Variables and Data Types

Introduction

Variables are used to store values so you can refer to them later in your code. Python is dynamically typed, which means you do not need to declare a type — Python works it out from the value you assign.

Assigning Variables

name = "Alice"
age = 30
salary = 75000.50
is_active = True

Variable names should be lowercase, with words separated by underscores. This is called snake_case and is the Python convention.

Core Data Types

String (str)

Text data, enclosed in single or double quotes.

firstname = "Alice"
greeting = 'Hello, World!'

You can combine strings using +:

full_name = "Alice" + " " + "Smith"
print(full_name)  # Alice Smith

Or use an f-string (the preferred modern approach):

firstname = "Alice"
lastname = "Smith"
print(f"Full name: {firstname} {lastname}")  # Full name: Alice Smith

Integer (int)

Whole numbers, positive or negative.

age = 30
year = 2024
negative = -10

Float (float)

Numbers with a decimal point.

salary = 75000.50
temperature = -3.5

Boolean (bool)

One of two values: True or False. Note the capital letter.

is_active = True
has_contract = False

NoneType (None)

Represents the absence of a value.

contract_end_date = None

Checking the Type of a Variable

name = "Alice"
age = 30
salary = 75000.50

print(type(name))    # <class 'str'>
print(type(age))     # <class 'int'>
print(type(salary))  # <class 'float'>

Type Conversion

You can convert between types when needed.

age_str = "30"
age_int = int(age_str)      # "30" → 30
price_str = str(19.99)      # 19.99 → "19.99"
whole = int(3.9)            # 3.9 → 3 (truncates, does not round)

Common Mistakes

• Forgetting quotes around strings: name = Alice → NameError
• Mixing types without converting: "Age: " + 30 → TypeError

# Correct way
age = 30
print("Age: " + str(age))
# Or, more cleanly:
print(f"Age: {age}")

Practice Exercises

• Create variables for your firstname, lastname, age, and salary. Print them all on one line using an f-string.
• Create a variable with the value "42". Convert it to an integer and add 8 to it. Print the result.
• Print the type of each of the four variables you created.
• Create a variable called is_employed and set it to True. Print it.