3.2 · Variables & Operators

Goal: declare variables, use the right operator for the job, and follow naming conventions.

Variables

A variable is a named storage for a value.

age = 17
name = "Alice"
height = 1.65
is_student = True

Python infers the type — no explicit declaration needed.

Naming rules

• Letters, digits, underscores; must start with a letter or _.
• Case-sensitive: Age ≠ age.
• Don't use Python keywords (if, for, class…).
• Convention: snake_case for variables and functions, UPPER_CASE for constants.

Good:

average_height = 165
PI = 3.14159

Bad:

a = 165          # what does 'a' mean?
PiValue = 3.14   # mixed conventions

Operators reference

Arithmetic

Operator	Meaning	Example	Result
+	Add	3+2	5
-	Subtract	3-2	1
*	Multiply	3*2	6
/	True division	5/2	2.5
//	Floor division	5//2	2
%	Modulus	5%2	1
**	Power	2**10	1024

Comparison (relational)

Operator	Meaning
==	Equal to
!=	Not equal to
> <	Greater / less
>= <=	Greater or equal / less or equal

Python vs HKEAA pseudocode

HKEAA pseudocode uses = for comparison and ← for assignment. Python uses == for comparison and = for assignment. Don't mix them in your answers.

Boolean (logical)

Operator	Meaning
and	True if both operands True
or	True if at least one True
not	Reverses Boolean

Assignment shortcuts

Shortcut	Means
x += 5	x = x + 5
x -= 3	x = x - 3
x *= 2	x = x * 2

Operator precedence (high → low)

1. **
2. * / // %
3. + -
4. Comparisons < <= > >= == !=
5. not
6. and
7. or

Use parentheses when in doubt.

Worked examples

total = 0
for i in range(1, 11):
    total += i              # 1+2+...+10
print(total)                # 55

a, b = 7, 3
print(a // b, a % b)        # 2 1   (quotient, remainder)

x = 5
print(x ** 2)               # 25
print((x + 3) * 2)          # 16

Common student mistakes

• Confusing = (assignment) with == (comparison).
• Forgetting that 5 / 2 = 2.5 (Python 3 true division); use // for integer division.
• Using & and | (bitwise) instead of and / or (Boolean).
• Reusing variable names that shadow built-ins (list, sum, str).

Exam-style question

Q (4 marks): Trace the value of result after each line:

x = 7
y = 3
result = (x % y) * 2 + (x // y) ** 2

Sample answer:

• x % y = 1
• (1) * 2 = 2
• x // y = 2
• 2 ** 2 = 4
• result = 2 + 4 = 6

Key takeaways

• Names matter — pick meaningful ones.
• Know your operators and their precedence.
• Python's = is assignment; == is comparison.

➡️ Next: 3.3 Input / Output