python中为什么self.number_served下面就是错的❌

class Restaurant:
    def __init__(self, restaurant_name, cuisine_type):
        self.restaurant_name = restaurant_name
        self.cuisine_type = cuisine_type
        self.number_served = 0

    def describe_restaurant(self):
        print(f"Our restaurant's name is {self.restaurant_name}.")
        print(f"Our restaurant provides {self.cuisine_type}'s style cuisine.")

    def open_restaurant(self):
        print("Our restaurant is opening now!")

    def get_number_served(self):
        return self.number_served

restaurant = Restaurant('xiaohua', 'Chinese food')
print(f"There have been {restaurant.get_number_served()} people served here.")