The Exception Hierarchy

We’ve already seen several of the most common built-in exceptions, and we’ll probably encounter the rest over the course of our regular Python development. As we noticed earlier, most exceptions are subclasses of the Exception class. But this is not true of all exceptions. The Exception class actually extends a class called BaseException. In fact, all exceptions must extend the BaseException class or one of its subclasses.

Types of BaseException

There are two key built-in exception classes:

• SystemExit
• KeyboardInterrupt

The later one derive directly from the BaseException class instead of the Exception class.

The SystemExit class

The SystemExit exception is raised whenever the program exits naturally, typically because we called the sys.exit() function somewhere in our code. For example, when the user selected an exit menu item, clicked the Close button on a window, entered a command to shut down a server, or the OS sent a signal to the application to terminate. This exception is designed to allow us to clean up code before the program ultimately exits.

If we do handle the SystemExit exception, we would normally re-raise the exception, since catching it could stop the program from exiting. Imagine a web service with a bug that is holding database locks and can’t be stopped without rebooting the server.

The KeyboardInterrupt class

We don’t want a SystemExit exception to be accidentally caught in generic except Exception: clauses. This is why it derives directly from BaseException. The KeyboardInterrupt exception is common in command-line programs. It is thrown when the user explicitly interrupts program execution with an OS-dependent key combination (normally, Ctrl + C). For Linux and macOS users, the kill -2 <pid> command will also work. This is a standard way for the user to deliberately interrupt a running program and, like the SystemExit exception, it should almost always respond by terminating the program. Also, like SystemExit, it can handle any cleanup tasks inside the finally blocks. Here is a class diagram that fully illustrates the hierarchy: