Exceptions are part of a function's API, and more broadly are part of certain protocols. For example, objects which implement the sequence protocol should raise an IndexError exception for indices which are out of range.

The exceptions which are raised are as much a part of a function's specification as the arguments it accepts and must be documented appropriately.

There are a handful of common exception types in Python, and usually when you need to raise an exception in your own code, one of the built-in types is a good choice. Much more rarely, you'll need to define new exception types, but we don't cover that in this book. (See the next book in this series The Python Journeyman for how to do that.)

If you're deciding which exceptions your code should raise, you should look for similar cases in existing code. The more your code follows existing patterns, the easier it will be for people to integrate and understand. For example, suppose you were writing a key-value database.

It would be natural to use KeyError to indicate a request for a non-existent key because this is how dict works. Which is to say that "mapping" collections in Python follows certain protocols, and exceptions are part of those protocols.

Let's look at a few common exception types.