There are some situations where we might need to be sure a particular piece of code always executes , even if our program reaches an exception. For example, if we are reading or writing to a file, we should always close that file when we are done, even if we encounter an error. To do that, we can add a Finally block to our programs in Python.
Consider this sample program. It includes a Try-Except statement already. To add a Finally block, we simply add i t at the end of the Except statements, just like any other. Inside of that Finally block, we can include any code needed. In this case, we’re just going to print a message showing that we’ve executed the code in that block. You’ll also notice that we’ve included another print statement outside of the Try-Except block as well, just so we know that we’ve completely exited that block of code.
To really understand how a Finally block works, let’s step through this program with several different values provided as input, and see what happens. First, we can provide the input of “5” to the program. In this case, it is able to successfully read the input and convert it to an integer. Then, it will check to see if the input is less than 0. Since it isn’t, we’ll move forward, and eventually reach the end of the Try-Except block. So, it will now execute the code in the Finally block, and print that output to the screen, and then it will exit the block and print the “After Try” message as well. This is a pretty simple case, since we didn’t reach any exceptions.
Let’s try it again, but this time we’ll provide the input -5. Once again, it is able to read that input and store it as an integer just fine. However, once it reaches the If-Then statement, it realizes that the input is less than 0, so it will enter the block and throw a new ValueError. When that happens, the Try-Except block will immediately catch and try to handle the exception. So, we’ll jump down a bit to the exception handler for ValueError, which will print the error message contained in the exception. Once we’ve handled the exception, the code will then move into the Finally block and print that output. So, as we can see, the Finally block will be executed regardless of whether the Try-Except block encountered an exception or not. At the very end, it will print the output outside of the block before exiting the program.
Here’s one more input we can trace. This time, we’ll provide some text instead of a number. So, when we try to convert the input to an integer, we’ll immediately cause a ValueError. That will move our execution down to the exception handler for ValueError again, so we’ll print the error there. Once again, we’ll immediately move to the Finally block and then the code outside the Try-Except block.
So, as we have seen through these examples, the Finally block is a very handy addition to a Try-Except block that allows us to add additional code that will execute at the end of the block, regardless of whether we’ve encountered an exception or not. We’ll see this used in many cases to handle closing files or resources used in our Try-Except statements, though later in this chapter we’ll learn about the With statement, which is another way to handle this situation.
There is one other block we can add to our Try-Except statements in Python, which is the Else block. We can add it to our code just like we did for the Finally block. Note that the Else block should always be placed after the exception handlers, but before the Finally block. In addition, we can also place any code we want inside of the block, but once again we’ll use a print statement so we can see when that code is executed.
So, let’s go back to our earlier examples and see how this changes things. First, we’ll provide 5 as input once again. So, we can successfully read the input, and find that it is greater than 0. After that, since we got all the way through the Try block without an exception, we can executed the code in the Else block, printing that output to the screen, followed by the Finally block and then the code after the Try-Except statement.
However, when we provide -5 as input, we’ll run into an exception. So, the code will move into the exception handler, then immediately following that it will reach the Finally block and exit the Try-Except statement. Since there was an exception that was caught by the Try-Except statement, the program will skip the Else statement entirely.
The Else block can be used in a variety of interesting ways. For example, we can accept and read user input in the Try-Except block, but only save the data when we reach the Else block, since we know that the data collected didn’t cause any exceptions. It can be a very powerful feature of the Python programming language.