Nested if statements push the real work to the right, and an else can end up a screen away from its condition. Handle the special cases first, each with its own return or raise, and the main path runs unindented at the end.

def shipping(order):
    if not order["items"]:
        return 0
    if order["country"] != "UK":
        return 20
    if order["total"] >= 50:
        return 0
    return 5

order = {"items": ["mug"],
         "country": "UK", "total": 30}
print(shipping(order))

Each guard answers one question and gets out of the way. By the last line, the reader knows every special case has been dealt with.

• Invalid input goes first, and raises.
• Then the cases with a fixed answer.
• Then the general rule.

Guards also make a new rule easy to add: it is one more guard, not another level of nesting.

Rewrite ticket_price(age, day, member) with guard clauses, keeping every price and error. Then add a new rule: children under 3 get in free (price 0).

The explanation opens here once every check passes.