如何检查用户输入的操作数是否有效？（蟒蛇）

问：

我正在编写代码以在 python 中实现一个简单的计算器。我正在尝试检查用户是否输入了有效的操作数。我输入的代码如下。

calc = False
while calc == False:
    firstNum = 0
    operand = ''
    secondNum = 0
    firstNum = input('Please enter a number: ')
    if firstNum.isdigit() == False:
        print('Please enter a number.')
        continue
    else:
        firstNum = int(firstNum)
    operand = input('Please enter an operand (+, -, *, or /): ')
    if operand != '/' or operand != '*' or operand != '+' or operand != '-':
        print('Please enter a valid operand.')
        continue
    secondNum = input('Please enter another number: ')
    if secondNum.isdigit() == False:
        print('Please enter a number.')
        continue
    secondNum = int(secondNum)
    if secondNum == 0 and operand == '/':
        print('You cannot divide by 0. Please try again.')
        continue
    if operand == '+':
        calc = True
        print(f'{firstNum} + {secondNum} = {firstNum + secondNum}')
    elif operand == '*':
        calc = True
        print(f'{firstNum} * {secondNum} = {firstNum * secondNum}')
    elif operand == '-':
        calc = True
        print(f'{firstNum} - {secondNum} = {firstNum - secondNum}')
    else:
        calc = True
        print(f'{firstNum} / {secondNum} = {firstNum / secondNum}')

calc = False
while calc == False:
    firstNum = 0
    operand = ''
    secondNum = 0
    listOfOperands = ['+', '-', '/', '*']
    firstNum = input('Please enter a number: ')
    if firstNum.isdigit() == False:
        print('Please enter a number.')
        continue
    else:
        firstNum = int(firstNum)
    operand = input('Please enter an operand (+, -, *, or /): ')
    if operand != listOfOperands:
        print('Please enter a valid operand.')
        continue
    secondNum = input('Please enter another number: ')
    if secondNum.isdigit() == False:
        print('Please enter a number.')
        continue
    secondNum = int(secondNum)
    if secondNum == 0 and operand == '/':
        print('You cannot divide by 0. Please try again.')
        continue
    if operand == '+':
        calc = True
        print(f'{firstNum} + {secondNum} = {firstNum + secondNum}')
    elif operand == '*':
        calc = True
        print(f'{firstNum} * {secondNum} = {firstNum * secondNum}')
    elif operand == '-':
        calc = True
        print(f'{firstNum} - {secondNum} = {firstNum - secondNum}')
    else:
        calc = True
        print(f'{firstNum} / {secondNum} = {firstNum / secondNum}')

代码的当前版本始终拒绝操作数并返回到 while 循环的顶部。如果用户没有输入 +、-、* 或 /，我想返回到循环的顶部，如果他们输入了四个字符之一，我想继续完成循环。