Variables and Numbers

By the end of this lesson you'll be able to:

• Create a variable in Python and store a number inside it
• Name variables using snake_case and choose clear, descriptive names
• Explain the difference between an integer and a float

Imagine you are playing a video game and you need to remember your score. You could try to hold it in your head — but what if it changes? What if you need that number in five different places in your code? A variable solves this problem. A variable is a labeled storage location in your program's memory. You give it a name, put a value inside it, and Python remembers that value for you.

Welcome to Chapter 3!

Variables are one of the most powerful ideas in all of programming — and you've already used them in Scratch! Today we learn how Python does the same thing with just a few keystrokes. Let's code it together!

What Is a Variable?

Picture a sticky note on your desk. The front has a name written on it — like score — and on the back you write the current value, like 10. Whenever you need to know the score, you flip it over and read it. If the score changes, you erase the old number and write a new one — but the name on the front stays the same.

In Python, a variable works exactly like that sticky note. You create a variable by writing its name, then an equals sign, then the value you want to store. This is called variable assignment:

score = 10

• The name on the left (score) is the variable's label.
• The equals sign (=) means "store this value in this name."
• The value on the right (10) is what gets stored.

Notice that = in Python does not mean "is equal to" the way it does in math. It is a one-way instruction: "take the value on the right and put it in the box on the left." The value flows from right to left, always.

After you store a value, you can read it back using print():

score = 10
print(score)

Scratch Bridge

In Scratch, you clicked the "Set score to 10" block to create a variable and give it a starting value. In Python, score = 10 does exactly the same thing — same idea, just written as text instead of snapped together as blocks!

Sample Code

Here is a short program that creates two variables and prints both of them. Read through it before you run it.

score = 10
lives = 3
print(score)
print(lives)

What Do You Think Will Happen?

Before you click Run, look at those four lines. The program stores two numbers and then prints them. What do you think the output will look like — will both numbers appear, or just one? And in what order? Make your prediction, then click Run to find out!

Try It Now

Edit the code below and click Run to see the result right on this page. No account needed — everything runs in your browser.

▶ Run ↻ Reset

Were you right? Python printed 10 on the first line and 3 on the second — one value per print() call, in the order they appeared.

If you see a red error message: Check your spelling carefully. Python is case-sensitive, so Score = 10 and score = 10 are two completely different variables. Make sure there are no extra spaces around the = sign.

How It Works

When Python runs score = 10, it sets aside a small storage location in memory, writes 10 into it, and attaches the label score. When Python then sees print(score), it looks up that label, retrieves the stored value, and displays it.

The table below explains each line of the program:

Line	What it does
score = 10	Creates a variable named score and stores the integer 10 inside it
lives = 3	Creates a second variable named lives and stores 3 inside it
print(score)	Looks up the value in score and prints it
print(lives)	Looks up the value in lives and prints it

Each variable is independent. Changing one does not affect the other. You can have as many variables as you like — your program's memory can hold thousands of them.

Naming Your Variables

Python lets you name a variable almost anything — but there are rules you must follow:

• Start with a letter or underscore — not a digit. score is fine; 1score causes an error.
• Use only letters, digits, and underscores — no spaces, hyphens, or symbols like @ or !.
• Avoid Python keywords — words like print, for, if, and while are reserved. Python needs those words for itself.

Python is also case-sensitive: Score, SCORE, and score are three completely separate variables. Mixing upper and lower case accidentally is one of the most common sources of bugs for beginners.

Snake_case: Python's Naming Style

When a variable name has more than one word, Python programmers join the words with underscores and keep all letters lowercase. This style is called snake_case (picture the underscores as a snake lying flat between the words).

The table below shows common naming mistakes and their snake_case fixes:

Problematic name	What's wrong	Better name
myscore	Words run together — hard to read	my_score
MyScore	Capital letters belong to a different naming style	my_score
my score	Spaces are not allowed in variable names	my_score
my-score	Python reads the dash as subtraction — my - score — not a name!	my_score
x	Too vague — what does x mean?	player_score
s	Single letters give no information	lives_remaining

Meaningful Variable Names

The best variable names describe exactly what the variable holds. A name like player_score tells you immediately what the number represents. A name like x gives you no information at all.

This matters because programs grow. A program you write today might be 10 lines — easy to keep in your head. But programs grow to hundreds and thousands of lines. When that happens, a good name is the difference between code you can read and code that feels like a puzzle.

Naming Pitfalls!

Watch out for the dash! If you write my-score instead of my_score, Python reads the dash as a minus sign and thinks you are subtracting a variable called score from a variable called my. Use an underscore (_), never a dash (-), to join words in a name. Other common traps: single-letter names like x that tell you nothing, capital letters that break snake_case, and accidentally using a Python keyword as your variable name.

Integers: Whole Numbers

Now that you know how to create and name variables, let's look more carefully at the kinds of numbers Python understands.

A whole number — one with no decimal point — is called an integer, or int for short. Integers can be positive, negative, or zero. You use integers for things you count: the number of lives in a game, the number of sides on a shape, a player's score.

sides = 6
lives = 3
temperature = -5

All three of those variables hold integers.

Floats: Decimal Numbers

A number with a decimal point is called a float — short for "floating-point number." Floats are used for measurements and values that don't land exactly on a whole number.

price = 4.99
height = 1.75
pi = 3.14159

The name "float" comes from how computers store these numbers internally — the decimal point can "float" to different positions. You don't need to know the internal details. What matters is when to use each type:

Kind	Python type	When to use	Examples
Whole number	int	Counting, game scores, number of repetitions	0, 7, -12, 1000
Decimal number	float	Measurements, money, percentages, averages	3.14, 0.5, -2.7, 99.0

One important detail: dividing two integers with / always gives you a float, even when the result is a whole number.

result = 10 / 2
print(result)

That prints 5.0, not 5 — note the decimal point. Python uses // (floor division) if you want a whole-number result: 10 // 2 gives 5.

Variable Reassignment

One of the most powerful things about variables is that you can change them. You reassign a variable by writing its name again on the left side of = with a new value on the right. Python discards the old value and stores the new one under the same label.

Here is a program that changes a score partway through:

score = 10
print(score)

score = 25
print(score)

Read through that before you run it. The first print shows the original value. The second assignment replaces it. The second print shows the updated value.

Sample Code — Reassignment

score = 10
print(score)

score = 25
print(score)

Try It Now — Reassignment

▶ Run ↻ Reset

Python printed 10 first and 25 second. The name score stayed the same; only the value changed.

You can also use the current value of a variable to calculate a new one:

score = 10
score = score + 5
print(score)

Python evaluates the right side first: score + 5 is 10 + 5, which equals 15. Then it stores 15 back into score. This pattern — "take the old value, add something, store the result back" — appears constantly in real programs.

See It: Boxes in Memory

The MicroSim below lets you watch the sticky-note boxes as a script runs. Before you click anything, look at the script on the left and predict how many boxes will exist when it finishes — and what value each box will hold. Then click Next Step to check yourself, one line at a time.

Explore the Variable Memory Model MicroSim

Do not miss the Copy Surprise program in the dropdown: after b = a, does changing a change b? Make your prediction, then step through to find out.

None: The "Nothing" Value

Python has one special value that is neither an integer nor a float. It is called None, and its type is called NoneType.

None means "no value yet" or "nothing here." You will see it in two common situations:

1. When you want a variable to exist but you haven't decided its value yet.
2. When a function finishes without returning anything — Python automatically gives back None in that case.

winner = None
print(winner)
print(type(winner))

Running that program prints:

None
<class 'NoneType'>

None is not zero. None is not an empty string. None is its own special thing that means "the absence of a value." Think of it like a sticky note that exists on your desk but has nothing written on the back yet.

Later in the course you will use None to mark the start of a search, represent an empty box in a game board, or signal that a function didn't find what it was looking for.

What type() Tells You

Python has a built-in function called type() that tells you what kind of value is stored in a variable. You saw it used with None above. Here it is with an integer and a float:

score = 10
price = 4.99

print(type(score))
print(type(price))

Output:

<class 'int'>
<class 'float'>

The word class in the output is how Python describes categories of values. For now, just read <class 'int'> as "this is an integer" and <class 'float'> as "this is a float." You will learn much more about classes later in the course.

Learning Check

Your Turn — Add the Missing Line

The program below creates two variables — player_score and bonus_points. It prints player_score just fine, but something is missing: bonus_points never gets printed. Add one line so the output shows both numbers, one on each line.

▶ Run ↻ Reset

If your output shows 42 on the first line and 10 on the second, you got it! The missing line was print(bonus_points).

Experiments

Try these changes in the labs above. For each one, predict what will happen first — then run the code to check!

1. In Lab 1, change score = 10 to score = 750. You'll know it worked when the output shows 750 instead of 10.
2. In Lab 1, add a third variable called high_score set to 100. Add a print(high_score) line. You'll know it worked when you see three numbers in the output.
3. In Lab 2 (the reassignment lab), change score = 25 to score = score * 2. You'll know it worked when the output shows 10 on the first line and 20 on the second.
4. Create a float variable called average_score and set it to 87.5. Print it. You'll know it worked when the output shows 87.5.
5. Create a variable called champion and set it to None. Print it. Then reassign champion to 1. Print it again. You'll know it worked when the output shows None on one line and 1 on the next.

You've Got Variables!

Variables, integers, floats, snake_case naming, reassignment, None — you now have the building blocks every Python program is built on. Every chapter from here uses what you just learned. Brilliant work, coder! Let's code it together!

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