Assertions and defensive checks

Stating what must be true

assert checks something you believe is already true, and raises AssertionError if it is not:

def average(numbers):
    assert len(numbers) > 0, "average of nothing is undefined"
    return sum(numbers) / len(numbers)

The distinction worth holding on to:

• assert catches programmer mistakes — things that should be impossible. Python strips assertions out entirely when run with -O, so never use them to validate input that comes from outside your program.
• raise handles expected bad input — a user typing letters into a number field.

Defensive checks at the top of a function are cheap and turn a confusing failure ten frames deeper into an obvious one right here.

Your turn: write set_temperature(celsius) that raises ValueError below absolute zero (−273.15) and above 1000, and otherwise returns the value rounded to one decimal.