Variables, types and operations

A program is a sequence of instructions that the computer executes in order, from top to bottom, like a cooking recipe. In this course we use Python, a readable and widely used language, but the ideas apply to every language.

Variables: labels on values

A variable is a name that refers to a value kept in memory. You create it with the = sign, which means “store this value under this name” (it is an assignment, not a mathematical equality).

prix = 12.5          # a decimal number
quantite = 3         # an integer
produit = "Cahier"   # text, between quotes
en_stock = True      # a boolean: True or False

quantite = quantite + 1   # we replace the old value: quantite is now 4

Choose meaningful names, in lowercase, with _ between words (prix_total rather than x). A name cannot start with a digit or contain a space.

Basic types

Every value has a type, which determines what you can do with it:

Type	Example	Usage
int	42	whole numbers
float	3.14	decimal numbers (a dot, not a comma)
str	"Bonjour"	text (string of characters)
bool	True	true / false

The type() function lets you check: type(prix) returns <class 'float'>.

Operations

a = 17
b = 5
print(a + b)    # 22  addition
print(a - b)    # 12  subtraction
print(a * b)    # 85  multiplication
print(a / b)    # 3.4 decimal division (always a float)
print(a // b)   # 3   integer division (quotient)
print(a % b)    # 2   modulo (remainder of the division)
print(a ** 2)   # 289 power

The modulo % is very useful: n % 2 == 0 tells you a number is even. Precedence follows the rules of math: 2 + 3 * 4 equals 14, and parentheses change the order.

Displaying a result

print() displays a value. To mix text and variables, the f-string is the clearest method:

total = prix * quantite
print(f"{quantite} x {produit} = {total} euros")   # 4 x Cahier = 50.0 euros
print(f"Rounded total: {total:.2f}")               # 2 decimal places

Watch out for types

With text, + glues strings together instead of adding: "3" + "4" gives "34". And "3" + 4 raises a TypeError. User input (input()) is always text: you must convert it with int() or float() before calculating.

age_texte = "30"
age = int(age_texte)    # conversion: 30 (int)
print(age + 1)          # 31

Key takeaway: name your variables well, check their type, and convert when necessary.