Turn a Task into a Function

At the end of Chapter 7, you built a menu that could add, list, and count items. It worked, but its loop had to explain every detail:

while True:
    command = input("Command: ").strip().lower()

    if command == "add":
        # ask for an item, store it, and confirm it
        ...
    elif command == "list":
        # handle an empty list or print every numbered item
        ...
    elif command == "count":
        # report the number of items
        ...

To understand the list command, a reader has to find that branch and read all the lines inside it. The loop is doing two jobs at once: choosing an action and explaining how every action works.

You already know what could improve that.

if command == "add":
    add_item(items)
elif command == "list":
    show_items(items)
elif command == "count":
    show_count(items)

Even before you know how those three functions are defined, you can read the plan. Each name answers the question, “What happens for this command?”

That is the main reason to write a function: it gives one understandable operation a name.

You already use functions

Chapter 3 introduced function calls:

name = input("What is your name? ")
letter_count = len(name)
print(letter_count)

input, len, and print are functions. You know three important parts of their behavior:

• A call uses parentheses, as in len(name).

• A call can receive an argument, such as name.

• A call can produce a return value, such as the number from len(name).

Until now, someone else wrote every function you called. This chapter gives you the other half of the picture: defining your own.

Definition first, call second

A function definition starts with def. It describes what should happen when the function is called:

The definition has two parts:

def show_count(items):

This first line gives the function its name and says what information it expects.

    print(f"Items stored: {len(items)}")

The indented body contains the work.

Defining the function does not perform that work. Python remembers the definition. The work happens later, at the call:

show_count(books)

That separation is useful. Define an operation once, then call it wherever the program needs it.

A name creates a useful level of detail

Imagine reading directions that say:

Move your right foot, shift your weight, move your left foot, shift your weight, and keep repeating.

Most people would simply say:

Walk to the door.

The shorter instruction is useful because the word walk names a familiar operation. A good function name does the same thing for code. It lets one part of a program state the plan while the function body holds the details.

Functions do not automatically improve a program. A vague name such as do_thing() hides information. A focused name such as show_items() reveals intent.

In this chapter, you will learn to write the details and choose the names.

def announce():
    print("The gate is open.")

print("Ready.")