Functions

Big idea

A function is a named block of code you write once and run whenever you call it by name.

Defining and calling

def greet():
    print("Hello!")
    print("Welcome to Python.")

• def — "define a function".
• greet — its name (snake_case, like variables).
• (): — the parentheses and colon. The indented block below is the function's body.

Defining a function doesn't run it. Calling it does — its name followed by ():

def greet():
    print("Hello!")
    print("Welcome to Python.")

greet()
print("---")
greet()

output
Hello!
Welcome to Python.
---
Hello!
Welcome to Python.

Giving it something to work with

A name in the parentheses is a parameter: a variable that gets its value from the call.

def cheer(name):
    print(f"Go, {name}!")

cheer("Ada")
cheer("Grace")

Go, Ada!
Go, Grace!

The value you pass in — "Ada" — is called the argument.

Why bother?

• No copy-paste. Fix a bug once, and every call is fixed.
• Names explain code. print_receipt(order) says what happens without reading every line.
• Small pieces. A big program becomes many short functions you can test one at a time.

Order matters

Python reads top to bottom, so a function must be defined before the line that calls it runs:

say_hi()

def say_hi():
    print("Hi!")

output
Hi!

NameError: name 'say_hi' is not defined — at the moment of the call, Python hadn't reached the def yet. Move the call below it.

Docstrings

A string on the first line of the body documents what the function does. Tools and editors show it when you hover the name:

def area(width, height):
    """Return the area of a rectangle."""
    return width * height

(return is next lesson.)

Watch it

Think of it like

A recipe card titled "Pancakes". Writing the card doesn't make pancakes. Saying "make Pancakes" — calling it — does, as many times as you like.

Key terms

• Function — A named, reusable block of code.
• Call — Running a function: greet().
• Parameter / argument — The name in the definition / the value passed in the call.