Bug detective: finding bugs

What you'll learn

Read error messages from the bottom up
Find bugs that don't produce an error message
Look inside a program with print() and “Steps”

Bugs are part of the game

A mistake in a program is called a bug. The name supposedly goes back to 1947, when a real moth got stuck inside a computer. Since then, finding mistakes has been called debugging.

Important: everyone makes mistakes, professionals too, every day. Good programmers aren’t the ones who never make mistakes – they’re the ones who find them quickly.

Clue 1: the error message

When Python stops, the message is your most important clue. Read it from the bottom up:

prices = [3, 5, 2]
total = sum(prices)
print("Total: " + total)

The last line says what happened: TypeError – text and a number don’t fit together.
Above it you can see which line it happened in.
Codelix marks the line in red and explains the error. Try to fix it: str(total) or a comma instead of +.

Codelix even spots typos: try typing pirnt("Hello") in an editor.

Clue 2: bugs without an error message

Nastier are bugs where the program runs through but produces the wrong thing:

# Should count how many numbers are bigger than 10
numbers = [5, 12, 30, 8, 11]
count = 0
for number in numbers:
    if number > 10:
        count = 1
print(count)

The only thing that helps here: think beforehand about what should come out (here: 3), then compare. Can you spot the bug?

Clue 3: look inside the program

If you don’t know what’s going on, let the program show you:

print() at suspicious places: print("number:", number, "count:", count) inside the loop shows every step.
The ⏯ Steps button in the editor plays the program line by line and shows all variables.

total = 0
for i in range(1, 5):
    total = total + i
    print("i =", i, "total =", total)

Detective rule: change only one thing at a time, then run it again. If you change five things at once, you won’t know what helped.

Quick check

What does this program print? And trickier: for which list would it give a wrong result?

values = [4, 7, 1]
biggest = 0
for v in values:
    if v > biggest:
        biggest = v
print(biggest)

Your task

Your turn

This program has three bugs. average(marks) should work out the average of a list of marks, and best_mark(marks) should find the best mark – where (as with German school marks) the smallest number is the best. Run the program, read the error message and fix one bug after another.

def average(marks):
    total = 0
    for mark in marks:
        total = mark
    return total / len(mark)


def best_mark(marks):
    best = marks[0]
    for mark in marks:
        if mark > best:
            best = mark
    return best


marks = [2, 1, 3]
print("Average:", average(marks))
print("Best mark:", best_mark(marks))

Something to think about
Read the last line of the error message. len() needs the whole list – is it really called mark?
Which command helps?
The total should grow in the loop. total = mark overwrites it every time – write total = total + mark.
Here the smaller number is the better mark. Which comparison sign do you need then?

Gap code – fill in the ___

                        def average(marks):
    total = 0
    for mark in marks:
        total = ___
    return ___


def best_mark(marks):
    best = marks[0]
    for mark in marks:
        if ___:
            best = mark
    return best


marks = [2, 1, 3]
print("Average:", average(marks))
print("Best mark:", best_mark(marks))

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

def average(marks):
    total = 0
    for mark in marks:
        total = total + mark
    return total / len(marks)


def best_mark(marks):
    best = marks[0]
    for mark in marks:
        if mark < best:
            best = mark
    return best


marks = [2, 1, 3]
print("Average:", average(marks))
print("Best mark:", best_mark(marks))