Challenge — Code Wars · Predict the Output

By the end of this lesson you can:

• Trace through a short Python snippet on paper, line by line, tracking every variable's value.
• Predict the output of common Level 1 patterns: loops, list mutations, function returns, string slicing, boolean conditions.
• Spot the moments where Python's rules trip up your intuition — empty list as False, integer division differences, list aliasing.

Open your notebook or a blank text file as a scoring sheet. For each snippet:

1. Read it — slowly, line by line.
2. Write your prediction on paper or the scoring sheet. Be specific — exact text, exact order, with quotes/brackets if they appear.
3. Run the snippet and compare.
4. Score yourself: ✓ for an exact match, ✗ for any mismatch.

Aim for 6/8 to call it a good day. 7/8 means you're reading Python like a native. 8/8 — start helping your classmates.

Snippet 1 · for-loop basics

for i in range(3):
    print(i, "Mango")

0 Mango
1 Mango
2 Mango

Snippet 2 · slice with negative end

word = "Selamat"
print(word[1:-1])

elama

Snippet 3 · list mutation in a loop

nums = [1, 2, 3]
total = 0
for n in nums:
    total = total + n
    nums.append(n)        # mutates the list we're iterating!

print(total)
print(nums)

(infinite loop — never reaches the print)

Snippet 4 · boolean short-circuit

x = 5
y = 0

if x > 0 and y > 0:
    print("both")
elif x > 0 or y > 0:
    print("either")
else:
    print("neither")

either

Snippet 5 · function return capture

def shout(name):
    print(name.upper())

result = shout("Aisyah")
print("Result is", result)

AISYAH
Result is None

Snippet 6 · empty list is falsy

bag = []

if bag:
    print("Heavy.")
else:
    print("Empty.")

bag.append("torch")

if bag:
    print("Heavy.")
else:
    print("Empty.")

Empty.
Heavy.

Snippet 7 · integer math vs float math

print(7 / 2)
print(7 // 2)
print(7 % 2)
print(7 ** 2)

3.5
3
1
49

Snippet 8 · list aliasing trap

a = [1, 2, 3]
b = a
c = list(a)

a.append(4)

print(a)
print(b)
print(c)

[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3]

1. Trace, don't skim

For every snippet, write down the value of every variable after every line. It feels slow at first — three minutes per snippet — but speeds up dramatically with practice. After 50 such traces, you do it in your head in seconds.

2. Slow down at boundaries

Bugs live at the edges: the first and last iterations of loops, empty inputs, off-by-one indices, equality checks (> vs >=). When tracing, always run the snippet in your head on the smallest possible input first — range(0), [], 0. If the snippet survives the edge cases, the middle is usually fine.

3. Trust the printer

Many wrong predictions come from imagining what the programmer meant instead of what they wrote. name.upper() with no print outputs nothing — it computes a value and throws it away. Predict literally what the code says, not what feels right.

Make a tally on your scoring sheet. For every snippet you got wrong, note which of the eight bug families from PY-L1-42 was hiding there. Common mappings:

• Snippet 1, 7 → off-by-one or range/divide trap (memorise edge rules)
• Snippet 2 → string slicing rules (re-read PY-L1-24)
• Snippet 3, 8 → list aliasing/mutation traps (re-read PY-L1-31, PY-L1-40)
• Snippet 4, 6 → boolean logic gotchas (re-read PY-L1-09, PY-L1-10)
• Snippet 5 → print vs return (re-read PY-L1-29)

Pick the one you missed most badly. Open the referenced lesson tonight and re-read the worked example.

Predicting output is reading code with conviction. The same skill that makes you a good debugger makes you a good code reviewer, and eventually a good architect. Today's eight snippets cherry-picked the Level 1 ideas that most often trip beginners up: range boundaries, slice semantics, the =/== distinction, the immutability of strings, the aliasing of lists, the truthiness of containers, the difference between / and //, and the silent None from forgotten returns. Tomorrow is the speed round — same kind of snippets, but you're asked to write them yourself, against the clock.