Week 3 Coding Lab: Typed Text and Sliced Labels at a Food-Truck Window

Part 1 — The Big Picture

This week you learned that whatever a person types reaches your program as a string, that an f-string fills its braces with values, and that indexes and slices pick characters out of a string by position. In this lab you put those ideas to work on a food-truck order screen. You predict what six short programs print, run them, and study every gap between your prediction and the run. Then you read three broken programs, fix each with the smallest change that works, write a tiny program of your own, and check a chatbot's answers to the same questions by running the code, because a chatbot predicts likely text and does not run your program.

Time budget: Plan for about 90 minutes.

Part 2 — The Task

Guiding question: When a customer types 2 at the food-truck window, which of these six predictions will you get wrong, and how will each run show you whether the program treated the typed 2 as text or as a number?

Workshop data: A food-truck order screen: six short programs and three broken ones

Source: Six short Python programs and three broken programs about an illustrative campus food-truck order screen, written for this lab and printed in full in Part 4; the menu item and quantities are invented and describe no real truck or person.

• Rows: 9 short programs in all: 6 programs to trace in the first table and 3 broken programs in the second table
• Language: Python 3, run in a free browser runner; no file to download and nothing to install
• Context: an illustrative food-truck order of a typed quantity and one menu item

Part 3 — Materials & Procedure

Tools and materials

• A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting

Ways of working

• write-run-and-verify
• trace-and-predict-output
• find-and-fix-the-bug

Procedure

1. Open a free browser Python runner in one tab and Python Tutor at pythontutor.com in another. In the runner, delete any sample code so the editor is blank. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: An empty editor with a Run button and a separate output panel.
2. In the first table in Part 4, write your prediction for each of the six programs before you run anything. Then type each program into the runner exactly as printed, including its setup lines qty = "2" and item = "veggie wrap", press Run, and copy the output into the Actual output column. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: Six actual outputs, one per row, each copied character for character from the output panel; at least one is likely to differ from your prediction.
3. Type each broken program from the second table into the runner and press Run. For an error, scroll to the bottom of the error message and copy the error type from its last line. For a program that runs, compare what it printed with what its description says it should print. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: The second table shows what each broken program printed, or the error type on its last line; one of the three runs without any error but prints the wrong total.
4. Fix each broken program with the smallest change that makes it do what its description in Part 4 says, run the fixed version, and record the new output. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: Each fixed program runs without an error and prints the total, order line, or letter its description asks for.
5. Paste the setup line item = "veggie wrap" and the line print(item[7:11]) into Python Tutor, click the button to visualize it, and step forward one line at a time. Watch item get its value before the slice prints. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: Python Tutor shows item holding the string before the second line prints wrap, which matches what the runner showed.
6. Write your own two-line program: line 1 stores the typed value 4 as the string cups = "4", and line 2 uses an f-string to print the label Cups: followed by twice the number of cups, converting with int() inside the braces. Predict its output, run it, and fill in the third table. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: A two-line program that runs without an error, with your predicted and actual output side by side.
7. Open your approved chatbot, paste the critique prompt from Part 6 as one message, then check each of its claims by running the code it talks about. If you are using the equivalent route, mark up the printed flawed analysis in the same way. Tool: A free browser-based Python runner (an online editor that runs Python 3 with no account and nothing to install), Python Tutor at pythontutor.com for step-by-step visualization, the official Python documentation at docs.python.org, and one approved chatbot used as a pair-programmer whose code the student runs before trusting. Check: The fourth table lists each chatbot claim, what your run showed, and a correction wherever the two disagree.

Fallback route

When to use it: Use this route if your browser Python runner will not open or will not run code.

Run every program in Python Tutor instead, which also runs Python 3 in the browser: paste each program with its setup lines, click the button to visualize it, and step to the end to read its output or its error. Complete every table, analysis question, and the critique step, and submit the same items for equal credit.

This route earns the same credit as the primary route.

Part 4 — The Scaffold

Predict first and run second, so the gap between the two columns is your own evidence of what Python really does with typed text.

Student scaffold table 1

Program (setup: qty = "2" and item = "veggie wrap")	Your prediction	Actual output from the run	Same or different?
print(qty * 3)
print(int(qty) * 6)
print(f"{qty} {item}s")
print(item[0])
print(item[7:11])
print(len(item))

Read each broken program from its run before you change a single character.

Student scaffold table 2

Broken program and what it should do	What printed, or the error type on the last line	Your one-change fix	Output after the fix
Bug 1, should print Total: and the cost of two wraps at 6 dollars each: print("Total:", qty * 6)
Bug 2, should print Order for 2 wraps: print("Order for " + 2 + " wraps")
Bug 3, should print the last letter of the menu item: print(item[11])

Write your own small program, then let the run confirm or correct what you expected it to print.

Student scaffold table 3

Line you wrote	Output you expected	Output of the run
Line 1
Line 2

Check every chatbot claim against a run so the run, not the chatbot, has the last word.

Student scaffold table 4

Chatbot claim, quoted	What your run showed	Correct as stated, or your correction

Part 5 — Analysis Questions

1. Which of your six predictions in the first table differed from the run? For each one, write the step-by-step trace that reaches the actual output and name the rule that explains the gap, such as typed text staying text or a slice dropping its stop.
2. For Bug 2 and Bug 3, which error type did each last line name, and what did each program print before it stopped? Explain why both are runtime errors rather than syntax errors.
3. Bug 1 runs without any error message but prints the wrong total. Explain why that makes it a logic error, and why the conversion with int() has to happen before the multiplication rather than after it.
4. Choose one: (a) describe how you would decide whether to trust a chatbot's claim about what a slice prints, using one claim from your critique as the example; or (b) describe how predicting before running changed what you noticed about typed text in this lab, using one row of your first table as the example.
5. Every run in this lab used the typed value 2 written as a literal. Name one thing these runs could not tell you about what a real customer might type, and say how you would check it.

Part 6 — AI-Critique Moment

Choose one equal-credit critique route. Route A: ask an approved AI tool to critique your completed work, then check its response against your own evidence. Route B is the no-AI equivalent printed below.

Approved tools: Claude, Gemini, ChatGPT, Grok, Microsoft Copilot.

Prompt to paste:

I'm doing a beginner Python lab about a food-truck order program, written for a class activity. Please do six things, and keep each answer short. First, with qty = "2" and item = "veggie wrap", predict exactly what each of these lines prints, and explain your reasoning for the first two: print(qty * 3); print(int(qty) * 6); print(f"{qty} {item}s"); print(item[0]); print(item[7:11]); print(len(item)). Second, this line is supposed to print a total of 12 dollars for two wraps at 6 dollars each, but it prints the wrong thing. Fix it and show your fixed version: print("Total:", qty * 6). Third, this line stops with an error. Name the error type, then fix it so it prints Order for 2 wraps: print("Order for " + 2 + " wraps"). Fourth, this line is supposed to print the last letter of item but stops. Fix it: print(item[11]). Fifth, does Python have a built-in function called substr() for cutting a piece out of a string? Sixth, write one line that prints the last four letters of any menu item stored in item, and show it works for item = "veggie wrap".

Test the response for these likely failures:

• Chatbots often predict that print(qty * 3) shows 6, treating the typed 2 as a number.
• A chatbot may quote the correct output 12 for print(int(qty) * 6) while explaining that Python joins the 2 six times, so its explanation and the printed result disagree.
• A chatbot may fix the total line by writing print("Total:", int(qty * 6)), which runs without an error but converts the already-repeated text.
• A chatbot may fix the last-letter line as print(item[len(item)]) and call it working without running it.
• A chatbot may describe a substr() function as if it were built into Python and show how to call it.
• Asked for the last four letters, a chatbot may write print(item[7:11]), which works for veggie wrap but fixes the positions to that one name.
• A chatbot may say the last letter of veggie wrap is at index 11 because len(item) is 11, counting positions from 1.

Write 2-3 sentences that name one thing the chatbot got right and at least one claim you corrected, and for each correction give the output or error type your own run showed.

Equivalent route (no AI required)

No AI account is required for this lab. Instead of the chatbot, critique this flawed analysis, printed here in full: “With qty = "2", print(qty * 3) prints 6, because Python multiplies the typed number. print(int(qty) * 6) prints 12 because Python joins the 2 six times. To fix the total line, write print("Total:", int(qty * 6)); this version is correct. To fix the last-letter line, write print(item[len(item)]); this version is working. Python's built-in substr() function cuts a piece out of a string. To print the last four letters of any menu item, write print(item[7:11]). The last letter of veggie wrap is at index 11, because the item has 11 characters.” Mark it up: quote each flawed sentence, run the code it describes in your browser Python runner, and write the corrected claim beside it with the run's output or error type as evidence, including a run of the four-letter line on a shorter item such as tacos. This route assesses the same objectives as the chatbot critique, namely reading typed input as text, slicing and indexing a string, and checking every claim about code by running it (Objective 2 and Objective 1), and it takes comparable effort, about 20 minutes of the lab. It is scored by the same rubric for the same credit, and its marked-up analysis replaces the chatbot report and share link in your submission. You do not need to say which route you chose.

Part 7 — What to Submit

• All four completed scaffold tables
• Written answers to all five analysis questions
• Your two-line cups program with its run output
• The chatbot critique report with its share link, or the equivalent route's marked-up flawed analysis

Deliverable:

A short Coding Lab report on an illustrative food-truck order screen: four completed tables (six predictions beside their runs, three broken programs read and fixed, your own two-line program, and the checked chatbot claims), written answers to five analysis questions, and a critique of a chatbot's answers to the same questions, or, on the equivalent route, a marked-up correction of the printed flawed analysis. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text. Choose a written, recorded, or visual form for your answers, such as screen captures of your runs with a short note for each; every form is graded by the same rubric.

Rubric

The rubric below shows how each submitted part is evaluated; bands are full, partial, or none.

Workshop evaluation criteria

Criterion	Full	Partial	None
Predicting and running the six programs	All six predictions are recorded before the runs, and all six actual outputs match what the programs print, copied character for character.	The table is mostly complete, but an actual output is missing or miscopied, or predictions appear to have been filled in after running.	No predictions or runs are recorded, or the outputs are invented rather than run.
Explaining the gaps between prediction and run	Every prediction that differed from its run is traced step by step to the actual output, with the governing rule named, such as input text staying a string, a slice dropping its stop, or indexes starting at 0.	The gaps are listed, but a trace is missing or the rule behind a gap is not named.	No gap is explained, or the response blames the runner instead of the rule.
Finding and fixing the three bugs	Each bug is read correctly from its run: what printed or the error type, a one-change fix, and the fixed program's output, with Bug 1 identified as a logic error and the conversion placed before the multiplication.	Two of the three bugs are read and fixed correctly, or all three are fixed but an error type or the run evidence is missing.	The bugs are not attempted, or errors are named without running the programs.
Writing, running, and verifying a program	The two-line cups program runs without an error, stores the entered value as a string, converts it with int() inside an f-string, and its expected and actual outputs are recorded side by side.	The program runs but misses one requirement, or the expected or actual output is not recorded.	No program is submitted, or it was not run.
Critiquing the AI analysis or the printed flawed analysis	The chatbot's answers, or on the equivalent route the printed flawed analysis, are quoted and at least two claims are corrected with evidence from the student's own runs.	The critique names errors but corrects only one, or corrects without run evidence, on either route.	No critique is submitted on either route, or chatbot or flawed text is pasted without any checking.
Reflection and limitation	The chosen reflection option is answered with a specific example from this lab, and the limitation answer names something these runs cannot show and a sensible way to check it.	The reflection or the limitation is general, without an example from this lab or a way to check it.	The reflection and limitation questions are not answered.

Quality Gate

• Every prediction in the first table was written before its program was run.
• Every actual output and error type is copied from your own run, never from memory or a chatbot.
• Each error is named by its type from the last line of the error message, not by the wording after the colon.
• Each fix changes as little as possible and its fixed program was run.
• No output, count, or claim is invented anywhere in the submission.
• The critique quotes the chatbot or the flawed analysis and corrects at least two claims with evidence from your runs.