Debugging Basics

Bugs are a normal part of programming. The skill isn't avoiding them — it's finding and fixing them efficiently. Python gives you helpful error messages if you know how to read them.

What you'll learn

• How to read a Python traceback (error message)
• Common error types and what they mean
• Using print() to trace what your code is doing

Instructions

The starter code has 4 intentional bugs. Your job is to find and fix all of them so the program runs and prints:

All bugs fixed!
Result: [2, 4, 6, 8, 10]

Run the code first — read the error message carefully. Fix one bug at a time, re-running after each fix.

Key concepts

Reading a traceback:

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    print(total)
NameError: name 'total' is not defined

Read from the bottom up:

1. The error type and message (NameError: name 'total' is not defined)
2. The line number (line 5)
3. The code that caused it (print(total))

Common errors:

| Error | What it means | |-------|--------------| | SyntaxError | Python can't parse your code (typo, missing colon, bad indentation) | | NameError | You used a variable that doesn't exist | | TypeError | You used the wrong type (e.g., adding a string and an int) | | IndexError | You tried to access a list index that's out of range |

Print debugging: When the error isn't obvious, add print() calls to see what values your variables hold at each step.

Hints

• Fix one error at a time and re-run.
• The first error Python shows you might hide others — fix it and run again.
• Read the error type first, then the line number. That tells you exactly where to look.
• Compare what you expect a variable to be vs what it actually is.