Chapter 2

Variables and Input

Chapter 1 used values directly: print(4.50 + 12.00). That's fine for a one-off, but a real script needs to hold onto a value, use it more than once, and change it. This chapter gives values names, and takes a value from whoever is running your script instead of writing it into the code by hand.

Naming a value

A variable is a name bound to a value. You create one with =, assignment:

price = 4.50
print(price)

4.5

price is not a container that holds 4.50 the way a box holds a book; it's a label pointing at the value 4.50. That distinction barely matters yet, but it matters a lot once you meet lists in Chapter 5, so it's worth saying now.

A name can be reassigned to a completely different value at any time:

price = 4.50
print(price)
price = 12.00
print(price)

4.5
12.0

Nothing links the old value to the new one. price just points somewhere else now.

Naming rules: letters, digits, and underscores, never starting with a digit, and Python's own words (print, if, for, and the rest) are off limits. By convention, Python variable names are lowercase with underscores between words, like total_spent, not TotalSpent or totalSpent. This book follows that convention throughout.

total_spent = 4.50 + 12.00
print(total_spent)

16.5

Assigning more than one name at once

Python lets you assign several names in one line, matching values to names by position:

amount, category = 4.50, "coffee"
print(amount)
print(category)

4.5
coffee

This is called unpacking, and it works with any group of values on the right that has exactly as many items as names on the left:

amount, category, note = 4.50, "coffee"

That's ValueError: not enough values to unpack (expected 3, got 2), another honest error rather than a guess. You'll use unpacking for real in Chapter 9, pulling three fields out of one line read from a file.

It's also the standard way to swap two variables, without a temporary third name to hold one of them while the swap happens:

first = "coffee"
second = "transit"
first, second = second, first
print(first)
print(second)

transit
coffee

f-strings

Chapter 1 built a message by concatenating with + and converting numbers with str(). An f-string does both at once. Put an f right before the opening quote, and anything inside {} is evaluated and inserted:

item = "coffee"
price = 4.50
print(f"{item} costs {price}")

coffee costs 4.5

You can put more than a bare variable inside the braces, including a full expression:

price = 4.50
quantity = 3
print(f"{quantity} at {price} comes to {price * quantity}")

3 at 4.5 comes to 13.5

f-strings are how this book builds every message from here on. str() and + still work, and you'll see them in other people's code, but an f-string is shorter and doesn't need a manual str() call.

Formatting a number inside an f-string

Add a colon and a format spec after the expression to control how it's shown, without changing the underlying value:

price = 4.5
print(f"{price:.2f}")
print(f"{price:>10.2f}")

4.50
      4.50

.2f means "fixed-point, two digits after the decimal," which is why 4.5 prints as 4.50 here, even though Chapter 1 showed that Python drops that trailing zero by default. >10.2f adds a minimum width of 10 characters, right-aligned, useful for lining up a column of amounts. You'll use .2f starting in Chapter 11, to make this book's tracker print dollar amounts that always show two decimal places.

Taking input from a person

input() pauses a script, shows the person running it whatever prompt text you give it, and waits for them to type a line and press enter:

name = input("What's your name? ")
print(f"Hello, {name}.")

$ python greet.py
What's your name? Ada
Hello, Ada.

input() always returns a string

Whatever the person types, input() hands it back as a str, even if it looks like a number:

raw_price = "4.50"
print(type(raw_price))

<class 'str'>

(That's a stand-in for raw_price = input("Price: "), exactly as described above: the same string input() would have returned if someone typed 4.50.)

You can't do arithmetic on it as-is, and the mistake is sneakier than an error:

raw_price = "4.50"
print(raw_price * 2)

4.504.50

That's str * int, which repeats the string, not 9.0. No error, no warning, just a wrong-looking result if you weren't expecting it. This is worse than a crash, because nothing tells you it's wrong. Convert first, always.

Converting with int() and float()

int() and float() convert a string to a number, if the text actually looks like one:

raw_price = "4.50"
price = float(raw_price)
print(price)
print(type(price))

4.5
<class 'float'>

raw_count = "3"
count = int(raw_count)
print(count * 2)

6

If the text doesn't look like a number, conversion raises ValueError:

price = float("twelve")

That's ValueError: could not convert string to float: 'twelve'. It's an honest error: Python is telling you exactly what it tried and exactly why it gave up. Right now that error would crash your script. Chapter 10 shows you how to catch it and respond instead of crashing, once you've met if and functions, which make a good response possible. For now, just recognize the message when you see it.

Putting it together

A tiny script for logging a coffee purchase, start to finish:

name = input("What did you buy? ")
raw_price = input("How much? ")
price = float(raw_price)
print(f"Logged: {name} for {price}")

$ python log_one.py
What did you buy? coffee
How much? 4.50
Logged: coffee for 4.5

And the same logic, tested for real, with "coffee" and "4.50" standing in for what input() would have returned:

name = "coffee"
raw_price = "4.50"
price = float(raw_price)
print(f"Logged: {name} for {price}")

Logged: coffee for 4.5

Practice

Try each of these before you read the solution under it.

1. Assign item = "cappuccino" and price = 4.20, then print "cappuccino is 4.2" using an f-string.
2. A stand-in for input() gives you raw_quantity = "3". Convert it to an int and print double that number.
3. raw_total = "12.5x" is meant to be a price but has a typo. Try converting it with float() and note, in a comment, what error you'd expect before running it.
4. Given price = 8 (a whole number, not 8.00), print it formatted as a two-decimal amount with a $ in front, using an f-string: $8.00.

Solutions

1.

item = "cappuccino"
price = 4.20
print(f"{item} is {price}")

cappuccino is 4.2

2.

raw_quantity = "3"
quantity = int(raw_quantity)
print(quantity * 2)

6

3. float() cannot parse a trailing letter, so this raises ValueError.

raw_total = "12.5x"
total = float(raw_total)

4. .2f formats even a plain int with two decimal places, since it's evaluated inside the braces as part of the expression.

price = 8
print(f"${price:.2f}")

$8.00

Where this leaves you

You can name a value with a variable, build a message with an f-string, and you know why input() always hands you back a string and how to convert it, including what happens when the text doesn't cooperate. Chapter 3 uses comparisons and booleans to make a script choose between paths instead of always doing the same thing.