Choosing a function or a procedure

Choosing between a function and a procedure depends on whether the code that calls it needs a value back. A function is the right choice when the calling code needs a result to use, such as a calculation, a true/false check, or a value to store in a variable. A procedure is the right choice when the task only needs to do something, such as displaying output, and no value needs to be returned.

A function must send its result back with return. Printing the result inside the function is not the same: the caller has no returned value to store or use. If a function works out the area of a board, it should return the area so the main program can store it or compare it. If a procedure shows a menu on screen, printing is the whole job, so there is nothing to return.

def board_area(length, width):
    return length * width

def show_menu():
    print("Start game")
    print("Quit game")

area = board_area(5, 3)
print(area)
show_menu()

Output:

15
Start game
Quit game

What the caller needs	Choose	Reason
A number for another calculation	Function	The caller needs the result
True or False for an `if` condition	Function	The returned Boolean is the check
A value to store in a variable	Function	The returned value can be assigned
A menu shown on screen	Procedure	Displaying is the whole task
The basket total display updated after an item is scanned	Procedure	No result is needed back

In Python, practical uses include a calculation function, a conversion function, a grading function, a procedure that displays instructions, and a quiz app decomposed into sub programs, with a procedure that shows the current question and a function that checks whether the chosen answer is correct.

Practise this