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: [4, 8, 12, 16, 20]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 definedRead from the bottom up:
- The error type and message (
NameError: name 'total' is not defined) - The line number (
line 5) - 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.
- Once the crashes are fixed, compare the function's condition with its name:
double_evensshould select even numbers.