Learning Python, 6th Edition

Chapter 33. Exception Basics

This part of the book deals with exceptions—events that signal conditions and modify the flow of control through a program. In Python, exceptions are triggered automatically on errors, and they can be both triggered and intercepted by your code. They are processed by four statements we’ll study here, the first of which comes in multiple flavors that qualify as different statement forms by some measures:

try/except/else/finally

Catch and recover from exceptions raised by Python, or by you

raise

Trigger an exception manually in your code

assert

Conditionally trigger an exception in your code

with

Use context managers that automate exception handling

We’ve met some of these briefly before, but full coverage of this topic was saved until the end of the main part of this book because you need to know about classes to code exceptions of your own. Still, with a few exceptions (pun intended), you’ll find that exception handling is simple in Python because it’s integrated into the language itself as another high-level tool. Before we dig into the “how,” though, let’s get clear on the “why.”

Why Use Exceptions?

In a nutshell, exceptions let us jump out of arbitrarily large chunks of a program. Consider the hypothetical pizza-making robot we discussed earlier in the book. Suppose we took the idea seriously and actually built such a machine. To make a pizza, our culinary automaton would need to execute a plan, which we would implement as a Python program: it ...