Lesson 01 · Variables and data types¶

What is a variable?¶

A variable is a labeled box in which you store a value. The label is the name of the variable, and the contents of the box is its value.

age = 14
name = "Andrew"
is_honor_student = True

Every time you write age, Python gives you the value 14.

Creating a variable¶

# syntax: variable_name = value
score = 100
city = "Bucharest"
temperature = 36.6

The = sign is the assignment operator — it doesn't mean "equal", but "store the value on the right into the variable on the left".

score = 100
print(score)   # 100
score = 250
print(score)   # 250

The 4 basic types¶

1. int — whole numbers¶

age = 14
student_count = 32
min_temperature = -5

No decimal point, no quotes.

2. float — decimal numbers¶

height = 1.75
grade = 9.5
pi = 3.14159

3. str — strings of characters (text)¶

first_name = "Anna"
message = 'Welcome!'
sentence = "I am 14 years old and I live in Cluj."

Single '...' and double "..." quotes work the same.

4. bool — logical values¶

is_adult = False
got_high_grade = True

Only two possible values: True or False (capital letter is mandatory!).

The type() function¶

print(type(14))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("hello"))   # <class 'str'>
print(type(True))      # <class 'bool'>

Naming rules¶

Writing conventions¶

# snake_case — recommended for variables
number_of_students = 32
max_temperature = 37.5

# UPPERCASE — for constants (values that don't change)
PI = 3.14159
SPEED_OF_LIGHT = 299792458

Displaying variables¶

name = "Helen"
age = 15

print(name)           # Helen
print(age)            # 15
print(name, age)      # Helen 15

Exercises¶

Exercise 1 — Define variables¶

Create variables for: your name (str), your age (int), your math grade average (float), and whether you are in 9^th grade (bool).

name = "Michael"
age = 15
math_average = 9.25
is_grade_9 = True

Exercise 2 — Which type?¶

What type is each value?

x = 42
y = "42"
z = 42.0
w = False

Exercise 3 — Track the value¶

What does the following program display?

points = 10
points = points + 5
points = points * 2
print(points)

30

Exercise 4 — Find the errors¶

What's wrong?

student count = 30
2score = 100
if = "word"

Mini-project: Personal record card¶

Create variables for a person and display them in a formatted way.

Expected output:

=== Personal record card ===
Name: Robert Popescu
Age: 13
Grade: 7
Overall average: 9.8
Is honor student: True

name = "Robert Popescu"
age = 13
grade = 7
average = 9.8
is_honor_student = True

print("=== Personal record card ===")
print("Name:", name)
print("Age:", age)
print("Grade:", grade)
print("Overall average:", average)
print("Is honor student:", is_honor_student)

Summary¶

• A variable stores a value under a name you choose
• Basic types: int, float, str, bool
• type(x) tells you the type of value x
• Use snake_case for naming variables

Next step: → Lesson 02: Operations and expressions