Variables and Data Types Explained

By Thread Academy · 7 September 2026 · Computer Science

Almost every program needs to remember something: a score, a name, a total. Programs remember things using variables — named storage locations that hold a value while the program runs. If you understand variables and the types of values they hold, you understand a large part of how programs work.

What a variable is

Think of a variable as a labelled box. You choose a name, put a value inside, and later you can read the value or replace it with a new one. In Python, assignment uses the = symbol, and it works like this:

score = 0
score = 10
print(score)

The first line creates the variable score and stores 0 in it. The second line replaces the old value with 10. The third line outputs 10. A variable's value can change during a program — that is exactly why it is called variable.

Good names matter. score tells the reader what the value means; x does not. Choose names that describe the data they hold.

The five main data types

Every value stored in a variable has a data type, which determines what kind of value it is and what operations make sense for it. The five core types you need to know are:

Integer

Whole numbers, positive or negative, with no decimal point. Used for counts and anything that cannot sensibly be fractional.

age = 16
temperature = -3
lives_remaining = 5

Real (float)

Numbers with a decimal point, used for measurements and anything that needs fractions. In Python this type is called a float.

price = 4.99
average = 72.5
gravity = 9.81

Char

A single character: one letter, digit, or symbol. In Python, chars are handled as one-character strings, but the concept of a single character matters in many languages.

grade = "A"
initial = "k"

String

A sequence of characters: words, sentences, or any text. Strings are written in quotes.

name = "Ada Lovelace"
message = "Hello, world!"
postcode = "1026"   # digits stored as text — see below

Boolean

One of only two values, True or False, used for yes-or-no decisions and the results of comparisons.

is_logged_in = True
game_over = False

Why data types matter: type errors

The data type controls which operations are valid. Adding two integers makes sense; adding a number to text usually does not. Watch what happens:

score = 10
name = "Ada"

total = score + name   # TypeError: cannot add a number and a string

This is a type error: the program tries to perform an operation on data types that do not fit together. Python stops and reports the error, which is genuinely helpful — it tells you exactly where your thinking went wrong.

Types also protect against silent mistakes. A postcode of "1026" looks like a number, but treating it as a string is correct: you would never add two postcodes together. Choosing the right type is part of describing your data accurately.

Sometimes you need to convert between types deliberately. Python provides functions such as int(), float(), and str() for this:

age_text = input("How old are you? ")   # input() always returns a string
age = int(age_text)                      # convert to an integer for maths
print("Next year you will be " + str(age + 1))

Notice the careful conversions: input arrives as text, becomes an integer for the calculation, and converts back to text for printing.

Constants

Some values should never change while a program runs: the number of days in a week, the price of an item in a fixed catalogue, or a school's name. These are constants.

DAYS_IN_WEEK = 7
SCHOOL_NAME = "Thread Academy"

By convention, constant names are written in CAPITAL_LETTERS so that any reader can see they are not meant to change. Using a named constant instead of a bare number is better for two reasons: it documents what the value means, and if it ever does need updating, you change it in one place.

Key takeaways

• A variable is a named storage location whose value can change while a program runs.
• Python assignment uses =: score = 10 creates or updates the variable score.
• The five core data types are integer, real (float), char, string, and Boolean.
• Data types decide which operations are valid; mismatched types cause type errors.
• Converting types deliberately with int(), float(), or str() avoids surprises.
• Constants are values that must not change; name them in CAPITALS to mark that clearly.

Putting it together: a small program

Here is a short program that uses almost everything in this article. It greets a user, works out a total price, and decides whether the order qualifies for free delivery.

FREE_DELIVERY_LIMIT = 25.0
VAT_RATE = 0.20

customer = "Bilal"
subtotal = 18.50
vat = subtotal * VAT_RATE
total = subtotal + vat

print("Customer: " + customer)
print("Total: " + str(round(total, 2)))
print("Free delivery: " + str(total >= FREE_DELIVERY_LIMIT))

The variables customer, subtotal, vat, and total change as the program runs, while FREE_DELIVERY_LIMIT and VAT_RATE stay fixed. Each value has the type that fits it: text is a string, prices are floats, and the free-delivery answer is a Boolean produced by checking whether total is greater than or equal to FREE_DELIVERY_LIMIT. Notice the str() conversions before printing, and round() to keep the money to two decimal places. Read the program again slowly, and check that every variable has a sensible type for the job it does — that habit will save you from most type errors before they happen.