Exiting a Function

00:00 Now that you’ve seen how to send information to a function, let’s take a look at how a function sends information back, and that’s through the return statement.

00:10 If a function doesn’t somehow change something in the calling environment, then why do you have it? So, a function is supposed to do something meaningful.

00:18 It’s supposed to have some effect on the overall execution of the program. One of the ways that it can do that is through side effects, as we talked about in the last lesson. More commonly, however, is it can return a value. We’ve asked it to do something, it has done that, and it is communicating back its result.

00:40 We do that with a return statement, and the return statement in Python does two things for us. One, it tells us we’re at the end of the function, or we don’t need to do any more processing within the function.

00:51 We’ve done everything we needed to do for this particular situation. And then we can also provide a value when we are done processing to communicate back to the calling environment the results of its task.

01:08 Exiting a function, one of its purposes:

01:12 When you put a return statement inside a function, you are indicating that this is where the function should stop. We typically don’t have statements to be executed after the return statement.

01:23 There are exceptions, and we’ll see that. So here in this example, we have a function f() which is going to print 'foo', then on another line is going to print 'bar', and that’s all we want it to do, and we’re done. Now, if there’s no value to return, then sometimes we leave it off.

01:41 You’ll notice in all of the functions that I showed you in all of these previous lessons, I never wrote a return statement.

01:49 When Python executes a function that doesn’t have a return statement, the return happens after the last statement that’s written. We call that “falling off the end.” In this example right here, this return statement really wouldn’t be needed because there’s nothing else to worry about, it’s not giving us a value back, so we could leave that off and the function would behave the same way. Other times, we want to exit a function based on some type of test.

02:21 So, here’s a function that only wants to print a meaningful value of x, something non-negative, but up to only 100. And so we’re going to do a couple tests. If we provide a negative number, then the evaluation of that if statement will be True and the function will return right there.

02:39 There’s nothing else to execute. If it gets past that, there’s another if statement. We check to see if it’s bigger than 100, and if it’s bigger than 100, we don’t want to proceed any further either.

02:52 If it fails both of those, then we have a number between 0 and 100 inclusive and we print that value. Notice that there’s no else in this. Because the function immediately stops executing with the return statement, it will only get to the second if statement provided the first one is False.

03:12 So, we could put else and elif appropriately in here, but it’s not necessary, because the return statements cause the function to stop executing at that very point. In the next lesson, you’ll learn a little bit more about the other use of a return statement, and that’s returning data to the calling environment.