Defining and calling functions

A function lets you name a block of code and run it on demand. You write it once with def, then call it whenever you need it.

Defining a function

Use the def keyword, a name, parentheses, and a colon. The indented lines below are the function body:

Python

def say_hello():
    print("Hello!")
    print("Welcome.")

Defining a function does not run it — it just teaches Python what the function does.

Calling a function

To actually run the body, call the function by writing its name followed by parentheses:

Python

def say_hello():
    print("Hello!")

say_hello()   # Hello!
say_hello()   # Hello!

Each call runs the whole body again, so you can reuse the same logic as many times as you like.

Why functions help

Without a function, you'd repeat the same lines everywhere. With one, you write the logic once and call it by name — and if you need to change it, you only edit it in one place.

Key takeaways

Define a function with def name(): and an indented body.
Defining does not run the code; calling with name() does.
Call a function as many times as you need to reuse its logic.