Python Variables

Variables & Dynamic Typing

Variables act as references assigned to specific data objects in memory.
They store data that can be referenced, modified, and manipulated during program execution.
Unlike statically typed languages, Python variables do not require explicit type declaration.
The data type is dynamically inferred by Python based on the assigned value.

x = 5
name = "Alex"  
print(x)
print(name)

5
Alex

Rules for Naming Variables

Names can contain alphanumeric characters and underscores (_).
The first character cannot be a digit.
Names are case-sensitive (e.g., myVar and myvar point to different objects).
Reserved keywords (such as if, else, for) cannot be used as identifiers.

Valid Identifiers:

age = 21
_colour = "lilac"
total_score = 90

Assigning Values & Object Reference

Basic Assignment: Values are bound to variable names using the assignment operator =.
Shared Reference: Executing y = x binds y to the same memory object referenced by x.
Garbage Collection: Objects with a reference count of zero are automatically reclaimed by Python's memory manager.

Try it yourself

Experiment with variables and dynamic typing:

x = 10
y = x
x = 20
print(y)  # y still refers to the original object

Variables Assessment

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Variables & Dynamic Typing

Rules for Naming Variables

Assigning Values & Object Reference

Python Operators

Operators are special symbols or keywords used to perform specific mathematical, relational, or logical computations on values and variables.

Arithmetic Operators

Used to perform standard mathematical calculations.

+ (Addition): Adds values on either side of the operator.
- (Subtraction): Subtracts right operand from left operand.
* (Multiplication): Multiplies values.
/ (Division): Divides left operand by right operand (returns a float).
// (Floor Division): Division that results in a whole number adjusted down to the nearest integer.
% (Modulus): Divides left operand by right operand and returns the remainder.
** (Exponent): Performs exponential (power) calculation on operators.

print(10 / 3)   # Output: 3.3333333333333335
print(10 // 3)  # Output: 3
print(2 ** 3)   # Output: 8

Comparison (Relational) Operators

Used to compare values, yielding a boolean outcome (True or False).

== (Equal to)
!= (Not equal to)
> (Greater than)
< (Less than)
>= (Greater than or equal to)
<= (Less than or equal to)

Logical Operators

Used to combine conditional evaluations.

and: Returns True if both operands are true.
or: Returns True if at least one operand is true.
not: Reverses the boolean state of its operand.

x = 5
print(x > 3 and x < 10)  # Output: True

Try it yourself

Experiment with operators:

a = 7
b = 2
print(a + b)   # 9
print(a // b)  # 3
print(a % b)   # 1
print(a ** b)  # 49

Operators Assessment

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Arithmetic Operators

Comparison (Relational) Operators

Logical Operators

Python Control Flow

Control flow lets your program make decisions and run different code based on conditions.

Boolean Expressions

Comparison operators compare values and produce a boolean result (True or False).

== (equal to), != (not equal to)
> (greater than), < (less than)
>= (greater than or equal to), <= (less than or equal to)

age = 18
print(age >= 18)   # Output: True
print(age == 21)   # Output: False

True
False

Logical Operators

and: True only if both operands are true.
or: True if at least one operand is true.
not: Reverses the boolean value.

score = 85
print(score > 50 and score < 100)  # Output: True
print(not score > 90)              # Output: True

if / elif / else

Python evaluates conditions top to bottom and runs the first block whose condition is true.

temperature = 30
if temperature > 35:
    print("Very hot")
elif temperature > 20:
    print("Warm")
else:
    print("Cool")

Warm

Indentation & Code Blocks

Python uses indentation (spaces or tabs) to define blocks — there are no braces.
All statements in a block must be indented consistently (usually 4 spaces).
Inconsistent indentation raises an IndentationError.

Nested Conditionals

You can place an if inside another if to check multiple levels of conditions.

age = 25
has_id = True
if age >= 18:
    if has_id:
        print("Entry allowed")
    else:
        print("ID required")

Entry allowed

Truthiness

In conditions, some values are treated as False (falsy) and most others as True (truthy).

Falsy: 0, 0.0, "", [], None, False.
Truthy: any non-zero number, non-empty string/list, and True.

name = ""
if name:
    print("Hello, " + name)
else:
    print("No name given")

No name given

Try it yourself

Experiment with conditionals:

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"
print(grade)

Control Flow Assessment

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Boolean Expressions

Logical Operators

if / elif / else

Indentation & Code Blocks

Nested Conditionals

Truthiness

Python Loops

Loops let you repeat a block of code many times, which is essential for working with collections and repetitive tasks.

for loops

A for loop iterates over each item in a sequence (a range, list, or string).

for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

apple
banana
cherry

range()

range() produces a sequence of numbers to loop over.

range(stop): 0 to stop-1.
range(start, stop): start to stop-1.
range(start, stop, step): with a custom step.

for i in range(3):
    print(i)          # 0 1 2

for i in range(1, 6, 2):
    print(i)          # 1 3 5

while loops

A while loop repeats as long as its condition is true. Be careful to update the condition so it eventually becomes false.

count = 0
while count < 3:
    print(count)
    count += 1

0
1
2

break / continue / pass

break: exits the loop immediately.
continue: skips the rest of the current iteration and moves to the next.
pass: does nothing (a placeholder for an empty block).

for i in range(10):
    if i == 3:
        continue   # skip 3
    if i == 6:
        break      # stop at 6
    print(i)       # 0 1 2 4 5

Nested loops

You can place a loop inside another loop. The inner loop runs fully for each iteration of the outer loop.

for row in range(2):
    for col in range(3):
        print(row, col)

0 0
0 1
0 2
1 0
1 1
1 2

Common loop patterns

Accumulation: summing values with total += x.
Counting: incrementing a counter inside the loop.
Indexed iteration: using enumerate() to get both index and value.

total = 0
for n in [2, 4, 6]:
    total += n
print(total)   # Output: 12

for idx, val in enumerate(["a", "b"]):
    print(idx, val)   # 0 a / 1 b

Try it yourself

Experiment with loops:

total = 0
for n in range(1, 6):
    total += n
print(total)  # 15

for i in range(3):
    print(i)

Loops Assessment

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