Learning Python, 2nd Edition

Chapter 26. Designing with Exceptions

This chapter rounds out Part VII, with a collection of exception design topics and examples, followed by this part’s gotchas and exercises. Because this chapter also closes out the core language material of this book, it also includes a brief overview of development tools, by way of migration to the rest of this book.

Nesting Exception Handlers

Our examples so far have used only a single try to catch exceptions, but what happens if one try is physically nested inside another? For that matter, what does it mean if a try calls a function that runs another try? Technically, try statements can nest in terms of both syntax, and the runtime control flow through your code.

Both these cases can be understood if you realize that Python stacks try statements at runtime. When an exception is raised, Python returns to the most recently entered try statement with a matching except clause. Since each try statement leaves a marker, Python can jump back to earlier trys by inspecting the markers stacked. This nesting of active handlers is what we mean by “higher” handlers—try statements entered earlier in the program’s execution flow.

For example, Figure 26-1 illustrates what occurs when try/except statements nest at runtime. Because the amount of code that can go into a try clause block can be substantial (e.g., function calls), it will typically invoke other code that may be watching for the same exception. When the exception is eventually raised, Python jumps ...