字符串格式自定义对象

我有一个基本的购物车。我正在尝试学习字符串格式——自定义对象: https://pyformat.info/#custom_1

我想做的是:

print("Your cart contains: {0:short}".format(cart))

应返回:

Your cart contains: cucumbers, tissues, tomatoes, toothpaste

我的 总体安排 方法是错误的。我不断得到:

Traceback (most recent call last):
  File "C:/Users/Mark/PycharmProjects/main/main.py", line 54, in <module>
    print("Your cart contains: {0:short}".format(cart))
TypeError: __format__ must return a str, not NoneType

我的代码:

class Item(object):
    def __init__(self, unq_id, name, price, qty, measure):
        self.unq_id = unq_id
        self.product_name = name
        self.price = price
        self.qty = qty
        self.measure = measure


class Cart(object):
    def __init__(self):
        self.content = dict()

    def __format__(self, format_type):
        if format == 'short':
            return ', '.join(item.name for item in self.content)

    def add(self, item):
        if item.unq_id not in self.content:
            self.content.update({item.unq_id: item})
            return
        for k, v in self.content.get(item.unq_id).items():
            if k == 'unq_id':
                continue
            elif k == 'qty':
                total_qty = v.qty + item.qty
                if total_qty:
                    v.qty = total_qty
                    continue
                self.remove_item(k)
            else:
                v[k] = item[k]

    def get_total(self):
        return sum([v.price * v.qty for _, v in self.content.items()])

    def get_num_items(self):
        return sum([v.qty for _, v in self.content.items()])

    def remove_item(self, key):
        self.content.pop(key)


if __name__ == '__main__':
    cart = Cart()
    cart.add(Item(1, "Cucumbers", 1., 1, 'kg'))
    cart.add(Item(2, "Tissues", 1., 2, 'dozen'))
    cart.add(Item(3, "Tomatoes", 1., 5, 'box'))
    cart.add(Item(4, "Toothpaste", 1., 5, 'box'))
    # print("You have %i items in your cart for a total of $%.02f" % (cart.get_num_items(), cart.get_total()))
    print("Your cart contains: {0:short}".format(cart))
    # cart.remove_item(1)
    # print("You have %i items in your cart for a total of $%.02f" % (cart.get_num_items(), cart.get_total()))