Challenge — Code Wars · Bug Hunt

By the end of this lesson you can:

• Read a short Python script and predict what it will print (or fail with) before running it.
• Spot common bug families: missing colon, = vs ==, off-by-one, None from missing return, wrong loop boundary.
• Fix a bug while explaining why the original was wrong — not just what made it work.

Five buggy scripts ahead. For each one:

1. Read it. Don't run it yet. Make a mental prediction — what will it print, or how will it crash?
2. Run it. Compare the actual output to your prediction. Where were you wrong?
3. Fix it. Make the smallest change that makes the script do what its comment claims it should do.
4. Explain in one sentence why the bug was a bug. Type the sentence as a comment under your fix.

Open bug_hunt.py in your editor. Copy each broken bug below into the file with the bug name as a header (# ===== Bug 1 =====). Fix in place. The whole exercise should take about 35-40 minutes.

Bug 1 · The countdown that doesn't

This is supposed to count down from 5 to 1, then print "Go!":

# Should print 5, 4, 3, 2, 1, Go!

n = 5
while n > 0
    print(n)
    n = n + 1
print("Go!")

Predict: what happens when you run it? Then fix.

n = 5
while n > 0:           # missing colon
    print(n)
    n = n - 1          # was: n + 1 (infinite loop!)
print("Go!")
# Bug: missing colon = SyntaxError; n + 1 = counts UP forever.

Bug 2 · The function that does nothing

# Should print "HELLO" for each name.

def shout(name):
    name.upper()

shout("Aisyah")
shout("Faiz")

def shout(name):
    print(name.upper())     # was: name.upper() with no print

shout("Aisyah")
shout("Faiz")
# Bug: .upper() returns a new string but doesn't change name —
# we have to print (or assign) the result.

Bug 3 · The always-true grade

# Should print "Pass" for score >= 50, "Fail" otherwise.

score = 30

if score = 50:
    print("Pass")
else:
    print("Fail")

score = 30

if score >= 50:                # was: score = 50 (assignment!)
    print("Pass")
else:
    print("Fail")
# Bug: = is assignment, == is equality, >= is the boundary check we want.

Bug 4 · The list mystery

# Should print [1, 2, 3] then [1, 2, 3, 4] — independent lists.

a = [1, 2, 3]
b = a
b.append(4)

print(a)
print(b)

a = [1, 2, 3]
b = list(a)        # was: b = a (same list, not a copy!)
b.append(4)

print(a)           # [1, 2, 3]
print(b)           # [1, 2, 3, 4]
# Bug: b = a aliases the SAME list. Use list(a) or a[:] to copy.

Bug 5 · The teacher's broken average

# Should print the average of a list of scores.

def average(nums):
    total = 0
    for n in nums:
        total = total + n
        return total / len(nums)

print(average([10, 20, 30]))     # expect 20.0

def average(nums):
    total = 0
    for n in nums:
        total = total + n
    return total / len(nums)     # un-indent: must be AFTER the loop
# Bug: return was inside the loop, so it fired on the first item.
# It returned 10/3 instead of 60/3.

print(average([10, 20, 30]))     # now correctly 20.0

for n in [-2, 3, -5, 7, 0]:
    if n > 0:
        print(n)
    else:
        continue
        print("skipped a non-positive")     # what's wrong?

Three habits that turn anyone into a fast bug-finder. Use these on the next two Code Wars rounds too.

1. Read the comment, then check the code matches it

Every script has a comment at the top saying what it's supposed to do. Most bugs are obvious the moment you compare the code to its stated purpose — "the comment says count down, the code counts up". Don't skip the comment.

2. Track every variable's value on the first pass

Walk through the first iteration of any loop with pencil-and-paper. Write down what every variable holds after each line. Most bugs are off by exactly one — and an off-by-one is invisible if you only ever read the variable names. Watch the values.

3. Trust nothing about indentation

Python uses indentation to mean structure. A statement two spaces too far in or out lives in a different code block entirely. Whenever a function or loop misbehaves, the first thing to check is whether every line is at the right indent level.

You've fixed all five (or four — be honest). Before moving on, answer in your head:

• For each bug, can you name which of the eight families above it belongs to?
• For each bug, did you write a one-line comment explaining why it was wrong?
• Could you write a brand-new buggy version of each bug — and would a classmate be able to fix it?

Yes? Move on. No? Re-read the worked example bug you struggled with and re-write the fix from scratch.

Bug hunting is a skill, not a vibe. The same eight families turn up again and again — once you know their shapes you start spotting them before you even read the rest of the line. Today you fixed five (and maybe six or seven), but more importantly you started reading code with bug-hunter eyes: comparing to the comment, tracking values, checking indents. The next two Code Wars lessons sharpen the same skill from different angles — predicting output without running, then writing solutions against the clock. Bring the file you fixed today; we'll start tomorrow's lesson with a quick "explain your worst bug" round.