Week 24 — Debugging — reading an error message on purpose¶

At a glance


Date	Friday, February 19, 2027
Unit	3 — From Blocks to Text
Big idea	Error messages are clues; a traceback is the computer pointing to where it got confused.
Goal	Students intentionally break short Python programs, read the error, and fix one bug at a time.
Student accounts needed	None. MakeCode Arcade runs in the browser without sign-in.
Prep time	5 minutes

Why this lesson looks the way it does¶

This may be the highest-leverage lesson in the unit. Many students learn syntax by fear: red text means 'I am bad at this.' Today reverses that. Red text means the computer left a clue. Model calm: break code on purpose, read the last line out loud, fix one thing, and run again.

The mixed-age strategy stays explicit: Starters never face a blank page, Builders work from a template, and Extenders add features or write from scratch. Driver/Navigator pairing is not just classroom management here; it shares the typing load and makes students verbalize syntax.

CSTA 2026 standards¶

Concept area	Connection
Algorithms & Design	Students design, test, and revise small programs within constraints
Programming	Students read and write Python syntax for the week's concept
Data & Analysis	Students use program output or test results as evidence
Systems & Security	Students work in no-account browser tools and avoid private information
Computing & Society	Students explain technical work clearly to an audience

"I can…" objectives¶

By the end of today, students can say:

I can read the main Python pattern for today's lesson.
I can make and test a small change to working code.
I can explain one important line to my partner.
I can use evidence from running the code before claiming it works.

Teacher prep — the 5-minute version¶

Open https://arcade.makecode.com and switch to Python.
Paste the mini-lesson code below and run it once.
Put this on the board: Run → Predict → Change one thing → Test.
Pair students Driver/Navigator and plan the 0:24 swap.
Keep Week 18–22 examples nearby for students who need a remix base.

Minute-by-minute run sheet¶

0:00–0:04 — Anchor¶

"Last Friday we named our own instructions with functions. Today we keep building in Python, but the win is still the same: understand a small program well enough to change it and test it."

0:04–0:12 — Mini-lesson¶

Break this code live by removing the colon, then read the final error line before fixing it.

score = 0
if score == 0:
    print("Start!")

"The important line is the last line. SyntaxError: expected ':' is not saying the whole program is bad. It is saying, 'I expected a colon here.'"

Then break indentation and read: IndentationError: expected an indented block after 'if' statement.

0:12–0:36 — Paired build¶

"Driver, type only what your pair understands. Navigator, your job is to stop the pair from making five changes before testing one."

Tiered challenges¶

🟢 Starter (grades 5–6): modify working starter code

Fix the missing quote, then change the message.

hero = sprites.create(img("""
    . 4 4 .
    4 4 4 4
    . 4 4 .
"""), SpriteKind.player)
hero.say_text("Hello)

🟡 Builder (grades 7–8): write from a template

Fix the two syntax bugs.

energy = 2
if energy > 1
    print("Ready")
else:
    print("Wait")

Then change the condition so the other message prints.

🔴 Extender (grades 9–10): write from scratch or add features

Find the scope bug with a test, then fix it.

coins = 0

def add_coin():
    coins = coins + 1
    print("Coins: " + str(coins))

add_coin()

Add a second call and predict the output.

"Do not show us perfect code. Show us an error you fixed. Read the error message and tell us which word helped you."

0:42–0:45 — Teaser + prompt¶

"Next Friday we connect this to AI. An AI assistant can write code quickly, but you cannot verify code you do not understand."

🤔 Think About This¶

Notice one error message this week outside coding. What clue did the message give you?

"Do not show us perfect code. Show us an error you fixed. Read the error message and tell us which word helped you."

🔌 Unplugged Plan B — if the internet is down¶

Time	Activity
0:00–0:04	Same anchor: connect to last Friday and name today's target.
0:04–0:12	Board model with paper code cards. Underline names, circle punctuation, and box indented blocks.
0:12–0:26	Pairs run a paper "human interpreter" trace. One student reads exactly; one student records output or error.
0:26–0:36	Pairs modify one card, trade with another pair, and test again.
0:36–0:42	Share one working change and one bug found by reading carefully.
0:42–0:45	Same teaser and Think About This.

Common problems & fixes¶

Problem	What students may see	Fix
Missing colon after `if`, `for`, or `def`	`SyntaxError: expected ':'`	Add a colon at the end of the header line.
Inconsistent indentation	`IndentationError: unindent does not match any outer indentation level`	Make lines in the same block line up exactly.
No indentation after a header	`IndentationError: expected an indented block after 'if' statement`	Indent the line that belongs inside the block.
Mismatched quotes	`SyntaxError: unterminated string literal`	Check every string starts and ends with the same quote mark.
Using `=` in a condition	`SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?`	Use `=` to store; use `==` to compare.
Misspelled name	`NameError: name 'socre' is not defined`	Match the spelling where the variable was created.

Vocabulary¶

Debugging — finding and fixing problems in code
Traceback — an error report that points to where Python got confused
Syntax error — code punctuation or structure Python cannot read
Indentation error — spaces do not match the program structure
Name error — Python sees a name it does not know

Reflection — fill in after class¶

Which students could explain their code without prompting?
Which syntax issue slowed the room down?
Did pairs test one change at a time?
What should carry into next week?

Next week: Week 25 — Coding with an AI assistant — and checking its work