Python变量和函数未定义

Question

为了修复NameError（名称未定义的问题），我使用了像这样的全局关键字，但我不认为这是一个很好的解决方案。我想把代码的最后一部分放到一个函数（main（））中，然后再次使用purchase_item函数发生未定义的错误，有人可以帮我改进这段代码吗？

from RetailItem import RetailItem

类CashRegister：

global Item_List
global Cashier_List
global total

Item_List = [ RetailItem("Item 1","Jacket", 12, 59.95),
              RetailItem("Item 2", "Designer Jeans", 40, 34.95),
              RetailItem("Item 3", "Shirt", 20, 24.95) ]
Cashier_List = []
total = 0

def purchase_item(RetailItem):
    global Cashier_List
    Cashier_List.append(RetailItem)

def get_total():
    global Cashier_List
    global total
    for o in Cashier_List:
        total += o.getPrice()
    return total

def show_items():
    global Cashier_List
    for o in Cashier_List:
        print(o)
def clear():
    global Cashier_List
    del Cashier_List[:]


print("Here are available items:")
for o in Item_List:
    print(o)
while True:
    x = input("Select items by number to buy or enter \"n\" to finish shopping: ")
    if x != "n":
        try:
            purchase_item(Item_List[int(x)-1])
            print("ADDED!")
        except ValueError:
            print("Invalid number, try again!")
        except IndexError:
            print("Invalid number, try again!")
    else:
        break
print("......The total price is ${:0.2f}\n......This is your checkout items:".format(get_total()))
show_items()