Learning Python, 6th Edition

Chapter 36. Exception Odds and Ends

This chapter rounds out this part of the book with the usual collection of stray topics, common-usage examples, and design concepts, followed by this part’s gotchas and exercises. Because this chapter also closes out the fundamentals portion of the book at large, it includes a brief overview of concepts and development tools to help as you make the transition from Python language beginner to Python application developer.

Nesting Exception Handlers

Most of 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. This was mentioned earlier in brief but merits clarification here.

Both of 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. Because each try statement leaves a marker on the top of a LIFO stack, Python can jump back to earlier trys by inspecting the stacked markers. This nesting of active handlers is what we mean when we talk about propagating exceptions up to “higher” handlers—such handlers are simply try statements entered earlier in the program’s execution flow.

Figure 36-1 illustrates what occurs when try statements ...