Lesson 04-03: The return Statement

Learning Target: I can utilize functions that return values.

Functions Represent Values

Every function has a value representation. Let’s try printing a function instead of just calling it.

Notice that in addition to printing the results (3 and 0 respectively), it prints None every time.

Even though not every function takes in data (through arguments), every function sends back, or returns a value, and that value is None by default. You can override it by using the return keyword.

Using The return Keyword

Let’s take our example from above, but instead of printing the sum of two numbers, we’ll return it.

You might notice that nothing got printed! That’s because there are no print statements anywhere. Since the function add2 returns a value, the expression add2(1,2) “becomes” that value, and represents 3. add2(10,10) represents the value 20. We can then print these value representations as if they were numbers themselves:

This is extremely useful as functions are often used for calculations, without actually printing it. Take the following as an example - a function that checks if a number is prime or not that will return True or False.

While this might look similar to other examples we’ve used in the past, now we can use the is_prime() for any general purpose! For example, if we wanted to print all the prime numbers up to 50, we can run the following:

Writing functions with return statements allow us to create useful, general-purpose, re-usable functions!

You might notice in the example above, there are multiple places where we use the return keyword. It’s important to know that when you invoke the return keyword, the function ends immediately afterwards.

Quick Note on Returning True/False

Let’s say you had a function that checks whether a number is even or not:

#method #1
def is_even(n):
    if n % 2 == 0:
        return True
        return False

The entire body of the function can just be replaced with:

#method #2
def is_even(n):
    return n % 2 == 0

In all cases, method #2 is better to write. However, it may look confusing at first - so for the rest of this chapter, I will be writing these types of functions as #1. You may find method #1 easier to read, or easier to understand - and that’s OK! You can do either one, although later on, after this chapter, you will be expected to write them like method #2.

Checks For Understanding

def random_function(word):
    if word == "hello":
        return "world"
        return "hello"
    return "goodbye"


    u0403-1: In the function above, what would be returned from calling random_function(“world”)?
  • (A) "hello"
  • (B) "world"
  • (C) "goodbye"


    u0403-2: In the function above, what would be returned from calling random_function(“HELLO”)?
  • (A) "hello"
  • (B) "world"
  • Remember, == is case sensitive!
  • (C) "goodbye"


In the function above, which line of code is unreachable (will never get run)? Remember: What does the return keyword do?

Next Section - Lesson 04-04: Variable Scope