Variables and Data Types

A variable is a name that points to a value. Think of it as a labelled box: you put something in, then use the label to get it back later.

age = 30
name = "Ada"
print(name, "is", age)

Output:

Ada is 30

The = sign here does not mean "equals" like in math. It means assign: take the value on the right and store it under the name on the left. You can reassign a variable at any time:

score = 10
score = score + 5
print(score)   # 15

Naming rules

• Use letters, digits, and underscores. A name can't start with a digit.
• Names are case-sensitive: age and Age are different variables.
• Python style is snake_case, like total_price.
• You can't use reserved words such as if, for, or while as names.

The basic data types

Every value has a type. Use the built-in type() function to check:

print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("hi"))      # <class 'str'>
print(type(True))      # <class 'bool'>
print(type(None))      # <class 'NoneType'>

A few things worth knowing:

• True and False must start with a capital letter.
• None is Python's way of saying "nothing here". It's useful as a placeholder, for example result = None before you've calculated anything.
• 5 is an int but 5.0 is a float, even though they look alike.
• "5" is a string, not a number. You can't do math with it.

Type conversion

Sometimes you need to turn one type into another. Use the type's name as a function:

int("42")      # 42
float("3.5")   # 3.5
str(100)       # "100"
int(3.9)       # 3  (cuts off the decimal, doesn't round)
bool(0)        # False

This matters because input() always gives you text, even if the user typed a number:

n = int(input())    # read a line and turn it into an integer
price = float(input())

To round a number, use round(number, digits):

print(round(3.14159, 2))   # 3.14
print(round(2.5))          # 2

Putting it together

celsius = 20
fahrenheit = celsius * 9 / 5 + 32
print(fahrenheit)         # 68.0
print(type(fahrenheit))   # <class 'float'>

Dividing with / always produces a float. You'll meet more operators in the next lesson.

Now try it yourself. The next step is two puzzles where you store values in variables and convert between types.