Здесь, как len (покупка) переоценена __len __ (self)?

Question

class Purchase:
    def __init__(self, basket, buyer):
        self.basket = list(basket)
        self.buyer = buyer

    def __len__(self):
        return len(self.basket)

purchase = Purchase([1, 2, 3], 'Python')

print(len(purchase))