Parameters and arguments

Functions become really useful when you can give them information to work with. The names in the definition are parameters; the values you pass when calling are arguments.

def greet(name):          # name is a parameter
    print(f"Hello, {name}!")

greet("Asha")             # "Asha" is an argument
greet("Leo")

Several parameters

def describe_pet(animal, name):
    print(f"I have a {animal} called {name}.")

describe_pet("dog", "Rex")
describe_pet("cat", "Mittens")

Arguments are matched to parameters by position: the first argument goes to the first parameter, and so on. Swap them and you get nonsense:

def describe_pet(animal, name):
    print(f"I have a {animal} called {name}.")

describe_pet("Rex", "dog")

The right number of arguments

Every parameter needs an argument (unless it has a default, coming up soon):

def add(a, b):
    print(a + b)

add(2)

Arguments can be any expression

def show_total(price, quantity):
    print(f"Total: {price * quantity:.2f}")

base = 40
show_total(base * 1.18, 3)
show_total(round(19.999, 2), len(["a", "b"]))

Passing a list

When you pass a list, the function receives the same list, not a copy. If the function changes it, the caller sees the change:

def add_task(tasks, task):
    tasks.append(task)

my_tasks = ["email"]
add_task(my_tasks, "call Mei")
print(my_tasks)

This is sometimes exactly what you want. When it isn't, pass a copy (add_task(my_tasks.copy(), ...)) or have the function build and return a new list.

Numbers and strings behave differently because they are immutable: a function cannot change the caller's number, it can only make a new one.

def double(n):
    n = n * 2        # makes a new number; the caller's variable is untouched
    print("inside:", n)

x = 5
double(x)
print("outside:", x)