5.10. 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
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) 2.23606797749979
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
>>> result = print_twice('Bing') Bing Bing >>> print(result) None
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
statement in our function. For example, we could make a very simple
addtwo that adds two numbers together and
returns a result.
When this script executes, the
addtwo function was called with 3 and 5
as arguments. Within the function, the parameters
b were 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 then assigned to the variable
x and printed out.