如何正确使用try和except？

Question

我正在尝试编写一个计算器，使用运算符 try 和 except。我的目标是让用户只输入 int （用于操作数）和 str （用于运算符）变量，如果其中一个等于“=”，它会给出结果。不幸的是，我的代码无法正常工作：它接受不正确的变量并仅在输入其他变量后才给出警告字符串。对不起我的英语不好，这不是我的母语

我的密码是：

result = None
operand = None
operator = None
wait_for_number = True

while True:
    try:
        operand1 = int(input("Enter 1st operand: "))
    except ValueError:
        print("Enter only numbers please")
        continue
    
    try:
        operator = str(input("Enter operator (+ -  * /): "))
    except IndentationError:
        print("Please enter only (+ -  * /) :")
        continue
    try:
        operand2 = int(input("Enter 2st operand: "))
    except ValueError:
        print("Enter only numbers please")
        continue

    if wait_for_number == True:
        try:
            if operator == "+":
                result = operand1 + operand2
            if operator == "-":
                result = operand1 - operand2
            if operator == "*":
                result = operand1 * operand2
            if operator == "/":
                result = operand1 / operand2
            elif operator == "=":
                break
        except ZeroDivisionError:
            break
    if operand1 == "=" or operand2 == "=" or operator == "=": 
        print(result) 

我的目标是得到这样的结果：

>>> 3
>>> +
>>> 3
>>> 2
2 is not '+' or '-' or '/' or '*'. Try again
>>> -
>>> -
'-' is not a number. Try again.
>>> 5
>>> *
>>> 3
>>> =
Result: 3.0

我尝试使用休息时间，添加

operand1 = None / operand2 = None / operator = None

。我预计它会让用户只输入需要的变量，但它没有帮助。

我该怎么办？你能告诉我我的代码有什么问题吗？

Answer 1

希望这对您有进一步的帮助：

# read first operand until correct value is entered
operand1 = None
while operand1 == None:
    try:
        operand1 = int(input("Enter 1st operand: "))
    except Exception as ex:
        continue

# read operator until correct value is entered
operator = None
while operator == None:
    try:
        operator = str(input("Enter 1st operand: "))

        if operator not in ['+', '-', '*', '/']:
            raise ValueError('')
    except Exception as ex:
        continue

# read second operand until its a number
operand2 = None
while operand2 == None:
    try:
        operand2 = int(input("Enter 2st operand: "))
    except Exception as ex:
        continue

# make calculation
if operator == "+":
    result = operand1 + operand2
elif operator == "-":
    result = operand1 - operand2
elif operator == "*":
    result = operand1 * operand2
elif operator == "/":
    result = operand1 / operand2

print(result)

您想尝试捕获每个值并循环直到每个值正确。你也可以像你一样把它作为一个整体来做，但是每次你犯错的时候，比如为操作数 2 输入“a”，你的程序会再次期待整个公式。

您可以在

continue

要求新值的地方打印消息，指定它必须是数字或运算符。

祝你好运！