13.1. What is an exception?¶
An exception is a signal that a condition has occurred that can’t be easily handled using the normal flow-of-control of a Python program. Exceptions are often defined as being “errors” but this is not always the case. All errors in Python are dealt with using exceptions, but not all exceptions are errors.
13.2. Exception Handling Flow-of-control¶
To explain what an exception does, let’s review the normal “flow of control” in a Python program. In normal operation Python executes statements sequentially, one after the other. For three constructs, if-statements, loops and function invocations, this sequential execution is interrupted.
For if-statements, only one of several statement blocks is executed and then flow-of-control jumps to the first statement after the if-statement.
For loops, when the end of the loop is reached, flow-of-control jumps back to the start of the loop and a test is used to determine if the loop needs to execute again. If the loop is finished, flow-of-control jumps to the first statement after the loop.
For function invocations, flow-of-control jumps to the first statement in the called function, the function is executed, and the flow-of-control jumps back to the next statement after the function call.
Do you see the pattern? If the flow-of-control is not purely sequential, it always executes the first statement immediately following the altered flow-of-control. That is why we can say that Python flow-of-control is sequential.
13.2.1. Raising and Catching Errors with try and except¶
The try/except control structure provides a way to process a run-time error and continue on with program execution. Until now, any run-time error, such asking for the 8th item in a list with only 3 items, or dividing by 0, has caused the program execution to stop. In the browser ActiveCode windows, you get an error message in a box below. When you are executing python programs from the command-line, you also get an error message saying something about what went wrong and what line it occurred on. After the run-time error is encountered, the python interpreter does not try to execute the rest of the code. You have to make some change in your code and rerun the whole program.
With try/except, you tell the python interpreter:
- Try to execute a block of code, the “try” clause.
If the whole block of code executes without any run-time errors, just carry on with the rest of the program after the try/except statement.
- If a run-time error does occur during execution of the block of code:
skip the rest of that block of code (but don’t exit the whole program)
execute a block of code in the “except” clause
then carry on with the rest of the program after the try/except statement
try:
<try clause code block>
except <ErrorType>:
<exception handler code block>
The syntax is fairly straightforward. The only tricky part is that after the word except, there can optionally be a specification of the kinds of errors that will be handled. The catchall is the class Exception. If you write except Exception: all runtime errors will be handled. If you specify a more restricted class of errors, only those errors will be handled; any other kind of error will still cause the program to stop running and an error message to be printed.
The code below causes an error of type IndexError, by trying to access the third element of a two-element list.
The code below causes an error of type ZeroDivisionError, or less specifically ArithmeticError.
Let’s see what happens if we wrap some of this problematic code in a try/except statement. Note that this won't print doesn’t print: when the error is encountered, the rest of the try block is skipped and the exception block is executed. When the except block is done, it continues on with the next line of code that’s outdented to the same level as the try: continuing is printed.
If we catch only IndexEror, and we actually have a divide by zero error, the program does stop executing.
There’s one other useful feature. The exception code can access a variable that contains information about exactly what the error was. Thus, for example, in the except clause you could print out the information that would normally be printed as an error message but continue on with execution of the rest of the program. To do that, you specify a variable name after the exception class that’s being handled. The exception clause code can refer to that variable name.