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4.10: Fruitful functions and void functions

Some of the functions we are using, such as the math functions, yield results; for lack of a better name, I call them fruitful functions. Other functions, like print_twice, perform an action but don't return a value. They are called void functions.

When you call a fruitful function, you almost always want to do something with the result; for example, you might assign it to a variable or use it as part of an expression:

x = math.cos(radians)
golden = (math.sqrt(5) + 1) / 2

When you call a function in interactive mode, Python displays the result:

>>> math.sqrt(5)

But in a script, if you call a fruitful function and do not store the result of the function in a variable, the return value vanishes into the mist!


This script computes the square root of 5, but since it doesn't store the result in a variable or display the result, it is not very useful.


Void functions might display something on the screen or have some other effect, but they don't have a return value. If you try to assign the result to a variable, you get a special value called None.


>>> result = print_twice('Bing')
>>> print(result)

The value None is not the same as the string "None". It is a special value that has its own type:

>>> print(type(None))
<class 'NoneType'>

To return a result from a function, we use the return statement in our function. For example, we could make a very simple function called addtwo that adds two numbers together and returns a result.

def addtwo(a, b):
    added = a + b
    return added

x = addtwo(3, 5)

# Code:

When this script executes, the print statement will print out "8" because the addtwo function was called with 3 and 5 as arguments. Within the function, the parameters a and bwere 3 and 5 respectively. The function computed the sum of the two numbers and placed it in the local function variable named added. Then it used the return statement to send the computed value back to the calling code as the function result, which was assigned to the variable x and printed out.