Print debugging done well
Read the labelled lines, find where the data shrinks, then fix the condition (use v is not None) and run again.
Debugging is finding the first place where what your program holds differs from what you believed. You can do it with well-labelled prints, with breakpoint() and the pdb debugger, with your editor's debugger, and with a systematic method such as bisection.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 7 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write a function first_negative(numbers) that returns the first negative number, or None if there is none. Call it with [3, 8, -2, 5] and print the result with print(f"{result=}"). It should print result=-2.
The function def total(items): return sum(item["price"] * item["qty"] for item in items) fails with KeyError: 'qty' on one item. Take a list of three dictionaries where one has no "qty" key. Write a loop that prints the position (counting from 0) of each item that is missing qty. Hint: enumerate gives you the position.
Using the bisect idea, find the first whole number from 1 to 1000 whose square is at least 50000. Count the checks as you go, and print the number and the count on one line. Hint: copy the while low < high loop and change the test to mid * mid >= 50000.
Explain it without notes
Describe a systematic way to debug a program that gives a wrong answer.
What does breakpoint() do, and what are the basic pdb commands?
Why is the line shown in a traceback not always where the bug is?
What is bisection and how does it apply to debugging?
Expected output
data=[5, 0, 7, None, 3]
cleaned=[5, 7, 3]
len(data)=5 len(cleaned)=3
lost=[0, None]