Using variables and constants

What you'll learn

• What variables and constants are, and why programs need them.
• How to use assignment in Python 3 to store and update values.
• How to choose clear names and suitable data types.
• How to decide when a value should be a variable and when it should be a constant.

The big idea: programs work with data

A program is a set of instructions that a computer can run. Most useful programs need to remember data while they run: a score, a username, a total price, whether a password was correct, and so on.

Instead of writing every value directly into the program, we usually give important values a name. This makes the program easier to read, test and change.

Values, names and identifiers

A value is a piece of data, such as 42, "Amira" or True.

An identifier is the name you give to something in a program, such as a variable or constant.

In Python, sensible identifiers usually use snake case, where words are lowercase and separated by underscores:

player_score = 0
number_of_lives = 3
user_name = "Sam"

Python identifiers:

• can contain letters, digits and underscores
• cannot start with a digit
• are case-sensitive, so score and Score are different names
• should not be Python keywords such as if, while or for

Assignment: storing a value in a variable

In Python, the assignment operator is =. It means “store the value on the right in the name on the left”.

score = 0

This creates a variable called score and stores the integer value 0 in it.

The important rule is:

1. Python works out the expression on the right-hand side.
2. Python stores the result in the variable on the left-hand side.
3. If the variable already had a value, the old value is replaced.

score = 0
score = score + 10
score = score * 2

Data types: what kind of value is being stored?

A data type describes what kind of data a value is. The type matters because it affects what operations the program can do with the value.

Common GCSE data types include:

Python uses the type name float for real numbers.

You should choose variables that match the data you need to store. For example, a number of attempts should be an integer, but a username should be a string.

age = int(input("Enter your age: "))
height = float(input("Enter your height in metres: "))
name = input("Enter your name: ")

Updating variables

Variables are useful because they can change. Two very common uses are counters and running totals.

A counter stores how many times something has happened.

attempts = attempts + 1

A running total stores a total that builds up over time.

total_cost = total_cost + item_price

total_score = 0
questions_answered = 0

total_score = total_score + 5
questions_answered = questions_answered + 1

total_score = total_score + 3
questions_answered = questions_answered + 1

Constants: fixed values with clear names

A constant is used when a value has a fixed meaning in the program. Python does not have a special built-in constant command, so GCSE Python programs normally use a naming convention: constants are written in capital letters with underscores.

MAX_ATTEMPTS = 3
PASS_MARK = 50
VAT_RATE = 0.20

This tells other programmers: “this value should not be changed later in the program”.

Constants are especially useful for values that might need changing in the future. If the pass mark changes from 50 to 55, you only need to update one line.

A magic number is a value written directly into code without a clear explanation.

Less clear:

if mark >= 50:
    print("Pass")

Clearer:

PASS_MARK = 50

if mark >= PASS_MARK:
    print("Pass")

Choosing between variables and constants

Ask yourself: “Should this value change while the program runs?”

Use a variable if the value may change, comes from the user, or is calculated by the program.

Use a constant if the value is fixed for the whole program.

A possible Python solution is:

ADULT_TICKET_PRICE = 12
CHILD_TICKET_PRICE = 7
BOOKING_FEE = 2

adult_tickets = int(input("Number of adult tickets: "))
child_tickets = int(input("Number of child tickets: "))

ticket_total = (adult_tickets * ADULT_TICKET_PRICE) + (child_tickets * CHILD_TICKET_PRICE)
amount_to_pay = ticket_total + BOOKING_FEE

print("Amount to pay: £", amount_to_pay)

Writing programs that use variables and constants appropriately

For Edexcel GCSE, you need to be able to write programs that make appropriate use of variables and constants. That means more than just knowing the definitions.

Your code should:

• store input values in clearly named variables
• store calculated results in variables when they are needed later
• use constants for fixed values such as limits, rates, fees and thresholds
• avoid repeating the same literal value many times
• use data types that fit the data being processed
• update variables correctly using assignment

mark = int(input("Enter mark: "))

if mark >= 50:
    print("Pass")
else:
    print("Fail")

print("Pass mark:", 50)

PASS_MARK = 50

mark = int(input("Enter mark: "))

if mark >= PASS_MARK:
    print("Pass")
else:
    print("Fail")

print("Pass mark:", PASS_MARK)

A quick style checklist

Good variable and constant use is partly about correctness and partly about readability.

Prefer this:

MAX_LOGIN_ATTEMPTS = 3

attempts = 0
password_correct = False

Avoid this:

x = 0
pc = False

The second version might run, but it is harder for another person to understand.