Subtle bug in that toppings has a mutable default argument [1].
Sometimes I just want to monkeypatch the list to be immutable in my app.
def make_pizza(crust=THIN, toppings=[], cheese=REGULAR, sauce=TOMATO):
// first call: toppings=["peperoni"]
// second call: toppings=["peperoni", "sausage"]
// third call: toppings=["peperoni", "sausage"]
make_pizza(toppings=["peperoni"])
make_pizza(toppings=["sausage"])
make_pizza()