Variables

A name for a value

Think of a sticky label you put on a box. The label is the name, and whatever is in the box is the value. A variable is exactly that: a name that refers to a value. You create one with a single equals sign:

campaign = "Spring Sale"
budget = 5000
print(campaign)
print(budget)

Read budget = 5000 as "store 5000 under the name budget", not as "budget equals 5000". The = sign is an instruction, not a statement of fact. Python works out the value on the right first. Then it sticks the name on the left onto it.

Once a name exists, you can use it anywhere you'd use the value itself. That includes a calculation that creates another variable:

budget = 5000
spent = 1200
remaining = budget - spent
print(remaining)

Variables don't update themselves

That last example trips up almost everyone once. remaining = budget - spent does the subtraction at that moment, and stores the result: 3800. It does not set up a formula the way a spreadsheet cell does. Change spent afterwards, and remaining still holds 3800 until you run the calculation again.

For marketers: a spreadsheet cell like =B2-C2 recalculates whenever B2 or C2 changes. A Python variable is more like typing the result into the cell as a plain number.

Changing a variable

Giving a new value to a name that already exists replaces its old value. The right side is worked out first, so a variable can appear on both sides:

clicks = 120
clicks = clicks + 30   # take the current value, add 30, store the result
print(clicks)

Adding to a variable is so common that Python has a shorthand: +=. The same works for the other operators too: -=, *= and /=.

clicks = 120
clicks += 30
clicks -= 10
print(clicks)

a = 10
b = a
a = 20
print(b)

Types: what kind of value it is

Every value has a type, and the type decides what you can do with it. You can add two numbers, but you can't add a number to a sentence. The four types you'll meet first:

type() tells you a value's type:

print(type("Spring Sale"))
print(type(5000))
print(type(0.035))
print(type(True))

Quotes make a string. "5000" is four characters of text that happen to be digits, so you can't do arithmetic with it. Python won't guess what you meant:

budget = "5000"
print(budget + 100)

The error says Python can only join a string to another string. You'll see this a lot when data arrives from a file or an API as text. The next lesson, on numbers, shows how to convert between types with int(), float() and str().

Naming variables

Python's rules for names:

• Letters, digits and underscores only. No spaces or hyphens.
• Can't start with a digit: campaign2 is fine, 2campaign is not.
• Case matters: budget and Budget are two different names.
• A few words are reserved because Python uses them, such as if, for, class and True.

Nearly all Python code follows one style, called snake_case: lowercase words joined by underscores, like cost_per_click or email_open_rate. Choose names that say what the value is. ctr is fine in a short script about ads. x isn't: in three weeks, neither you nor an AI assistant reading your code will know what it holds.

A misspelled name gives a NameError, Python's way of saying it has never seen that name:

budget = 5000
print(Budget)

Read the last line of any error first. Here it names the exact thing Python couldn't find.

sends = 24000
clicks = 1920
purchases = 132

click_through_rate = clicks / sends
conversion_rate = purchases / clicks

print("CTR:", click_through_rate)
print("Conversion rate:", conversion_rate)

Why a variable isn't a spreadsheet cell

In a spreadsheet, a cell can hold a formula, so it changes when its inputs change. A Python variable doesn't work like that. It holds the value that was worked out at the moment that line ran.

price = 10
total = price * 3
price = 20
print(total)

This prints 30, not 60. total was calculated when price was 10, and nothing tells it to look again. If you want a new total, run the calculation again after changing the price.

Two more traps

One equals sign isn't a question. In math class, = says two things are equal. In Python, a single = is an instruction: put this value under this name. You'll meet ==, which really does ask whether two things are equal, in the lesson on conditions.

Quotes make text. age = "42" stores the characters 4 and 2, not a number. Add 1 to it and Python stops with a TypeError, because it won't guess whether you meant arithmetic. When you see that error, check for stray quotes first.

Names can't contain spaces or hyphens. ad-spend looks like a name to you, but Python reads it as ad minus spend. Write ad_spend instead. Clear names are worth a few extra letters, since you'll read them far more often than you type them.