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For loops: write the steps once, run them many times

A for loop is your shortcut for doing something again and again. Write the steps once, and the loop repeats them for you.

The big idea

A for loop runs its pushed-in lines once for each item in a list, or once for each number in a count.

See it in code

1Start simple

A loop can walk through a list. Each trip around, the loop variable bake holds the next item, and the pushed-in line runs:

python
menu = ["brownie", "cookie", "muffin", "croissant"]

for bake in menu:
    print("*", bake)
Two lines print the whole menu:
* brownie
* cookie
* muffin
* croissant

Four items, four trips, four lines. Add a fifth bake to the list and the loop prints it too, with no extra typing.

2A step further

A loop can also just count. range(4) hands the loop the numbers 0, 1, 2, 3, one per trip:

python
for count in range(4):
    print("Into the oven: cookie number", count)
print("Tray is full!")
Counting cookies onto a tray:
Into the oven: cookie number 0
Into the oven: cookie number 1
Into the oven: cookie number 2
Into the oven: cookie number 3
Tray is full!

The loop counted from 0, not 1. The last line isn't pushed in, so it ran once, after the loop finished.

3In the paint studio

Now use the counter as a position. Each trip, count is one bigger, so each button lands a little further to the right:

python
button_count = 4
button_start = 155
button_gap = 30

paper("wheat")
box(x=200, y=255, width=150, height=130, color="silver", corner=18)
box(x=200, y=140, width=100, height=80, color="silver", corner=16)
circle(x=180, y=130, size=16, color="deep sky blue")
circle(x=220, y=130, size=16, color="deep sky blue")
smile(x=200, y=150, size=40, color="black", width=4)
for count in range(button_count):
    circle(x=button_start + count * button_gap, y=235, size=18, color="red")
One loop paints every button:
A silver robot with blue eyes and a smile, and a row of four red buttons across its chest, painted by a loop.

count * button_gap is 0, then 30, then 60, then 90. One line of paint became four buttons, each in its own spot.

How a for loop paints a row of buttons A flow chart. Start leads to "Paint a button", then "Move over a bit", then a decision "Any left?". If yes, the arrow loops back to paint another button. If no, it reaches Done. yes — keep going no Start Paint a button Move over a bit Any left? Done!
Each trip around the loop: paint one button, move over, then ask “any left?” Keep going until the answer is no.
The same idea, everywhere

A loop doesn't care what it repeats. The same idea prints a menu, paints a row of buttons, builds a wall of bricks in a game, or adds up a day of sales.

Try it yourself

Change button_count = 4 to button_count = 2 and run it. Then try 7. What happens when the buttons run past the robot's body? Can button_gap fix it?

The common mistake

Forgetting to use the counter. Write x=button_start with no count and every button lands in the same spot. It looks like one button, painted four times.

What it unlocks

Next, learn how range() counts, put a loop inside a loop to fill rows and columns, and keep a running total as a loop goes.

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

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