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Functions: write a recipe once, use it anywhere

A baker doesn't invent brownies from scratch every morning. They follow a recipe. A function is a recipe for your code: steps you write once and run by name.

The big idea

def writes a function, a named recipe of steps. Calling the name, with parentheses, runs those steps.

See it in code

1Start simple

def starts the recipe and gives it a name. The pushed-in lines are its steps. Nothing happens until the last line calls it:

python
def open_shop():
    print("=== ELECTRIC BAKERY ===")
    print("We're open!")

open_shop()
Write the recipe, then call it:
=== ELECTRIC BAKERY ===
We're open!

The two print lines waited inside the recipe. open_shop() is the call. That's the moment the steps actually ran.

2A step further

A recipe can be used again and again. Each call runs every step, wherever the call sits in your program:

python
def ring_bell():
    print("Ding ding!")

ring_bell()
print("Order up: cookie")
ring_bell()
print("Order up: brownie")
One recipe, called twice:
Ding ding!
Order up: cookie
Ding ding!
Order up: brownie

You wrote "Ding ding!" once and heard it twice. Change the recipe and every call changes with it.

3In the paint studio

A recipe can paint, too. These five shapes make a ladybug, and now they have one name:

python
def ladybug():
    circle(x=200, y=178, size=18, color="black")
    circle(x=200, y=200, size=40, color="red")
    line(x1=200, y1=180, x2=200, y2=220, color="black", width=2)
    circle(x=191, y=200, size=8, color="black")
    circle(x=209, y=200, size=8, color="black")

paper("light sky blue")
rect(x=200, y=300, width=400, height=200, color="yellow green")
oval(x=200, y=205, width=110, height=56, color="forest green")
ladybug()
Five lines of paint behind one name:
A red ladybug with a black head and two spots, sitting on a green leaf in a garden under a light blue sky.

The last line is short and easy to read. It says what the program paints, and the recipe above holds the details.

The same idea, everywhere

Big programs are mostly recipes. A game has one for starting over and one for making the next wave. A shop has one for printing a receipt.

Try it yourself

Add a second ladybug() call at the bottom and run it. You still see one ladybug. Why? Both calls paint in the very same spot. That puzzle is solved by giving a recipe inputs.

The common mistake

Writing a recipe and never calling it. def only writes the steps down. If nothing appears when you run your program, check that the call is there.

What it unlocks

A recipe gets far more useful when you can give it inputs, so each call does something a little different. After that, learn how a recipe can hand an answer back.

Older, or want more depth? Read the Teens version →

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