Learning Python, 2nd Edition

Chapter 24. Exception Basics

Part VII deals with exceptions, which are events that can modify the flow of control through a program. In Python, exceptions are triggered automatically on errors, and can be both triggered and intercepted by your code. They are processed by three statements we’ll study in this part, the first of which has two variations:

try/except

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

try/finally

Perform cleanup actions whether exceptions occur or not.

raise

Trigger an exception manually in your code.

assert

Conditionally trigger an exception in your code.

With a few exceptions (pun intended), we’ll find that exception handling is simple in Python, because it’s integrated into the language itself as another high-level tool.

Why Use Exceptions?

In a nutshell, exceptions let us jump out of arbitrarily large chunks of a program. Consider the pizza-making robot we talked about 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 implement as a Python program. It would take an order, prepare the dough, add toppings, bake the pie, and so on.

Now, suppose that something goes very wrong during the “bake the pie” step. Perhaps the oven is broken. Or perhaps our robot miscalculates its reach and spontaneously bursts into flames. Clearly, we want to be able to jump to code that handles such states quickly. Since we have no hope of finishing ...