Warm-up · Activity 1 of 7
Warm-up from B2.3: what does this print?
for n in [1, 3, 5]:
if n % 2 == 0:
print("even:", n)
break
else:
print("no even number")// B2.4 · ~30 min · Beginner
Pattern matching with match
After this lesson you can choose between cases with match, take a list apart with patterns that capture its items, and add conditions with guards.
Predict · Activity 2 of 7
Predict before you read on: what does this print?
status = 418 match status: case 200: print("OK") case 404: print("Not found") case _: print("Other")Practice · Activity 3 of 7
Fill in the symbol that lets one case match "sat" or "sun".
day = "sun" match day: case "sat" ____ "sun": print("weekend") case _: print("weekday")case "sat" "sun":Practice · Activity 4 of 7
split() turns the text into a list of words. What does this print?
command = "go north".split() match command: case ["quit"]: print("bye") case ["go", direction]: print("going", direction) case _: print("unknown")Practice · Activity 5 of 7
Match each pattern to what it matches.
Brain teaser · Activity 6 of 7
Brain teaser. QUIT is meant to be a constant. What does this print?
QUIT = "q" key = "x" match key: case QUIT: print("quitting", QUIT)Apply · Activity 7 of 7
Mini-task. Write a tiny calculator: read one line with input(), split it, and match the words. add 2 3 prints 5, neg 4 prints -4, quit prints bye, and anything else prints unknown command. Convert the captured words with int().
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
Commands for a text game
A text adventure reads commands such as go north or take lamp key. The program splits each command into words and matches the list: exact commands, a guard that allows only four directions, *things to take any number of items, | for two spellings, and a wildcard for everything else. Add your own commands to the list and run it again.
main.py
# A few commands a player might type.
commands = ["look", "go north", "go up", "take lamp key", "i", "", "dance now"]
inventory = []
for line in commands:
match line.split():
case ["look"]:
print("You are in a dark hall.")
case ["go", direction] if direction in ["north", "south", "east", "west"]:
print("You walk", direction)
case ["go", other]:
print("You cannot go", other)
case ["take", *things]:
for thing in things:
inventory.append(thing)
print("Taken:", things)
case ["inventory" | "i"]:
print("You carry:", inventory)
case []:
print("Say something.")
case [verb, *_]:
print("I do not know how to", verb)
Run it with
python main.pyOutput
You are in a dark hall.
You walk north
You cannot go up
Taken: ['lamp', 'key']
You carry: ['lamp', 'key']
Say something.
I do not know how to dance- go up fails the guard of the first go case, so match tries the next case, ["go", other].
- *things collects every word after take, here two, into a list.
- "".split() is the empty list, which case [] matches.
- case [verb, *_] catches every other non-empty command and keeps only the first word.
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
HTTP status text
The program reads an HTTP status code as a whole number. Use match to print OK for 200, Redirect for 301 or 302, Not found for 404, Server error for 500 and Unknown for anything else. Combine 301 and 302 in one case with |.
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
match code: with one case per status, each followed by an indented print.
Hint 2
case 301 | 302: matches either code.
Hint 3
Finish with case _: print("Unknown") as the last case.
Show a solution
One way to solve it. Yours can look different and still pass the checks.
code = int(input())
match code:
case 200:
print("OK")
case 301 | 302:
print("Redirect")
case 404:
print("Not found")
case 500:
print("Server error")
case _:
print("Unknown")
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
code = int(input())
# Match code and print its text.
print("Unknown")
test_main.py
from learnrun import run_main
def status(code):
return run_main(f"{code}\n").strip()
def test_ok_and_not_found():
"""200 is OK and 404 is Not found"""
got = status(200), status(404)
assert got == ("OK", "Not found"), f"for 200 and 404 the program printed {got!r}"
def test_redirects():
"""301 and 302 are both Redirect"""
got = status(301), status(302)
assert got == ("Redirect", "Redirect"), f"for 301 and 302 the program printed {got!r}"
def test_server_error():
"""500 is Server error"""
got = status(500)
assert got == "Server error", f"for 500 the program printed {got!r}, expected 'Server error'"
def test_unknown():
"""Any other code is Unknown"""
got = status(418)
assert got == "Unknown", f"for 418 the program printed {got!r}, expected 'Unknown'"
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
Area of a shape
The program reads a line such as square 5 or rect 3 4. Split it and match the words. Print the area of a square or a rectangle as a whole number. If a size is not above 0, print invalid, using a guard. For anything else print unknown shape.
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
case ["square", side]: captures the size as a string; int(side) turns it into a number.
Hint 2
A guard goes after the pattern: case ["square", side] if int(side) > 0:.
Hint 3
After the guarded cases, case ["square", _] | ["rect", _, _]: catches the right shapes with bad sizes.
Show a solution
One way to solve it. Yours can look different and still pass the checks.
words = input().split()
match words:
case ["square", side] if int(side) > 0:
print(int(side) * int(side))
case ["rect", width, height] if int(width) > 0 and int(height) > 0:
print(int(width) * int(height))
case ["square", _] | ["rect", _, _]:
print("invalid")
case _:
print("unknown shape")
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
words = input().split()
# Match words: ["square", side] or ["rect", width, height].
print("unknown shape")
test_main.py
from learnrun import run_main
def area(line):
return run_main(line + "\n").strip()
def test_square():
"""square 5 has area 25"""
got = area("square 5")
assert got == "25", f"for square 5 the program printed {got!r}, expected '25'"
def test_rect():
"""rect 3 4 has area 12"""
got = area("rect 3 4")
assert got == "12", f"for rect 3 4 the program printed {got!r}, expected '12'"
def test_invalid_sizes():
"""Sizes of 0 or less are invalid"""
got = area("square 0"), area("rect 3 -1")
assert got == ("invalid", "invalid"), f"for square 0 and rect 3 -1 the program printed {got!r}"
def test_unknown():
"""Other shapes and wrong word counts are unknown"""
got = area("circle 2"), area("rect 3")
assert got == ("unknown shape", "unknown shape"), f"for circle 2 and rect 3 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
A bare name used as a constant
QUIT = "q"
key = "x"
match key:
case QUIT:
print("quitting")
case _:
print("other key")
What Python prints
SyntaxError: name capture 'QUIT' makes remaining patterns unreachableWhy, and the fix
A bare name in a case captures any value, so every case after it could never run, and Python refuses the file. Compare with the literal instead: case "q":. Named constants work only as dotted names, such as keys.QUIT, which you will meet with modules and classes.
or instead of |
day = "sat"
match day:
case "sat" or "sun":
print("weekend")
What Python prints
SyntaxError: invalid syntaxWhy, and the fix
Patterns are not expressions, so or is not allowed in them. Alternatives are joined with a bar: case "sat" | "sun":.
The wildcard first
n = 1
match n:
case _:
print("anything")
case 1:
print("one")
What Python prints
SyntaxError: wildcard makes remaining patterns unreachableWhy, and the fix
case _: matches everything, and match stops at the first case that fits, so no case below it could ever run. Put the specific cases first and the wildcard last, as you would put else at the end of an if chain.
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.