Variables and Types

A variable is a name that refers to a value. Variables let your program remember things so you can use them later.

A good way to picture it: a variable is a label stuck on a value. age = 36 sticks the label age on the value 36. Whenever you write age, Python looks up which value that label is stuck on.

Creating variables

You create a variable by assigning a value to a name with =:

name = "Ada"
age = 36

print(name)
print(age)
print(name, "is", age, "years old")

Read age = 36 as "age now refers to 36". The = sign means "store this value", not "equals" as in maths.

Changing a variable

Variables can be reassigned at any time: the label moves to the new value.

Predict first: what will each print show? Decide, then press Run.

score = 10
print(score)

score = 25
print(score)

score = score + 5   # take the current value, add 5, store the result
print(score)

The last line looks odd if you think of = as "equals". Python works it out in two steps: first it calculates the right side (25 + 5), then it stores the result in score.

Naming rules

Variable names:

• can contain letters, digits and underscores (_),
• can't start with a digit,
• are case-sensitive (age, Age and AGE are three different variables),
• can't be a Python keyword such as if, for or class.

By convention, Python programmers use snake_case: lowercase words joined with underscores. Good names describe what the value is, like total_price rather than tp.

user_name = "sam"      # good
total_price = 9.99     # good
2nd_place = "Bo"       # SyntaxError: starts with a digit

Basic data types

Every value in Python has a type: is it a whole number, a decimal number, some text? These are the ones you'll use most:

int is short for integer and str for string. None is a special value meaning "nothing here yet".

Use the built-in type() function to find out the type of any value:

count = 42
price = 19.99
language = "Python"
is_ready = True
result = None

print(type(count))
print(type(price))
print(type(language))
print(type(is_ready))
print(type(result))

The type matters because it decides what you can do with a value. You can do maths with numbers, but not with text. Numbers get a lesson of their own next.

A variable can change type

You never declare a type in Python. A variable simply takes the type of whatever value it currently holds, and it can hold a different type later:

thing = 10
print(thing, type(thing))

thing = "ten"
print(thing, type(thing))

Converting between types

"25" (text) and 25 (a number) look similar, but they're different types. To change a value's type, use the type's name like a function:

text = "25"
number = int(text)          # str -> int
print(number + 5)

print(float("3.5") * 2)     # str -> float
print(str(100) + " points") # int -> str
print(int(9.99))            # float -> int (drops the decimals)

Mixing text and numbers without converting raises a TypeError. Run this to see it:

age = 30
print("I am " + age + " years old")

Python refuses to guess whether you meant maths or joining text. Fix it by converting with str(age), or by using an f-string.

f-strings

An f-string has an f before the opening quote. It lets you put variables, and even small calculations, inside curly braces {}:

name = "Ada"
age = 36

print(f"{name} is {age} years old")
print(f"Next year {name} will be {age + 1}")

f-strings convert the values for you, so they're the easiest way to mix text and numbers. You'll use them in almost every lesson from here on.

Swapping two variables

You can assign several variables in one line by separating them with commas. This gives a neat trick for swapping two values:

a = "left"
b = "right"

a, b = b, a
print(a, b)

Later, in the User Input lesson, you'll let the person running your program type in the values instead of writing them into the code.

Exercises

Exercise 1: Area of a rectangle

Create variables width and height with the values 7 and 4.5, calculate the area, and print it using an f-string: The area is 31.5.

width = 
height = 

Exercise 2: Add two numbers stored as text

The two variables below hold strings. Convert them and print their sum, which should be 42, not "1527".

first = "15"
second = "27"

print(first + second)

Exercise 3: Swap the values

Swap the values of cup_a and cup_b so the program prints coffee tea.

cup_a = "tea"
cup_b = "coffee"

# swap here

print(cup_a, cup_b)

Exercise 4: Check the types

Print the type of each value: 3, 3.0, "3" and 3 > 2. Can you predict each one before running?

# print the type of each value

Summary

• A variable is a name for a value, created with =.
• Common types: int, float, str, bool, None. Check one with type().
• A variable can hold any type of value, and can switch to a different type later.
• Convert with int(), float() and str(). Text and numbers don't mix without converting.
• Use f-strings like f"{name} is {age}" to build readable output.