python - 如何在输入无效之前调用函数？

Question

我是 python 的新手，我想知道如何在用户提供无效输入之前调用函数。这是一个代码示例:

start = input("For sum of squares, type 'squares'. For sum of cubes, type 'cubes'. "
              "\nIf you would like to raise a number to something other than 'squares' or 'cubes', type 'power'. "
              "\nIf you would like to exit, type 'exit':")

def main_function(start):
    while start.lower() != "exit":
        if start.lower() in "squares":
            initial = input("What is the initial constant for the sum of the squares: ")
            terms = input("Number of terms: ")

        if start.lower() in "cubes":
            initial = input("What is the initial constant for the the sum of the cubes: ")
            terms = input("Number of terms: ")



           if start.lower() in "power":
                initial = input("What is the initial constant for the the sum: ")
                terms = input("Number of terms: ")

        else:
            print("Program halted normally.")
            quit()

main_function(start)

我想让它做的是在用户输入正确输入时重新提示“开始”，然后让它再次运行该函数。我曾尝试将“start”放在“else”语句上方和下方的函数中，但它从不接受新输入。

Answer 1

我会这样做，在方法中定义开始输入并在循环内调用它，当它等于 "exit" 时而不是中断循环。

也可以使用 elif，这样如果第一个条件语句为 True 则您将不会检查其他条件语句，当然除非这是您想要的。

def get_start_input():
    return input("For sum of squares, type 'squares'. For sum of cubes, type 'cubes'. "
              "\nIf you would like to raise a number to something other than 'squares' or 'cubes', type 'power'. "
              "\nIf you would like to exit, type 'exit':")

def main_function():
    while True:
        start = get_start_input()
        if start.lower() == "squares":
            initial = input("What is the initial constant for the sum of the squares: ")
            terms = input("Number of terms: ")

        elif start.lower() == "cubes":
            initial = input("What is the initial constant for the the sum of the cubes: ")
            terms = input("Number of terms: ")

        elif start.lower() == "power":
            initial = input("What is the initial constant for the the sum: ")
            terms = input("Number of terms: ")

        elif start.lower() == "exit":
            print("Program halted normally.")
            break

main_function()

编辑:

正如 dawg 在评论中所写，最好在此处使用 == 而不是 in 因为您可以有多个匹配项和含糊不清的含义。