11. Variables and types

Chapter 7 wrote text directly inside print calls. That works, but any information needed in more than one place has to be retyped every time. Variables fix that. A variable is a name that stands in for a value, so you write the value once and reuse it.

Declaring a variable

The most common shape of a variable declaration in Python:

name = "Keiko"
print(name)

Two things are going on in that first line:

• name is the variable's name. You pick it.
• = is the assignment operator. It puts the value on the right into the variable on the left. It does not mean "equal to" the way it does in maths.

Now name stands in for the string "Keiko", so print(name) prints Keiko.

Changing a variable

A variable's value can change. Use = again:

score = 0
print(score)   # prints 0
score = 50
print(score)   # prints 50

Types

In Python a value also has a type, not just data. The five types you need right now:

• str — text, written between quotes: "hello", 'a', "" (the empty string).
• int — a whole number with no decimal point: 0, -7, 42.
• float — a number with a decimal point: 3.14, 0.5, -1.0.
• bool — exactly two values: True and False. No quotes. Note the capital letter — true (lowercase) is an error in Python.
• NoneType — the type of None, which represents the absence of any value.

type() is a built-in function that returns an object describing a value's type. To get just the type name as a short string, use type(x).__name__:

print(type("hello").__name__)   # str
print(type(7).__name__)         # int
print(type(3.14).__name__)      # float
print(type(True).__name__)      # bool
print(type(None).__name__)      # NoneType

Variable names

You pick variable names, but Python has rules about which are allowed:

• They can contain letters, digits, and the underscore _.
• They cannot start with a digit. level1 is fine; 1level is not.
• They are case sensitive. Score, score, and SCORE are three different names.
• They cannot be one of Python's reserved words: and, as, assert, async, await, break, class, continue, def, del, elif, else, except, False, finally, for, from, global, if, import, in, is, lambda, None, nonlocal, not, or, pass, raise, return, True, try, while, with, yield. You will meet most of these later.

Beyond the rules, follow these habits:

• Choose meaningful names. player_name beats n. hit_points beats hp once code gets long, though hp is fine for short scripts.
• Lower case with underscores is the standard Python style: max_score, enemy_count. This convention is called snake_case.

Printing a variable

print does not need quotes around variable names. Quotes are only for text you type directly. To combine a label with a variable, either pass them as separate arguments (Python adds a space between them) or use an f-string:

level = 7
print("Level:", level)        # Level: 7
print(f"Level: {level}")      # Level: 7

An f-string starts with f before the opening quote. Any variable or expression inside {} is evaluated and inserted into the string. This is the standard Python way to build strings with values in them.

Multiple assignment

Python lets you declare several variables and assign several values on one line. Names on the left pair with values on the right by position:

name, level = "Keiko", 7
print(name, level)   # Keiko 7

This groups related declarations neatly. It also enables a handy trick — swapping two variables in one step:

a, b = 1, 2
a, b = b, a
print(a, b)   # 2 1

Python reads the whole right side first, then assigns. So a, b = b, a takes the current b and a and puts them in opposite slots.

Homework

Homework starters are in exercises/11/homework/.

Stuck or finished? Open the homework solutions page.