Session 8: Lists — collections of values - From Scratch Programming Class

Session 8: Lists — collections of values

Phase 3 — Python basics · Session 8 of 16

What we’re learning today

Until now, every variable has held one thing — one number, one string, one True/False. Today you’ll learn how to hold many things in a single variable. That’s a list, and it’s arguably the most important data type in all of Python. By the end of class, you’ll have built a “favorites collector” program that lets the user enter as many things as they want and then shows them all back.

You’ll need to remember from last time

Variables can hold a value (number, string, boolean).
for loops — for i in range(N):.
while True: and break for “loop until something happens” patterns.
input(...) for getting input from the user.
f-strings for formatting output.

Part A: What lists are and how to use them

Open Thonny and start a new file. Save it as lists.py.

Making a list

A list is a value that holds multiple other values, in order. You make one with square brackets:

colors = ["red", "blue", "green", "yellow"]
print(colors)

Save. Run.

['red', 'blue', 'green', 'yellow']

The variable colors holds four strings, all in one list.

A list can hold any kind of value — strings, numbers, even booleans:

ages = [12, 14, 9, 16]
mixed = ["Alex", 12, True]
empty = []

The last one ([]) is an empty list — a list with nothing in it yet. Useful when you want to fill it up gradually.

Getting items by index

Lists use the same [index] syntax you learned for strings in Session 3. The first item is at index 0:

colors = ["red", "blue", "green", "yellow"]
print(colors[0])    # red
print(colors[1])    # blue
print(colors[-1])   # yellow (last)

Same indexing rules. Negative numbers count from the end.

How long is the list?

len() works on lists too:

colors = ["red", "blue", "green", "yellow"]
print(len(colors))    # 4

Adding to a list

The most common list operation: adding an item. Use the .append() method:

colors = ["red", "blue"]
colors.append("green")
colors.append("yellow")

print(colors)    # ['red', 'blue', 'green', 'yellow']

Each .append() adds one item to the end of the list.

Looping through a list

You can iterate through a list with for, just like you did with strings:

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

for color in colors:
    print(f"I like the color {color}.")

Output:

I like the color red.
I like the color blue.
I like the color green.
I like the color yellow.

The loop runs once for each item. Each iteration, color (the loop variable) holds the next item from the list.

This is the same syntax as for letter in name: from Session 3. Strings and lists both work with for ... in ....

Changing items

You can replace an item by assigning to its index:

colors = ["red", "blue", "green"]
colors[1] = "purple"
print(colors)    # ['red', 'purple', 'green']

This is one of the things lists can do that strings can’t — strings are immutable (you can’t change their characters), but lists are mutable (you can).

Checkpoint: You’ve created a list, accessed items by index, used .append() to add to it, and used a for loop to print all the items. This is the natural stop point if class is cut short.

Part B: A favorites collector

Time to use lists for something real.

What you’re building

A program that asks the user to enter their favorite things, one at a time. They keep entering until they type “done.” Then the program shows back the complete list.

Base goal

print("Favorites Collector!")
print("Enter your favorite things, one at a time. Type 'done' when finished.")

favorites = []

while True:
    item = input("Favorite: ")
    if item == "done":
        break
    favorites.append(item)

print(f"\nYou have {len(favorites)} favorites:")
for fav in favorites:
    print(f"  - {fav}")

Save. Run. Type a few favorites (pizza, soccer, blue, whatever). Type “done” when finished. The program shows the list.

Walk through what’s happening:

favorites = [] — start with an empty list.
The while True: loop keeps asking forever.
If the user types “done”, we break out of the loop.
Otherwise, we .append() what they typed to the list.
After the loop ends, len(favorites) and a for loop show the complete list.

That’s the base goal — a complete interactive program built around a list.

Stretch — number them

Right now the list shows:

- pizza
- soccer
- blue

Numbered would be nicer:

1. pizza
2. soccer
3. blue

Use range(len(favorites)) to get the indexes:

print(f"\nYou have {len(favorites)} favorites:")
for i in range(len(favorites)):
    print(f"  {i + 1}. {favorites[i]}")

The loop variable i goes 0, 1, 2… but we add 1 to display “1, 2, 3…” (humans number from 1).

favorites[i] looks up the item at index i. Same square-bracket syntax as accessing a single item.

Extension — categorize

Ask the user to categorize each favorite — food, color, activity, etc. Store both:

favorites = []
categories = []

while True:
    item = input("Favorite (or 'done'): ")
    if item == "done":
        break
    cat = input("What category? ")
    favorites.append(item)
    categories.append(cat)

print(f"\nYou have {len(favorites)} favorites:")
for i in range(len(favorites)):
    print(f"  {i + 1}. {favorites[i]} ({categories[i]})")

Two parallel lists. The favorite at index 0 has the category at index 0. They stay in sync because we always append to both at the same time.

(There are better ways to do this in Python — using a list of pairs, or a dictionary, or a list of dictionaries. We’ll get to dictionaries in Session 10. For now, two lists is fine.)

Extension — sorted output

Sort the list before showing it:

favorites.sort()
print(f"\nYour {len(favorites)} favorites (alphabetical):")
for fav in favorites:
    print(f"  - {fav}")

.sort() is a list method that sorts the list in place (it changes the list itself). Strings sort alphabetically; numbers sort numerically.

Wrap-up

Before we leave, share with the room:

What’s in your favorites list? Anything funny?
For the kids who built the categorized version — did the two parallel lists stay in sync?
For the kids who used .sort() — did the order surprise you? (Capital letters sort before lowercase letters.)

You learned today the most-used data structure in all of Python. Lists are everywhere. Files are read as lists of lines. Spreadsheet rows are lists of cells. Webpages are lists of elements. Programs that handle data, programs that handle collections of anything — they all use lists.

You also learned that a for loop knows how to walk through a list (or a string). Same for X in Y: syntax, different collections. From here on out, that pattern is your default for “do something with each item.”

If you missed this session

Open Thonny and start a new file. Save as lists.py. Then:

Make a list and try basic operations:

colors = ["red", "blue", "green"]
print(colors[0])     # red
print(len(colors))   # 3
colors.append("yellow")
print(colors)

Loop through it:

for color in colors:
    print(f"I like {color}.")

Build the favorites collector from Part B:
- Empty list
- while True loop
- input, check for “done”, append
- Print all at the end with a for loop

About 30-40 minutes. If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

.remove(item) — removes the first occurrence of an item from the list. colors.remove("red") deletes “red”.
.pop() — removes and returns the last item. colors.pop() takes off the last one.
in for membership — "red" in colors returns True/False. Same in you used for if letter in "aeiou": in Session 3.
+ between lists — [1, 2, 3] + [4, 5] is [1, 2, 3, 4, 5]. Combines two lists.
* between lists and ints — ["a"] * 5 is ["a", "a", "a", "a", "a"]. Useful for initialization.

What’s next

Next week we go deeper into lists — more methods, more patterns, slicing (the [start:end] syntax you saw with strings). After that, dictionaries — lists’ cousin that holds named pairs instead of just ordered values.

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