subclasses become more specific exception types within those categories. We also saw that the built-in exception superclasses we must inherit from provide usable defaults for printing and state retention, which we can override if desired.

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The next chapter wraps up this part of the book by exploring some common use cases for exceptions and surveying tools commonly used by Python programmers. Before we get there, though, here’s this chapter’s quiz.