Variables and data types in Python

In the previous lessons you already know what Python is, you have your environment set up, and you’ve written your first program with print(). Now it’s time to take the next step: learn to store and manipulate information.

Imagine you’re writing a video game. You need to save the player’s score, their name, whether they have lives remaining… That’s what variables are for: containers where you store information that can change while your program runs.

What is a variable?

A variable is a name you give to a piece of data so you can use it later. Think of it as a labeled box where you store something.

name = "Ana"
age = 25
score = 1500

You just created three variables:

• name contains the text "Ana"
• age contains the number 25
• score contains the number 1500

Creating a variable

In Python, creating a variable is as simple as writing a name, the = sign, and the value you want to store:

message = "Hello, world"

You don’t need to declare the data type (like in other languages). Python is smart enough to know that "Hello, world" is text.

Using a variable

Once created, you can use the variable by writing its name:

name = "Carlos"
print(name)  # Prints: Carlos

age = 30
print(age)   # Prints: 30

You can also use variables inside operations:

price = 50
discount = 10
total = price - discount
print(total)  # Prints: 40

Variables can change

That’s where the name comes from: they’re variable. You can change their value at any time:

counter = 0
print(counter)  # Prints: 0

counter = 1
print(counter)  # Prints: 1

counter = 100
print(counter)  # Prints: 100

Rules for naming variables

Python has some strict rules (and others that are good practices):

Mandatory rules

1. Only letters, numbers, and underscores: You can’t use spaces or special symbols

   full_name = "Ana Garcia"  # ✅ Correct
   full-name = "Ana Garcia"  # ❌ Error
   full name = "Ana Garcia"  # ❌ Error
   

2. Can’t start with a number

   age1 = 25      # ✅ Correct
   1age = 25      # ❌ Error
   

3. You can’t use Python reserved words (if, for, while, def, etc.)

   for = 5         # ❌ Error (for is a reserved word)
   my_for = 5      # ✅ Correct
   

Good practices (conventions)

1. Use snake_case: lowercase words separated by underscores

   user_name = "Ana"      # ✅ Preferred in Python
   userName = "Ana"       # ❌ Works but not pythonic
   UserName = "Ana"       # ❌ This is reserved for classes
   

2. Descriptive names: make it clear what the variable contains

   user_age = 25           # ✅ Clear
   a = 25                  # ❌ What is "a"?
   
   discounted_price = 45   # ✅ Descriptive
   dp = 45                 # ❌ Too short
   

3. In English (recommended): facilitates international collaboration

   user_age = 25           # ✅ Recommended
   

Basic data types

When you create a variable, Python automatically assigns it a data type depending on the value you give it. The three most important types to start with are:

1. Integers (int)

Numbers without decimals. Used for counting, indexes, ages, scores…

age = 25
players = 4
year = 2026
temperature = -5

You can do mathematical operations with them:

sum = 10 + 5        # 15
subtraction = 10 - 5       # 5
multiplication = 10 * 5    # 50
division = 10 / 5    # 2.0 (note: result is float)
floor_division = 10 // 3  # 3 (integer division, discards decimals)
remainder = 10 % 3       # 1 (remainder of division)
power = 2 ** 3    # 8 (2 to the power of 3)

2. Floating point numbers (float)

Numbers with decimals. Used for measurements, prices, percentages…

price = 19.99
temperature = 36.5
percentage = 0.15

Important: In Python you use the dot (.) as decimal separator, not the comma:

price = 19.99   # ✅ Correct
price = 19,99   # ❌ This creates a tuple, not a number

Operations with floats:

total = 10.5 + 2.3   # 12.8
half = 10.0 / 2      # 5.0

When you mix int and float, the result is always float:

result = 10 + 2.5    # 12.5 (float)

3. Text strings (str)

Any text goes between quotes. You can use single quotes '...' or double quotes "...", it doesn’t matter:

name = "Ana"
surname = 'Garcia'
message = "Hello, world"

Tip: Use double quotes by default. Only use single quotes when you need double quotes inside the text:

phrase = 'She said: "Hello"'

Concatenating strings

You can join texts with the + operator:

name = "Ana"
surname = "Garcia"
full_name = name + " " + surname
print(full_name)  # Ana Garcia

f-strings: the modern way to format text

Since Python 3.6 there’s a much more comfortable way to include variables inside text: f-strings (formatted strings). Put an f before the quotes and write the variables inside {}:

name = "Carlos"
age = 30

# Old way (works but ugly)
message = "Hello, my name is " + name + " and I am " + str(age) + " years old"

# Modern way with f-strings (much better!)
message = f"Hello, my name is {name} and I am {age} years old"
print(message)  # Hello, my name is Carlos and I am 30 years old

You can do operations inside the curly braces:

price = 100
discount = 20
print(f"Final price: {price - discount} euros")  # Final price: 80 euros

4. Booleans (bool)

Can only have two values: True (true) or False (false). Used for conditions, decisions, states…

is_adult = True
has_won = False
has_discount = True

Important: True and False start with a capital letter. It’s mandatory:

active = True    # ✅ Correct
active = true    # ❌ Error (NameError)

Booleans usually come from comparisons:

age = 25
is_adult = age >= 18    # True

temperature = 15
is_cold = temperature < 20    # True

points = 100
has_won = points >= 150       # False

Checking the type of a variable

If you’re not sure of a variable’s data type, use type():

name = "Ana"
print(type(name))    # <class 'str'>

age = 25
print(type(age))      # <class 'int'>

price = 19.99
print(type(price))    # <class 'float'>

active = True
print(type(active))    # <class 'bool'>

This is especially useful when debugging code and you want to understand what’s happening.

Converting between types

Sometimes you need to convert one data type to another. Python has functions for that:

str() — Convert to text

age = 25
age_text = str(age)
print("I am " + age_text + " years old")  # I am 25 years old

Or better with f-strings (which does the conversion automatically):

age = 25
print(f"I am {age} years old")  # I am 25 years old

int() — Convert to integer

price_text = "100"
price = int(price_text)
print(price + 50)  # 150

⚠️ Careful: If you try to convert something that isn’t a number, Python will throw an error:

number = int("abc")  # ValueError: invalid literal for int()

float() — Convert to decimal

price_text = "19.99"
price = float(price_text)
print(price)  # 19.99

bool() — Convert to boolean

number = 5
is_true = bool(number)
print(is_true)  # True

Important rule: In Python, 0, "" (empty string), None, empty lists [] and empty dictionaries {} are considered False. Everything else is True:

bool(0)      # False
bool(1)      # True
bool(-5)     # True
bool("")     # False
bool("Hi")   # True

Practical exercise

Open your REPL (type python in the terminal) and try this step by step:

# 1. Create variables with personal information
>>> name = "Your Name"
>>> age = 25
>>> height = 1.75  # in meters
>>> student = True

# 2. Display the information
>>> print(f"Name: {name}")
>>> print(f"Age: {age} years")
>>> print(f"Height: {height} meters")
>>> print(f"Is a student? {student}")

# 3. Do calculations
>>> birth_year = 2026 - age
>>> print(f"Approximate birth year: {birth_year}")

# 4. Check types
>>> print(type(name))
>>> print(type(age))
>>> print(type(height))
>>> print(type(student))

# 5. Convert data
>>> age_text = str(age)
>>> print("I am " + age_text + " years old")

# 6. Experiment with operations
>>> discount = 20
>>> original_price = 100
>>> final_price = original_price - (original_price * discount / 100)
>>> print(f"Original price: {original_price}€")
>>> print(f"Discount: {discount}%")
>>> print(f"Final price: {final_price}€")

Key concepts from this lesson

• Variables are named containers for storing data
• Use snake_case and descriptive names for your variables
• Basic data types are: int, float, str, bool
• f-strings (f"Text {variable}") are the modern way to format text
• Use type() to check a variable’s type
• You can convert types with int(), float(), str(), bool()

Next steps

In the next lesson we’ll learn about operators and expressions: how to do more complex calculations, compare values, and combine conditions. It’s the foundation for making decisions in your code.

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💡 Challenge: Create a program in a file calculator.py that:

1. Stores two numbers in variables
2. Calculates sum, subtraction, multiplication, and division
3. Displays the results with f-strings in a clear way

Example output:

Number 1: 10
Number 2: 3
Sum: 13
Subtraction: 7
Multiplication: 30
Division: 3.33

Never stop coding!