Working with Functions

The way in which functions work in Python is similar to the way a computer works. Both the functions found in software and computer hardware in general can be described using an INPUT-PROCESSING-OUTPUT model.

A computer takes some sort of data (numbers, strings, images, video) as INPUT, performs some sort of PROCESSING on the data (calculations involving numbers, modifying text in an email, editing pixels in an image), and then produces some sort of OUTPUT (text/graphics on the screen, a new file or email). A function in Python acts in much the same way, except that it may/may not take INPUT and it may/may not produce an OUTPUT.

To create and implement a function in Python, two steps need to be taken. First it must be defined using the keyword def.

Function Definition

An example of a function definition in Python is

def cube(n):
    return n*n*n

In general a function definition takes the form

def functionName( parameters ):
    statement
    statement
    statement
    return someValue

which consists of the name of the function, parameters (separated by commas) that may/may not be used as INPUT to the function, and then statements that are indented as they are in a loop or if statement. Finally, a value may/may not be returned as the OUTPUT of the function. This is simply the definition of the function.

Function Implementation

In order to implement the function we must use the function name, pass in the parameter values as INPUT, and possibly assign the OUTPUT value to a variable. The function above could be used in either of the following ways, both producing an output value of 64.

value = cube(4)
print( value )

or

print( cube(4) )

Using a INPUT-PROCESSING-OUTPUT model, here is what is happening with the cube() function.