Section 10.1. What Are Exceptions?

10.1. What Are Exceptions?

10.1.1. Errors

Before we get into detail about what exceptions are, let us review what errors are. In the context of software, errors are either syntactical or logical in nature. Syntax errors indicate errors with the construct of the software and cannot be executed by the interpreter or compiled correctly. These errors must be repaired before execution can occur.

Once programs are semantically correct, the only errors that remain are logical. Logical errors can either be caused by lack of or invalid input, or, in other cases, by the inability of the logic to generate, calculate, or otherwise produce the desired results based on the input. These errors are sometimes known as domain and range failures, respectively.

When errors are detected by Python, the interpreter indicates that it has reached a point where continuing to execute in the current flow is no longer possible. This is where exceptions come into the picture.

10.1.2. Exceptions

Exceptions can best be described as action that is taken outside of the normal flow of control because of errors. This action comes in two distinct phases: The first is the error that causes an exception to occur, and the second is the detection (and possible resolution) phase.

The first phase takes place when an exception condition (sometimes referred to as exceptional condition) occurs. Upon detection of an error and recognition of the exception condition, the interpreter performs an operation called raising an exception. Raising is also known as triggering, throwing, or generating, and is the process whereby the interpreter makes it known to the current control flow that something is wrong. Python also supports the ability of the programmer to raise exceptions. Whether triggered by the Python interpreter or the programmer, exceptions signal that an error has occurred. The current flow of execution is interrupted to process this error and take appropriate action, which happens to be the second phase.

The second phase is where exception handling takes place. Once an exception is raised, a variety of actions can be invoked in response to that exception. These can range anywhere from ignoring the error, to logging the error but otherwise taking no action, performing some corrective measures and aborting the program, or alleviating the problem to allow for resumption of execution. Any of these actions represents a continuation, or an alternative branch of control. The key is that the programmer can dictate how the program operates when an error occurs.

As you may have already concluded, errors during runtime are primarily caused by external reasons, such as poor input, a failure of some sort, etc. These causes are not under the direct control of the programmer, who can anticipate only a few of the errors and code the most general remedies.

Languages like Python, which support the raising andmore importantlythe handling of exceptions, empower the developer by placing them in a more direct line of control when errors occur. The programmer not only has the ability to detect errors, but also to take more concrete and remedial actions when they occur. Due to the ability to manage errors during runtime, application robustness is increased.

Exceptions and exception handling are not new concepts. They are also present in Ada, Modula-3, C++, Eiffel, and Java. The origins of exceptions probably come from operating systems code that handles exceptions such as system errors and hardware interruptions. Exception handling as a software tool made its debut in the mid-1960s with PL/1 being the first major programming language that featured exceptions. Like some of the other languages supporting exception handling, Python is endowed with the concepts of a "try" block and "catching" exceptions and, in addition, provides for more "disciplined" handling of exceptions. By this we mean that you can create different handlers for different exceptions, as opposed to a general "catch-all" code where you may be able to detect the exception that occurred in a post-mortem fashion.