You have been calling functions since lesson 1.2 — print(), len(), input(), range(). Now you'll write your own.

A function is a named block of code that runs only when you call it.

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

greet()
greet()

Breaking that down:

• def starts the definition.
• greet is the name — same rules as variable names, snake_case by convention.
• () holds the parameters (none yet).
• : and an indented body, exactly like if and for.

Defining a function does not run it. The body only executes when you write greet().

Why bother?

Suppose you print a separator line in three places:

print("-" * 20)
print("Section one")
print("-" * 20)
print("Section two")
print("-" * 20)

Now say you want = instead of -. You have to change three lines and hope you didn't miss one. With a function, there is exactly one place to change:

def separator():
    print("-" * 20)

separator()
print("Section one")
separator()
print("Section two")
separator()

That is the real point of functions: one idea lives in one place. Programmers call this DRY — Don't Repeat Yourself.

Order matters

Python reads top to bottom, so a function must be defined before you call it:

say_hi()

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

That raises NameError: name 'say_hi' is not defined. Define your functions first, then call them.

Calling from inside another function

Functions can call other functions. This is how programs get built out of small pieces:

def separator():
    print("-" * 20)

def header():
    separator()
    print("MY PROGRAM")
    separator()

header()

Try it yourself

Write a function called greet that prints two lines, then call it twice.

Checkpoint

• def name(): defines a function; name() calls it.
• The body must be indented, and defining is not the same as running.
• Functions must be defined before they are called.
• Functions keep one idea in one place, so you change it once.