Python Variables and Data Types

You create a variable by writing a name, an equals sign and a value:

name = "Maya"
age = 14
height = 1.62

The = sign means "store this value under this name", not "is equal to". You can change a variable later by assigning a new value, and the old one is replaced:

score = 10
score = score + 5   # score is now 15

Python works out the type from the value, so you never have to declare it in advance.

• Names can contain letters, digits and underscores, but can't start with a digit.
• Names are case-sensitive: age, Age and AGE are three different variables.
• You can't use Python's own keywords such as if, for or class as names.
• By convention, Python uses snake_case: lowercase words joined by underscores, like total_price or first_name.

Choose names that say what the value means. minutes_left is far easier to understand than m or x2.

Python lets you assign more than one variable in a single line. The values on the right are matched to the names on the left in order:

x, y = 10, 20
first = second = 0     # both names refer to 0
x, y = y, x            # swap: x is now 20, y is now 10

The swap works because Python builds the right-hand side, (20, 10), completely before assigning anything. In many other languages you need a temporary third variable to swap two values.

Python has no special keyword for constants. Instead, programmers write names that should never change in capitals, such as MAX_LIVES = 3 or VAT_RATE = 0.2. Python will still let you reassign them; the capitals are a signal to other people reading the code.

Use type() to check what type a value is: type(3.5) gives <class 'float'>. Notice that "42" in quotes is a string, not a number. The quotes are what make it text.

Python is dynamically typed: the type belongs to the value, not to the variable. The same name can hold a number now and text later.

result = 42
print(type(result))    # <class 'int'>
result = "forty-two"
print(type(result))    # <class 'str'>

This is convenient, but it also means Python won't warn you if you accidentally replace a number with a string. Clear names and consistent types keep programs predictable.

There is one more value you will see often: None. It means "no value yet" or "nothing here". A variable set to None exists, but it does not hold a useful value:

winner = None
if winner is None:
    print("No winner yet")

Check for it with is None, not == None. Functions that don't return anything also give back None, which you'll meet in the functions lesson.

An f-string has an f before the opening quote. Inside it, anything in {curly braces} is replaced by its value:

name = "Maya"
age = 14
print(f"{name} is {age} years old.")

This prints Maya is 14 years old. You can even put calculations inside the braces, such as {age + 1}. Add :.2f to show a number with two decimals: {price:.2f}.

input() shows a message, waits for the user to type something and press Enter, then gives you what they typed as a string, even if they typed a number.

age = input("How old are you? ")
age = int(age)          # turn "14" into 14
print(f"Next year you'll be {age + 1}")

To do maths with typed numbers, convert them with int() for whole numbers or float() for decimals. str() goes the other way and turns a number into text.