Debugging Tools

Use print debugging, VS Code breakpoints, and pdb to inspect running programs.

• Level: Beginner
• Estimated time: 35–50 minutes
• You will learn: Use print debugging, VS Code breakpoints, and pdb to inspect running programs.
• Practice in: Colab, VS Code, and the terminal

Open this lesson in Google Colab

Questions

• What problem does Debugging Tools help us solve in a small Python program?
• What should we predict before running the example?
• What value, output, or error should we inspect after changing one line?

Objectives

• Run a complete example for debugging tools in Colab.
• Explain the example line by line using plain language.
• Change one part of the code and predict the result before running it.
• Recognize one common mistake and use the error message as evidence.

Hands-on episode: Debugging Tools

Debugging is controlled observation. You choose a place to inspect, run the program to that place, compare actual values with expected values, and then change one thing.

We will learn this by running code, not by memorizing a definition first. Open the Colab notebook from the button above, find this section, and run each cell in order. Keep a small note beside the notebook with three columns: prediction, actual result, and what changed.

Example 1.1

Use print debugging to reveal a type problem before it causes confusion later.

price = "10"
print("price:", price, type(price))
total = int(price) * 2
print(total)

Run the cell once without editing it. If the result is different from your prediction, leave the prediction visible and write one sentence about the difference. That sentence is more useful than a perfect first guess.

Explain Example 1.1

• The first line stores text, not a number, because of the quotes.
• The temporary print shows both the value and its type, which is the clue.
• int(price) converts the string before multiplication, so the final result is numeric.

Now explain the example out loud or in a Markdown cell. Use short sentences: “this line creates…”, “this name stores…”, “this output appears because…”. If you cannot explain a line yet, run only the lines above it and inspect the values that exist at that moment.

Challenge 1.1

NoteChallenge

Remove int(...) and predict whether Python repeats the text, multiplies numerically, or raises an error. Then run it and explain the evidence.

Show a safe way to approach the challenge

1. Copy Example 1.1 into a new Colab cell.
2. Change exactly one value, name, condition, or line.
3. Write the expected output before running the cell.
4. Run the cell and compare the actual result with your prediction.
5. If the result surprises you, undo the change and try a smaller one.

Suggested first move: Remove int(...) and predict whether Python repeats the text, multiplies numerically, or raises an error.

Debugging checkpoint 1.1

WarningDebugging checkpoint

Printing too much creates noise. Print the smallest useful value, include a label, and remove the temporary print after learning from it. In VS Code, avoid stepping into library code at first; stay inside the code you wrote.

Do not debug by rewriting the whole example. Read the error type or surprising output, inspect the closest value with print(...) or type(...), then change one thing. This is the same routine you will use in larger projects.

Apply it

Use the same bug three ways: inspect it with print, pause on it with a VS Code breakpoint, and step through it with pdb. Write which tool felt clearest and why.

Finish by adding a Markdown cell that answers: What did this example teach me that I can reuse in a project?

Key points

• Learn the concept by running a complete, small example first.
• Predict before execution so your thinking becomes visible.
• Change one thing at a time so cause and effect stay clear.
• Treat errors as clues about the exact line or value Python could not handle.

Why this matters

Use print debugging, VS Code breakpoints, and pdb to inspect running programs.

This lesson combines related subtopics that belong together in one learning conversation. You will still pause for a quiz after each section, but you do not need to jump between separate pages while building one clear explanation.

NoteGuiding questions

By the end of this lesson, you should be able to answer:

• How do the sections in Debugging Tools fit together?
• Which small example demonstrates each section?
• Which debugging clue should I check first for each section?

NoteLearning objectives

You will practice how to:

• explain the shared concept for this lesson;
• use each section as one step in a larger workflow;
• complete 3 short section quizzes before moving on;
• connect examples, mistakes, and debugging routines.

Lesson map

• 1. Print Debugging — Inspect variable values and types with small temporary print statements.
• 2. Breakpoints in VS Code — Pause code in VS Code and inspect values step by step.
• 3. pdb Basics — Use Python’s built-in debugger from the terminal.

1. Print Debugging

Inspect variable values and types with small temporary print statements.

TipAnalogy

Print statements are sticky notes along a route: they show what happened at each checkpoint.

What this means

Print debugging adds temporary observations so you can see what the program knows at a point in time.

Example 1

Predict what will happen before you run the code.

price = "10"
print("price:", price, type(price))
total = int(price) * 2
print(total)

Step-by-step explanation

1. price = "10" — pause here and say what this line reads, creates, changes, or displays.
2. print("price:", price, type(price)) — pause here and say what this line reads, creates, changes, or displays.
3. total = int(price) * 2 — pause here and say what this line reads, creates, changes, or displays.
4. print(total) — pause here and say what this line reads, creates, changes, or displays.

After running the example, compare the actual output with your prediction. If they differ, do not erase your prediction. The difference is the part that can teach you the most.

Challenge

NotePractice

Change one input value, predict the new output, run the code, and explain the difference in one sentence.

Show one possible solution path

1. Copy Example 1 into Colab, Jupyter, or a .py file.
2. Mark the line you plan to change.
3. Write a one-sentence prediction.
4. Run the changed code.
5. If the result surprises you, restore the original and change a smaller part.

The goal is not to find the only correct answer. The goal is to create a small experiment where you can explain cause and effect.

Common mistakes

WarningCommon mistake

Too many print statements create noise. Print the smallest useful value, then remove or replace with logging later.

When you get stuck, use this debugging routine:

1. Read the last line of the error message or inspect the unexpected output.
2. Find the smallest line of code that could be responsible.
3. Print or inspect the value and type at that point.
4. Change one thing.
5. Run again and record what changed.

Check your understanding

This quiz checks the ideas in this section before you move on.

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2. Breakpoints in VS Code

Pause code in VS Code and inspect values step by step.

TipAnalogy

A breakpoint is a pause button on a movie: stop at the interesting scene and look around before continuing.

What this means

A breakpoint tells the debugger to pause before a chosen line so you can inspect program state.

Example 2

Predict what will happen before you run the code.

def total(prices):
    result = 0
    for price in prices:
        result += price
    return result

print(total([2, 3, 4]))

Step-by-step explanation

1. def total(prices): — pause here and say what this line reads, creates, changes, or displays.
2. result = 0 — pause here and say what this line reads, creates, changes, or displays.
3. for price in prices: — pause here and say what this line reads, creates, changes, or displays.
4. result += price — pause here and say what this line reads, creates, changes, or displays.
5. return result — pause here and say what this line reads, creates, changes, or displays.
6. Continue the same process for the remaining lines, one line at a time.

After running the example, compare the actual output with your prediction. If they differ, do not erase your prediction. The difference is the part that can teach you the most.

Challenge

NotePractice

Change one input value, predict the new output, run the code, and explain the difference in one sentence.

Show one possible solution path

1. Copy Example 1 into Colab, Jupyter, or a .py file.
2. Mark the line you plan to change.
3. Write a one-sentence prediction.
4. Run the changed code.
5. If the result surprises you, restore the original and change a smaller part.

The goal is not to find the only correct answer. The goal is to create a small experiment where you can explain cause and effect.

Common mistakes

WarningCommon mistake

Place the breakpoint before the suspicious line, not after all important values have already changed.

When you get stuck, use this debugging routine:

1. Read the last line of the error message or inspect the unexpected output.
2. Find the smallest line of code that could be responsible.
3. Print or inspect the value and type at that point.
4. Change one thing.
5. Run again and record what changed.

Check your understanding

This quiz checks the ideas in this section before you move on.

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3. pdb Basics

Use Python’s built-in debugger from the terminal.

TipAnalogy

pdb is a flashlight for terminal programs: small, built in, and useful when the room is dark.

What this means

pdb lets you pause and inspect Python programs without a graphical editor.

Example 3

Predict what will happen before you run the code.

python -m pdb script.py
# commands to try inside pdb: n, s, p variable, c, q

Step-by-step explanation

1. python -m pdb script.py — pause here and say what this line reads, creates, changes, or displays.
2. # commands to try inside pdb: n, s, p variable, c, q — pause here and say what this line reads, creates, changes, or displays.

After running the example, compare the actual output with your prediction. If they differ, do not erase your prediction. The difference is the part that can teach you the most.

Challenge

NotePractice

Change one input value, predict the new output, run the code, and explain the difference in one sentence.

Show one possible solution path

1. Copy Example 1 into Colab, Jupyter, or a .py file.
2. Mark the line you plan to change.
3. Write a one-sentence prediction.
4. Run the changed code.
5. If the result surprises you, restore the original and change a smaller part.

The goal is not to find the only correct answer. The goal is to create a small experiment where you can explain cause and effect.

Common mistakes

WarningCommon mistake

pdb commands are typed at the debugger prompt, not inside your Python file unless you deliberately add a breakpoint().

When you get stuck, use this debugging routine:

1. Read the last line of the error message or inspect the unexpected output.
2. Find the smallest line of code that could be responsible.
3. Print or inspect the value and type at that point.
4. Change one thing.
5. Run again and record what changed.

Check your understanding

This quiz checks the ideas in this section before you move on.

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Notebook and Colab practice

Open a blank notebook at https://colab.new. Use one section at a time: copy the Example 1, predict the result, run it, answer the section quiz, and then move to the next section. This is better than copying the entire page at once.

Instructor note

Teaching notes

• Treat each section as a short teaching episode.
• Pause for the section quiz before introducing the next section.
• Ask learners to compare sections: what stayed the same, and what changed?
• If time is short, teach the first two sections live and assign the rest as practice.

Key points

TipKey points

• Print Debugging: Print debugging adds temporary observations so you can see what the program knows at a point in time.
• Breakpoints in VS Code: A breakpoint tells the debugger to pause before a chosen line so you can inspect program state.
• pdb Basics: pdb lets you pause and inspect Python programs without a graphical editor.
• Use the section quizzes as gates: review before moving on if a quiz feels uncertain.

References

• Python errors tutorial: https://docs.python.org/3/tutorial/errors.html
• Python Tutorial: https://docs.python.org/3/tutorial/
• Quarto OJS documentation: https://quarto.org/docs/interactive/ojs/
• ipywidgets documentation: https://ipywidgets.readthedocs.io/en/stable/
• VS Code Python tutorial: https://code.visualstudio.com/docs/python/python-tutorial
• pdb documentation: https://docs.python.org/3/library/pdb.html