Expecting the Unexpected
Systems built with software can be fragile. While the software is highly predictable, the runtime context can provide unexpected inputs and situations. Devices fail, networks are unreliable, mere anarchy is loosed on our application. We need to have a way to work around the spectrum of failures that plague computer systems.
There are two broad approaches to dealing with the unforeseen. One approach is to return a recognizable error-signaling value from a function. A value, like None, could be used. Other library functions can then be used by an application to retrieve details of the erroneous condition. A variation on this theme is to pair a return from an OS request with a success or failure indicator. The other approach is to interrupt the normal, sequential execution of statements and divert to statements that handle exceptions. This second approach is what Python does: it eliminates the need to check return values for errors.
In this chapter,...