Parameters and defaults

Default values and keyword arguments

A parameter can have a default, which makes it optional:

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("Ada")                    # "Hello, Ada!"
greet("Ada", "Good morning")    # "Good morning, Ada!"
greet("Ada", greeting="Hi")     # naming it is clearer

Parameters with defaults must come after those without.

Naming arguments at the call site — greet(name="Ada") — makes calls readable when there are several, and lets you skip past ones you are happy with.

One serious trap: never use a mutable default like def f(items=[]). The list is created once, when the function is defined, and every call that relies on the default shares it. Use None and build inside:

def f(items=None):
    if items is None:
        items = []

Your turn: write format_price(amount, currency="£", decimals=2) returning something like "£9.99".

The mutable default trap

This is worth writing once so it sticks:

def broken(item, basket=[]):     # do not do this
    basket.append(item)
    return basket

broken("a")    # ["a"]
broken("b")    # ["a", "b"]  ← the same list, still there

Your turn: write add_item(item, basket=None) that appends to the given basket, or starts a fresh one when none is supplied — and does not share state between calls.