What Should Be Wrapped

In principle, you could wrap every statement in your script in its own try, but that would just be silly (the try statements would then need to be wrapped in try statements!). What to wrap is really a design issue that goes beyond the language itself, and it will become more apparent with use. But for now, here are a few rules of thumb:

 

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  • Operations that commonly fail should generally be wrapped in try statements. For example, operations that interface with system state (file opens, socket calls, and the like) are prime candidates for trys.
  • However, there are exceptions to the prior rule—in a simple script, you may want failures of such operations to kill your program instead of being caught and ignored. This is especially true if the failure is a showstopper. Failures in Python typically result in useful error messages (not hard crashes), and this is often the best outcome you could hope for.
  • You should implement termination actions in try/finally statements to guarantee their execution, unless a context manager is available as a with/as option. The try/finally statement form allows you to run code whether exceptions occur or not in arbitrary scenarios.
  • It is sometimes more convenient to wrap the call to a large function in a single try statement, rather than littering the function itself with many try statements. That way, all exceptions in the function percolate up to the try around the call, and you reduce the amount of code within the function.

The types of programs you write will probably influence the amount of exception handling you code as well. Servers, for instance, must generally keep running persistently and so will likely require try statements to catch and recover from exceptions. In-process testing programs of the kind we saw in this chapter will probably handle exceptions as well. Simpler one-shot scripts, though, will often ignore exception handling completely because failure at any step requires script shutdown.