Variables # Jump to this section

Variables let you store values (like numbers or text) so your program can use them later.

Basic Idea # Jump to this section

• Assign a value to a variable using =.
• Use the variable name to use the value later.

Example:

message = "Hello, world!"
print(message)

Output: Hello, world!

Python reads the code top to bottom, one line at a time.

Naming Rules # Jump to this section

Allowed

• user_name
• age1
• total_score

Not allowed

• 1name (cannot start with a number)
• user name (no spaces)
• if, for, class (reserved keywords)

Variable names are case-sensitive: age and Age are different.

Common Data Types # Jump to this section

Variables can store different kinds of information. Here are the most common:

• Integers (whole numbers): x = 10
• Floats (decimal numbers): pi = 3.14
• Strings (text): name = "John"
• Booleans (True/False): is_available = True

Make sure to always capitalize the first letter of a boolean (use True and False, you will get an error if you use true/false)

age = 21            # integer
country = "Canada"  # string

The hash symbol # starts a comment. All text following the # to the end of the line is a comment and does not get run as code. Comments are used to help explain code.

Reassigning and Updating # Jump to this section

You can change a variable by assigning a new value.

var = "value"
var = "new value"

score = 10
bonus = 5
score = score + bonus        # score is now 15

You can change the type by assigning a different kind of value

value = 5
value = "five"              # now `value` is a string

Changing a variable’s type is allowed, but usually not recommended since it can make programs harder to read/debug.

User Input # Jump to this section

Use input() to prompt the user to enter an input. You will have to type the response in the terminal where the output appears.

name = input("What's your name? ")

print("Your name is " + name)

input() always returns a string, even if the user types a number. Convert it if you need to do any math.

F-Strings # Jump to this section

F-strings are a way to simplify combining text and variables when using print(). Start the string with f and put expressions in {}.

name = "John"
age = 30
print(f"My name is {name} and I am {age} years old")

F-strings avoid manual string conversion and are easier to read.

# without f-strings
print("My name is " + name + " and I am " + str(age) + " years old")

# with f-strings
print(f"My name is {name} and I am {age} years old")

Anything inside {} is evaluated as Python code, so you can include expressions:

print(f"Next year I will be {age + 1}")

You can control how values look by adding a format after a colon : inside the braces.

pi = 3.14159265
print(f"Pi approx: {pi:.2f}")   # round to 2 decimal places

If you need to show braces in the output (for example in math or templates), double them:

print(f"Use false to show braces")  # output: Use { and } to show braces

Math with variables # Jump to this section

You can perform arithmetic using variables that hold numbers (integers and floats). The basic operators are:

• + addition
• - subtraction
• * multiplication
• / division (always returns a float)

There are also more specialized operators:

• // floor division (returns the quotient without the remainder)
• % modulus (remainder after division)
• ** exponentiation (raise to a power)

Examples:

a = 7
b = 3
print(a + b)    # 10
print(a - b)    # 4
print(a * b)    # 21
print(a / b)    # 2.3333333333333335  (float)
print(a // b)   # 2                  (floor division)
print(a % b)    # 1                  (remainder)
print(a ** b)   # 343                (7 to the power of 3)

Order of operations follows normal math rules: parentheses first, then **, then *, /, //, %, then + and -.

result = 2 + 3 * (4 ** 2)  # 2 + 3 * 16 = 50

Notes:

• Strings are not numeric, but you can use + to join them and * to repeat them:

name = "John"
greeting = "Hello, " + name    # greeting is now "Hello, John"
repeat = "ha" * 3                # "hahaha"

• Booleans behave like numbers in arithmetic (True == 1, False == 0) but use them carefully for clarity.

Augmented assignment combines operations to simplify math:

• += add and assign
• -= subtract and assign
• *= multiply and assign

Instead of counter = counter + 1, you can simplify it to counter += 1

counter = 0
counter += 1   # counter is 1

count = 5
count -= 1     

Same as count = count - 1, count is 4

value = 4
value *= 3

Same as value = value * 3, value is 12

Multiple Assignment and Swapping # Jump to this section

Assign multiple variables on one line, or swap values quickly:

x, y = 5, 10

# swap
x, y = y, x

# multiple assignment
first, second, third = "a", "b", "c"

Types and Conversion # Jump to this section

Use type() to check a variable's type. Convert between types with int(), float(), str():

number_string = "42"   # currently, the type is string
number = int(number_string)     # converts to integer '42'
print(type(number))       # output: <class 'int'>

price = 4.5            # currently a float
print(int(price))      # prints 4 (drops decimal part)

Be careful: converting invalid strings raises an error (you can't convert "hello" to an integer).

Constants (convention) # Jump to this section

A constant is a variable whose value cannot be changed. Python has no true constants, but by convention use ALL_CAPS for values you don't plan to change:

PI = 3.14159
MAX_USERS = 100

Challenge # Jump to this section

Write a short program that:

1. Asks the user (using input()) for the width and height of a rectangle.
2. Converts the inputs to numbers.
3. Calculates the area and prints a friendly message like: "The area is 24".

If you run into problems, remember that the error message can contain helpful information.

width = float(input("Width: "))
height = float(input("Height: "))
area = width * height
print("The area is", area)

Key Takeaways # Jump to this section

• Variables store values for later use
• Use = to assign values
• input() gathers user input & returns a string
• Convert types before doing math