Defining a function

def square(x):
    return x ** 2

print(square(5))

def starts a function definition; return sends a value back to the caller. A function with no return statement returns None.

Default arguments

def greet(name, greeting="Hello"):
    return greeting + ", " + name + "!"

print(greet("Ada"))
print(greet("Ada", "Welcome"))

Lists

A list is an ordered, changeable collection, written in square brackets:

fruits = ["apple", "banana", "cherry"]
print(fruits[0])       # apple -- indexing starts at 0
print(fruits[-1])      # cherry -- -1 is the last element
print(len(fruits))     # 3
fruits.append("date")  # adds to the end

Looping over a list

for fruit in fruits:
    print(fruit)

Why This Matters for Your Exams

Functions let you write a calculation once and reuse it everywhere -- the same principle behind not repeating yourself in any language. Get comfortable with return vs print: a function that prints its answer instead of returning it can't have that answer used in further calculations.

Try it yourself