为什么会出现“ NameError：名称'B'/'C'/'V'未定义”？

Question

Question = input("Welcome to the meal chooser program. The base cost is $9.99. Would you like to choose beef, chicken, or the vegetarian option?:")
if Question.casefold() == "beef":
    print("Thank you. Your total cost is","$",'{:.2f}'.format(B))
elif Question.casefold() == "chicken":
    print("Thank you. Your total cost is","$", '{:.2f}'.format(C))
elif Question.casefold() == "vegetarian":
    print("Thank you. Your total cost is","$", '{:.2f}'.format(V))
B = 9.99*1.02
C = 9.99*1.025
V = 9.99*1.03 

每当我运行此命令并输入鸡肉，牛肉或素食主义者时，都会提示我一条消息，提示未定义B，C和V，但它们是定义的。

Answer 1

代码自上而下线性运行（禁止跳跃，如函数调用和类初始化）。因为您在底部初始化变量，所以到达if-else块时，代码尚未创建值。如果将变量放在首位，则代码将正常运行。