PY-L1-15 · Treasure Chests · Lists Introduction

Learning Goals

3 min

By the end of this lesson you can:

Create a list with square brackets and read any item back using its index — starting from 0.
Count how many items a list holds using len().
Add a new item to the end of a list with .append() and see it appear when you print the list.

Warm-Up

5 min

Last lesson, we used nested for loops to paint shapes — rows of columns. Today we still loop, but over a collection of values instead of a range of numbers.

Quick predict-the-output puzzle. Read it carefully — what does Python print?

foods = ["pizza", "pancake", "hot dog"]
print(foods[0])
print(foods[2])

Show the answer

pizza
hot dog

The first item sits at index 0, not 1. That zero-counting habit will save you headaches all the way to Level 8 — every list, string and array in Python starts from 0.

Today's big idea

A list is one variable that holds many items in order. Square brackets to build it, square brackets to read from it, .append() to grow it, len() to measure it.

New Concept · The List

12 min

The treasure-chest analogy

Imagine a row of small numbered boxes inside one big chest. Each box holds one value. The chest itself has one name — the variable. To find a treasure, you call the chest's name and the box number.

drinks  = [ "hot chocolate" , "orange juice" , "iced cocoa" , "strawberry milk" ]
index →      0              1          2            3

The chest is called drinks. Four drinks live inside, in fixed positions. Reading drinks[2] gives you the third one — "iced cocoa" — because Python counts boxes from 0.

Tool 1 · Creating a list

Open a pair of square brackets, drop items inside separated by commas, save the whole thing in a variable. A list can hold any type — strings, numbers, even other lists later on.

prices = [5.50, 3.00, 8.00, 2.50]
names  = ["Mia", "Leo", "Ana"]
empty  = []

That last one — [] — is an empty list, zero items, ready to be filled. You'll start one of those whenever the items aren't known in advance.

Tool 2 · Reading by index

The index sits in square brackets right after the variable name. The first item is [0], the second is [1], and so on. Negative indices count from the back — [-1] is the last item.

names = ["Mia", "Leo", "Ana"]
print(names[0])    # → Mia
print(names[1])    # → Leo
print(names[-1])   # → Ana

Off-by-one warning

If a list has 3 items, the valid indices are 0, 1 and 2 — never 3. Asking for names[3] crashes with IndexError: list index out of range. The chest only has so many boxes.

Tool 3 · `len()` — how big is the chest?

The built-in len() returns the number of items in a list. Same function you met for strings in PY-L1-05 — it counts characters there, items here.

names = ["Mia", "Leo", "Ana"]
print(len(names))  # → 3

Tool 4 · `.append()` — drop a new item in

Lists can grow. .append() adds one new item to the end of the list. The dot syntax is new — it means "ask this list to do something for me".

names = ["Mia", "Leo"]
names.append("Ana")
print(names)
# → ['Mia', 'Leo', 'Ana']

Notice we don't write names = names.append("Ana"). .append() changes the list in place — it doesn't hand back a new one. Treating its return value as the new list is a classic first-week bug.

Worked Example · The Hawker Order Pad

12 min

Phase 1 · The first three orders

June runs a tiny stall at Market Pagi Taman Connaught. We're going to build her a Python order pad that starts with three dishes, lets her add a new one, and reports how many are on the pad.

Save as order_pad.py:

Code · phase 1

# order_pad.py — June's hawker order pad

orders = ["pizza", "fried rice", "noodles"]

print("First order:", orders[0])
print("Last order :", orders[-1])
print("Total items:", len(orders))

Output

First order: pizza
Last order : noodles
Total items: 3

Six lines. The list comes first, then three print lines — one using a positive index, one using -1 for the last, one using len() for the count. Notice how print happily takes a label and a value, separated by a comma.

Phase 2 · A new customer walks in

A customer asks for hot dog. We append it to the list and reprint the count — without retyping the whole list.

Code · phase 2 (add to the same file)

orders.append("hot dog")
print("Last order :", orders[-1])
print("Total items:", len(orders))

Output · phase 2

Last order : hot dog
Total items: 4

What changed?

One line of new code (orders.append("hot dog")) added one new item. orders[-1] still asks for the last item — but the "last" has shifted. len() went from 3 to 4 automatically. The list always knows how big it is; we just ask.

Why this matters

Every app you'll ever write — high-score tables, shopping carts, chat messages, exam questions — leans on this exact shape. A list that grows, and a way to read any item back by position.

Basic

5 min

Three exercises, twenty minutes total. Type the starter, then make the change. Don't open the hint until you've given it a real try — a broken first attempt is still progress.

01 🟢 Three drinks

Create a list called drinks holding three drinks. Print the first and the third drink on separate lines, and print how long the list is.

Hint

drinks = ["hot chocolate", "orange juice", "iced cocoa"]
print(drinks[0])
print(drinks[2])
print(len(drinks))

Remember: indexing starts at 0. The third drink lives at index 2, not 3.

Challenge 1

4 min

02 🟡 Grow the list

Start from the same drinks list. Ask the user for a new drink with input(), append it to the list, then print the whole list and the new total count.

Hint

drinks = ["hot chocolate", "orange juice", "iced cocoa"]
new = input("Add a drink: ")
drinks.append(new)
print(drinks)
print("Total:", len(drinks))

Try it twice with different inputs. The list grows each run — but only inside that single program run. We'll learn to save lists to a file in Level 2.

Challenge 2

4 min

03 🔴 Loop the chest open (stretch)

Create a list of five hawker dishes you'd order on a long weekend. Use a for loop to print each dish on its own line, numbered 1 to 5 at the start of the line.

1. pizza
2. fried rice
3. pancake
4. hot dog
5. ice cream

Hint

dishes = ["pizza", "fried rice",
          "pancake", "hot dog", "ice cream"]
for i in range(len(dishes)):
    print(i + 1, ".", dishes[i])

The trick is range(len(dishes)) — it gives you the indices 0, 1, 2, 3, 4. We add 1 to the printed number because humans count from 1, but Python lists count from 0.

Challenge 3 · Felix's Market Pagi Bill

8 min

Felix is helping his mum at the Saturday morning market in Bergen. She asks him to write a quick Python script that keeps a running list of prices and tells her the total at the end.

Your file market_bill.py must:

Start with an empty list called prices.
Use a for loop that runs exactly 4 times. Each pass:
- Ask the user for a price using input() — convert it to a float with float().
- append the price to prices.
After the loop, print the full list of prices.
Use sum() (a built-in, just like len()) on the list to print the total in the form Total: $19.50.

Stretch goal. Print the highest price too, using max() — another built-in that takes a list and gives back the biggest item.

Show one possible solution

# market_bill.py — Felix's morning market bill

prices = []
for i in range(4):
    p = float(input("Price ($)? "))
    prices.append(p)

print("Items:", prices)
print("Total: $", sum(prices))
print("Most expensive: $", max(prices))

The non-negotiables are: prices = [] to start empty, the for loop with range(4), float(input(...)) for each price, prices.append(p), and sum(prices) for the total. Your prompt wording and decimal formatting can vary — we'll learn to format prices with two decimal places when we meet f-strings in Level 2.

Recap

3 min

A list is one variable that holds many items in order. Square brackets build it. An index in brackets reads from it. len() measures it. .append() grows it. That tiny set of moves unlocks every collection-shaped problem in programming — leaderboards, inventories, message logs, shopping carts.

Vocabulary Card

list: An ordered collection of items, written with square brackets — ["a", "b", "c"]. Items keep the order you put them in.
index: The position number of an item in a list, starting from 0. my_list[0] is the first item; my_list[-1] is the last.
len(): A built-in function that returns the number of items in a list (or characters in a string).
.append(): A list method — written with a dot — that adds one new item to the end of the list. Changes the list in place; returns nothing.

Extra Mission

4 min

Create a new file called favourite_dishes.py — a personal favourites list of yours.

Rules:

Start with a list called dishes that already contains three of your favourite dishes.
Print the list, the first dish (using index 0), and the count using len().
Ask the user (input()) for two more dishes — one at a time — and append each one to the list.
Print the new list, the last dish (using index -1), and the new count.
Stretch. After all five dishes are in, use a for loop to print each dish on its own numbered line — exactly like Try-It-Yourself exercise 3.

Bring favourite_dishes.py next class — we'll loop straight through it (without writing a single [i]) in PY-L1-16.

Sample · favourite_dishes.py

# favourite_dishes.py — a growing list of favourites

dishes = ["pizza", "pancake", "fried rice"]

print("Starting list:", dishes)
print("First dish   :", dishes[0])
print("Count        :", len(dishes))

extra1 = input("Add a dish: ")
dishes.append(extra1)
extra2 = input("Add another: ")
dishes.append(extra2)

print("Updated list :", dishes)
print("Last dish    :", dishes[-1])
print("Count        :", len(dishes))

# Stretch — numbered list
for i in range(len(dishes)):
    print(i + 1, ".", dishes[i])

Your three starting dishes will be different — that's the whole point. The non-negotiables are: a starting list with three items, two input + append pairs, and at least one access by index. Your prompt wording can vary freely.

# market_bill.py — Felix's morning market bill prices = [] for i in range(4): p = float(input("Price ($)? ")) prices.append(p) print("Items:", prices) print("Total: $", sum(prices)) print("Most expensive: $", max(prices))

Learning Goals

Warm-Up

New Concept · The List

The treasure-chest analogy

Tool 1 · Creating a list

Tool 2 · Reading by index

Tool 3 · len() — how big is the chest?

Tool 4 · .append() — drop a new item in

Worked Example · The Hawker Order Pad

Phase 1 · The first three orders

Phase 2 · A new customer walks in

What changed?

Basic

Challenge 1

Challenge 2

Challenge 3 · Felix's Market Pagi Bill

Recap

Vocabulary Card

Extra Mission

Sample · favourite_dishes.py

Learning Goals

Warm-Up

New Concept · The List

The treasure-chest analogy

Tool 1 · Creating a list

Tool 2 · Reading by index

Tool 3 · len() — how big is the chest?

Tool 4 · .append() — drop a new item in

Worked Example · The Hawker Order Pad

Phase 1 · The first three orders

Phase 2 · A new customer walks in

What changed?

Basic

Challenge 1

Challenge 2

Challenge 3 · Felix's Market Pagi Bill

Recap

Vocabulary Card

Extra Mission

Sample · favourite_dishes.py

Tool 3 · `len()` — how big is the chest?

Tool 4 · `.append()` — drop a new item in

Tool 3 · `len()` — how big is the chest?

Tool 4 · `.append()` — drop a new item in