Warm-up · Activity 1 of 7
Warm-up from lesson B1.3: what does this print?
word = "Python"
print(word[-3:])// B1.4 · ~25 min · Beginner
Lists: store and change a sequence
After this lesson you can keep several values in one list, read and change items by position, add to the end, and tell when two names share one list.
Predict · Activity 2 of 7
Predict before you read on: one of the cubes is wrong, and the program fixes it. What does it print?
cubes = [1, 8, 27, 65, 125] cubes[3] = 64 print(cubes)Practice · Activity 3 of 7
Fill in the method that adds "c" at the end of the list.
letters = ["a", "b"] letters.____("c") print(letters)letters.("c")Practice · Activity 4 of 7
What does this print?
nums = [1, 2, 3, 4, 5] print(nums[1:3], nums[-1], len(nums))Practice · Activity 5 of 7
x = [["a", "b"], [1, 2, 3]]. Match each expression to its value.
Brain teaser · Activity 6 of 7
Brain teaser. Only b is changed. What does print(a) show?
a = [1, 2, 3] b = a b.append(4) print(a)Apply · Activity 7 of 7
Mini-task. Start a shopping list: items = ["milk", "bread"]. Add "eggs" at the end, then replace the first item with "oat milk". Print the list and, on a second line, Items: with the number of items.
Check your work against this list
Build it yourself
Read the worked example, then write the exercises. Your code runs in your browser or on your computer and is never uploaded.
Worked example
A week of temperatures
The program keeps a week of daily temperatures in a list, Monday first. It reads single days and the weekend with indexes and a slice, corrects one reading, adds the next day, and then puts two weeks into a nested list. Change a temperature and run it again.
main.py
# Temperatures for one week, Monday first.
temps = [12, 14, 13, 17, 19, 21, 18]
print("Days:", len(temps))
print("Monday:", temps[0], "Sunday:", temps[-1])
print("Weekend:", temps[5:])
temps[3] = 16 # Thursday's reading was corrected
temps.append(15) # next Monday
print("Corrected:", temps)
weeks = [temps[:7], [9, 11, 10, 12, 14, 15, 13]]
print("Week 2, Tuesday:", weeks[1][1])
Run it with
python main.pyOutput
Days: 7
Monday: 12 Sunday: 18
Weekend: [21, 18]
Corrected: [12, 14, 13, 16, 19, 21, 18, 15]
Week 2, Tuesday: 11- temps[5:] is a new list with the last two days: a slice of a list is a list.
- temps[3] = 16 changes the list in place; the other items stay where they are.
- append() made the list one longer: it now has 8 items.
- weeks[1][1]: first the second week, then the second day in it.
Change it and run it
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Exercises
Exercise 1 of 2
First, last and middle
scores = [72, 95, 88, 61, 79] holds test scores in the order they were written. Print three lines: First: 72 Last: 79 Middle: [95, 88, 61] Count: 5 The middle is every score except the first and the last. The checks also try other lists.
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Hints
Hint 1
scores[-1] is the last item, however long the list is.
Hint 2
scores[1:-1] starts at the second item and stops before the last one.
Hint 3
print("Count:", len(scores))
Show a solution
One way to solve it. Yours can look different and still pass the checks.
scores = [72, 95, 88, 61, 79]
print("First:", scores[0], "Last:", scores[-1])
print("Middle:", scores[1:-1])
print("Count:", len(scores))
Run it on your computer
Install Python 3.14 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.
main.py
scores = [72, 95, 88, 61, 79]
# First: ... Last: ...
# Middle: ...
# Count: ...
print("First:", scores[0], "Last:", scores[4])
test_main.py
import re
from learnrun import run_main
def run_with(**values):
with open("main.py", encoding="utf-8") as f:
code = f.read()
for name, value in values.items():
code, count = re.subn(rf"^{name} = .*$", f"{name} = {value!r}", code, count=1, flags=re.M)
assert count == 1, f"main.py needs the line {name} = ... at the top"
with open("variant.py", "w", encoding="utf-8") as f:
f.write(code)
return run_main(path="variant.py").strip()
def test_first_and_last():
"""The first line shows the first and the last score"""
got = run_with(scores=[72, 95, 88, 61, 79]).splitlines()[:1]
assert got == ["First: 72 Last: 79"], f"the first line is {got!r}, expected ['First: 72 Last: 79']"
def test_middle_and_count():
"""The middle scores and the count follow"""
got = run_with(scores=[72, 95, 88, 61, 79]).splitlines()[1:]
assert got == ["Middle: [95, 88, 61]", "Count: 5"], f"lines 2 and 3 are {got!r}"
def test_other_list():
"""A list of three scores works too"""
got = run_with(scores=[50, 60, 70]).splitlines()
assert got == ["First: 50 Last: 70", "Middle: [60]", "Count: 3"], f"for [50, 60, 70] the program printed {got!r}"
On macOS and Linux, type python3 wherever these commands say python, as in the first lesson.
Run the program:
python main.pyRun the checks (needs learnrun.py in the same folder):
python learnrun.py testExercise 2 of 2
Update a playlist
songs = ["Intro", "Blue", "Green"] is a playlist. Change it in place: add "Outro" at the end, then replace the second song with "Red". Print the list, then Songs: and the number of songs. For the starting list that gives: ['Intro', 'Red', 'Green', 'Outro'] Songs: 4
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Hints
Hint 1
songs.append("Outro") adds the song at the end of the same list.
Hint 2
The second song has index 1: songs[1] = "Red".
Hint 3
Change the list first and print it afterwards, so the output shows both changes.
Show a solution
One way to solve it. Yours can look different and still pass the checks.
songs = ["Intro", "Blue", "Green"]
songs.append("Outro")
songs[1] = "Red"
print(songs)
print("Songs:", len(songs))
Run it on your computer
Install Python 3.14 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.
main.py
songs = ["Intro", "Blue", "Green"]
# 1. Add "Outro" at the end.
# 2. Replace the second song with "Red".
print(songs)
print("Songs:", len(songs))
test_main.py
import re
from learnrun import run_main
def run_with(**values):
with open("main.py", encoding="utf-8") as f:
code = f.read()
for name, value in values.items():
code, count = re.subn(rf"^{name} = .*$", f"{name} = {value!r}", code, count=1, flags=re.M)
assert count == 1, f"main.py needs the line {name} = ... at the top"
with open("variant.py", "w", encoding="utf-8") as f:
f.write(code)
return run_main(path="variant.py").strip()
def test_playlist():
"""Outro is added and the second song becomes Red"""
got = run_with(songs=["Intro", "Blue", "Green"]).splitlines()[:1]
assert got == ["['Intro', 'Red', 'Green', 'Outro']"], f"the list printed is {got!r}"
def test_count():
"""The count includes the new song"""
got = run_with(songs=["Intro", "Blue", "Green"]).splitlines()[1:]
assert got == ["Songs: 4"], f"the second line is {got!r}, expected ['Songs: 4']"
def test_other_playlist():
"""A playlist of two songs works too"""
got = run_with(songs=["A", "B"]).splitlines()
assert got == ["['A', 'Red', 'Outro']", "Songs: 3"], f"for ['A', 'B'] the program printed {got!r}"
On macOS and Linux, type python3 wherever these commands say python, as in the first lesson.
Run the program:
python main.pyRun the checks (needs learnrun.py in the same folder):
python learnrun.py testCommon mistakes
An index past the end
items = ["a", "b"]
print(items[2])
What Python prints
IndexError: list index out of rangeWhy, and the fix
A list of two items has the indexes 0 and 1. Use items[-1] for the last item, or len(items) - 1 as its index. To add a third item, use append(); assigning to items[2] gives the same error.
A method from another language
items = ["a"]
items.push("b")
What Python prints
AttributeError: 'list' object has no attribute 'push'Why, and the fix
Python lists have no push method. The method that adds an item at the end is append: items.append("b").
Storing what append() returns
items = ["a", "b"]
items = items.append("c")
print(items[0])
What Python prints
TypeError: 'NoneType' object is not subscriptableWhy, and the fix
append() changes the list itself and returns None. Assigning that result to items throws the list away, and items[0] then fails because None has no items. Call it on its own line: items.append("c").
Python in the browser: Pyodide 314.0.7, MPL-2.0. Licence and source
Exit ticket
5 questions, no hints. Score 80% or more to complete the lesson.
Finish every activity above to unlock the exit ticket.