11.5. Function Output

The final part of the function machine is the function providing output. Let’s take a look at function output in greater detail.

11.5.1. Return Statements

Some functions return values that are useful. In particular, the type conversion functions convert input to the specified data type and return the result—calling int("3.14") returns the integer value 3.14.

11.5.1.1. Returning a Value

To return a value from functions that we create, we can use a return statement. A return statement has the form:

return some_val

where some_val is any value.

Example

This function has a single parameter, n, which is expected to be a positive integer. It returns the sum 1+2+…+n.

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def sum_to_n(n):
   sum = 0
   for i in range(n+1):
      sum += i
   return sum

print(sum_to_n(3))

Console Output

6

Notice that sum_to_n does not print anything; the output comes from the final line of the program, which prints the value returned by the function call sum_to_n(3).

Now that we have return statements in our coding toolbox, we will very rarely print anything within a function. If we want to see the value returned by a function then we must print it after calling the function.

Question

The function sum_to_n uses a pattern that we have seen previously. What is it called?

11.5.1.2. return Terminates Function Execution

When a return statement executes, the function terminates, regardless of whether or not there is any code following the return statement. This means that you must be careful to use return only when the work of the function has been completed.

Example

This print() statement in this function never executes, since the function returns before it is reached.

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def past_the_point_of_return():
   return "I'm done!"
   print("This will not be printed")

print(past_the_point_of_return())

Console Output

I'm done!

We can use the fact that return stops the execution of a function intentionally, to force a function to stop execution.

Example

This function prints out the integers 1…n using an infinite while loop, which nonetheless terminates when the return statement is executed.

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def count_to_n(n):
   count = 1
   while True:
      if count > n:
         return
      print(count)
      count += count

11.5.1.3. Boolean Functions

A function that returns a boolean value is known as a boolean function. Perhaps the simplest such function is one that tests an integer to determine if it is even.

Example

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def is_even(n):
   if n % 2 == 0:
      return True
   else:
      return False

print(is_even(4))
print(is_even(7))

Console Output

True
False

It is conventional to name boolean functions by starting with either is or has, which creates a nice semantic effect when reading the code. For example, reading is_even(4) communicates to the reader that the function should answer the question, “Is 4 even?” This is a convention so widely used by programmers that it extends to nearly every language.

Let’s return to the is_even function above, to see how we can use the power of return statements to make it even better.

Since return terminates the function, we can leave out the else clause and have the same effect. This is because if n is even, the return statement in the if block will execute and the function will end. If n is odd, the if block will be skipped and the second return statement will execute.

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

This updated version works exactly the same as our initial function.

Additionally, notice that the function returns True when n % 2 == 0 returns True, and it returns False when n % 2 == 0 returns False. In other words, the return value is exactly the same as the value of n % 2 == 0. This means that we can simplify the function even further by returning the value of this expression.

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

This version of is_even is better than the first two, not because it is shorter (shorter isn’t always better), but because it is simpler to read. We don’t have to break down the conditional logic to see what is being returned.

Most boolean functions can be written so that they return the value of a boolean expression, rather than explicitly returning True or False.

11.5.2. Check Your Understanding

Question

What does the following code output?

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def plus_two(num):
   return num + 2

a = 2

for i in range(4):
   a = plus_two(a)

print(a)
  1. 10
  2. 12
  3. 4
  4. 16

Question

What does the following function return?

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def repeater(str):
   repeated = str + str
   print(repeated)

repeater('Bob')
  1. "BobBob"
  2. Nothing (no return value)
  3. undefined
  4. The value of Bob