Unit 3: Loops & Lists

Do things again and again.

Unit 3 of 31 in Python for kids. Its 6 lessons are Repeating, Keep Going Until, Lists, Functions, Sending an Answer Back and Guessing Game — below is everything each one explains, and a question or two from it to try.

Every sample on this page was run through real Python before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

🔁 Repeating

Lazy is good

Never write the same line five times. A for loop repeats it for you. range(3) counts 0, 1, 2 — it starts at zero and stops *before* the number.

Python

for i in range(3):
    print(i)

It prints

0
1
2

Try it yourself

How many lines does this print?

Python

for i in range(4):
    print("hi")

What is the LAST number this prints?

Python

for i in range(5):
    print(i)

Answer them in the app

♾️ Keep Going Until

When you do not know how many times

A for loop needs to know the count up front. A while loop just keeps going as long as something stays True — perfect when you cannot know in advance.

Python

lives = 3
while lives > 0:
    print(lives)
    lives = lives - 1

It prints

3
2
1

Jumping out early

break leaves the loop immediately, whatever the test says. Handy when you find what you were looking for.

Python

for n in [3, 7, 2, 9]:
    if n > 5:
        print(f"found {n}")
        break

It prints

found 7

Try it yourself

What is the LAST number this prints?

Python

n = 1
while n < 20:
    print(n)
    n = n * 2

What goes wrong here?

Python

n = 5
while n > 0:
    print(n)

Nothing, it prints 5 four times
n never changes, so it prints forever
while needs a colon
It prints nothing

Answer them in the app

📋 Lists

A box that holds many things

A list holds lots of values in order, inside square brackets. Counting starts at 0, so the first thing is number 0.

Python

pets = ["cat", "dog", "fish"]
print(pets[0])
print(len(pets))

It prints

cat
3

Looping over a list

A for loop can walk through a list directly. Each turn, the variable holds the next item.

Python

for pet in ["cat", "dog"]:
    print(pet)

It prints

cat
dog

Try it yourself

What does colors[1] give?

Python

colors = ["red", "green", "blue"]

red
green
blue
1

What does this print?

Python

nums = [4, 8, 15]
print(len(nums))

Answer them in the app

🧰 Functions

Make your own commands

A function is code you name once and use whenever you like. def makes it. Nothing happens until you *call* it by writing its name with brackets.

Python

def cheer():
    print("Woohoo!")

cheer()
cheer()

It prints

Woohoo!
Woohoo!

Passing things in

Words inside the brackets are parameters — stuff you hand to the function each time you call it.

Python

def greet(name):
    print("Hi " + name)

greet("Zoe")

It prints

Hi Zoe

Try it yourself

What does this print?

Python

def hello():
    print("hi")

hi
hello
nothing — it was never called
an error

Answer it in the app

↩️ Sending an Answer Back

Printing is not answering

So far your functions have *printed*. A function that returns hands the answer back, so you can store it, add to it, or pass it on. That is what makes functions properly useful.

Python

def double(n):
    return n * 2

answer = double(5)
print(answer + 1)

It prints

A function with no return

Leave return out and Python quietly hands back None, which means "nothing here". That is the usual cause of a mysterious None appearing in your output.

Python

def shout(word):
    print(word + "!")

result = shout("hi")
print(result)

It prints

hi!
None

Try it yourself

What is the difference between print and return?

They are the same
print shows it on screen; return hands it back so the rest of the code can use it
return is faster
print only works with numbers

What does this print?

Python

def add(a, b):
    return a + b

print(add(2, 3) + add(1, 1))

Answer them in the app

🏆 Guessing Game

Asking the player something

input() waits for the player to type a line. It always gives you text, so wrap it in int() when you need a number.

Python

guess = int(input())
print(guess + 1)

It prints

If the player types 4, this prints 5

Try it yourself

What is the last thing this prints?

Python

for i in range(3):
    print(i * 2)

Answer it in the app