IB Computer Science Topic B2.1.3: How Programs Use Common Exception Handling Techniques Notes

Exception Handling: What a Program Does When a Line Fails

1. An exception is an error that a program hits while it is running, in code that compiled and started without complaint.
2. Left unhandled, the exception stops the program on that line, so nothing after it runs and anything not yet saved is lost.
3. Handling the exception lets the program report the problem in words a user can act on, then either recover and carry on or close down in an orderly way.
4. The work is anticipating which lines can fail and wrapping only those lines, rather than reacting after a crash.

Three Points of Failure You Are Expected to Anticipate

1. Unexpected inputs
   1. A user types letters where a number was expected, leaves a field blank, or enters zero as a divisor.
   2. The program has no control over what arrives, so any line that converts or divides input is a candidate for handling.
2. Resource unavailability
   1. A file has been moved or deleted, a disk is full, a database refuses the connection, or the network drops mid-transfer.
   2. The resource was there when the program was written and tested, which is what makes this class of failure easy to forget.
3. Logic errors
   1. The reasoning in the code is wrong, so a loop runs one step past the end of a structure or a value is used before anything was stored in it.
   2. These surface as exceptions such as an index out of range or a null reference, and the handler reports them rather than curing them.

try / catch in Java: Guard Only the Lines That Can Fail

1. The lines that might fail go inside the try block.
2. Each catch block names one exception type and holds the code that runs if that type is thrown.
3. Catch blocks are tested in order, so the most specific type is listed first and a general Exception last if you use one at all.
4. Once a catch block has run, execution continues after the whole try statement rather than returning into the try block.

try / except in Python: The Same Structure, Different Keywords

1. Python spells the guarded block try and each handler except, and the behaviour matches Java's.
2. Several except clauses can be listed, or one clause can name a tuple of types it will handle the same way.
3. Writing except ValueError as e binds the exception object so the message can be printed.
4. An optional else clause runs only when the try block finished without raising anything, which keeps the success path out of the guarded block.

finally: The Block That Runs Whatever Happened

1. A finally block runs after the try block and after any handler, whether an exception was raised, whether it was caught, and even if the try block returned a value.
2. Its purpose is cleanup, so it is where a file is closed, a database connection released or a lock given up.
3. Without it, a file opened just before a failure stays open, and the program leaks a resource each time the error occurs.
4. Python has finally as well as Java, so the block is available whichever language you sit.

What Handling Errors Buys the User

1. The program keeps running after a recoverable failure, so a single bad entry costs one retry rather than the whole session.
2. The message names the problem in the user's terms instead of printing a stack trace at them.
3. Files, connections and locks are released on the way out, so the next run starts from a clean state.
4. The handler records where the failure occurred, which shortens the search when the fault turns out to be in your own logic.
5. Related structures are examined in B2.5.1 Constructing Code to Perform File-Processing Operations, where every read and write is a candidate for a handler.