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Giving functions inputs: one recipe, many results

A cookie recipe that only makes chocolate chip is a bit dull. Leave a blank for the flavor and one recipe makes them all. Function inputs are those blanks.

The big idea

A parameter is a named slot in a function. Each call fills the slots with its own values, so one recipe can do many jobs.

See it in code

1Start simple

The name inside the parentheses, flavor, is a slot. Each call puts a value in it, and the recipe uses whatever it was handed:

python
def bake_tray(flavor):
    print(f"Baking a tray of {flavor} cookies!")

bake_tray("chocolate chip")
bake_tray("oatmeal")
Same recipe, two flavors:
Baking a tray of chocolate chip cookies!
Baking a tray of oatmeal cookies!

The recipe never changed. Only the value in the slot did, and that changed the sentence.

2A step further

A recipe can have more than one slot. Give a slot a default value with = and a call is allowed to leave it out:

python
def bake_tray(flavor, how_many=12):
    print(f"Baking {how_many} {flavor} cookies!")

bake_tray("chocolate chip")
bake_tray("oatmeal", 6)
A full tray unless you say otherwise:
Baking 12 chocolate chip cookies!
Baking 6 oatmeal cookies!

The first call gave no number, so the recipe used its default, 12. The second call filled both slots.

3In the paint studio

Slots can say where to paint and what color to use. This call style names each slot, so every value says what it is:

python
def butterfly(x, y, color="orchid"):
    wing_gap = 24
    oval(x=x - wing_gap, y=y, width=30, height=44, color=color)
    oval(x=x + wing_gap, y=y, width=30, height=44, color=color)
    oval(x=x, y=y, width=9, height=52, color="black")

paper("light sky blue")
rect(x=200, y=330, width=400, height=140, color="yellow green")
butterfly(x=90, y=120)
butterfly(x=300, y=90, color="orange")
butterfly(x=200, y=210, color="deep sky blue")
One butterfly recipe, three butterflies:
Three butterflies with oval wings in orchid, orange, and blue, flying over a green garden under a light blue sky.

The first call left out color, so that butterfly is orchid, the default. The other two named their own.

The same idea, everywhere

Every paint command works this way. circle(x=200, y=200, size=60, color="gold") is a call that fills four slots. Now you know how to write commands of your own.

Try it yourself

Add a fourth call: butterfly(x=320, y=250, color="gold"). Then add a size slot to the recipe with a default, and use it for the wings' width.

The common mistake

Leaving out a slot that has no default. butterfly(color="red") stops with a TypeError, because the recipe still needs x and y. A call must fill every slot that doesn't have a default.

What it unlocks

Inputs go in, and next, answers come back out. Put a call inside a for loop and one recipe can paint a whole parade.

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

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