The 15 Most Common Python Mistakes in HKDSE ICT

Learn From Others' Mistakes

We analysed thousands of HKDSE ICT scripts. These 15 mistakes appeared most often — and each cost students 1 to 3 marks. Know them. Avoid them.

1. Forgetting to Cast input()

n = input("Enter n: ")
print(n * 2)   # "55" instead of 10 if user enters 5

Fix: n = int(input(...))

2. Using = Instead of ==

if x = 5:  # SyntaxError
if x == 5:  # ✓

3. Indentation Inconsistency

Mixing tabs and spaces causes invisible errors. Use 4 spaces consistently.

4. Off-by-One in range()

# Want 1 to 10 inclusive
range(1, 10)    # Only 1 to 9 ✗
range(1, 11)    # 1 to 10 ✓

5. Modifying a List While Iterating

lst = [1,2,3,4,5]
for x in lst:
    if x % 2 == 0:
        lst.remove(x)   # ✗ Skips elements

Fix: iterate over lst[:] (a copy) or use list comprehension.

6. Confusing append() and extend()

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

7. Integer vs Float Division

print(10 / 3)    # 3.3333...
print(10 // 3)   # 3 (integer division)

8. Returning Inside a Loop Too Early

def find_max(lst):
    for x in lst:
        return x   # ✗ Returns first element only
# Correct logic needs max comparison

9. Not Handling Empty Input

scores = []
print(sum(scores)/len(scores))  # ZeroDivisionError

if len(scores) > 0:
    print(sum(scores)/len(scores))

10. String vs List Confusion

s = "hello"
s[0] = "H"   # ✗ strings are immutable

s = list("hello")
s[0] = "H"
"".join(s)   # "Hello"

11. Missing return Statement

def double(x):
    x * 2   # ✗ Does nothing — no return

def double(x):
    return x * 2   # ✓

12. Global vs Local Variables

count = 0
def increment():
    count += 1   # ✗ UnboundLocalError

def increment():
    global count
    count += 1   # ✓

13. File Not Closed

f = open('data.txt')
# ... use f ...
# forgot f.close() — loses a mark

# Better:
with open('data.txt') as f:
    ...

14. Incorrect String Comparison

grade = input("Grade: ")
if grade == "A":  # ✓
if grade is "A":  # ✗ Works sometimes, unreliable

15. Forgetting .strip() When Reading Files

with open('data.txt') as f:
    for line in f:
        if line == "END":   # ✗ line is "END\n"
            break
        # Fix:
        if line.strip() == "END":  # ✓
            break