尝试/排除错误 Python？

Question

我的代码的输出应该是这样的：

加密：

Enter 1 to encipher or 2 to decipher: 1
Enter text you wish to encipher: My dog has fleas.
Enter the number of characters to shift: 7
The encrypted text is: Fr whz atl yextl.

解密：

Enter 1 to encipher or 2 to decipher: 2
Enter text you wish to decipher: Fr whz atl yextl.
The most likely shift is: 7
My dog has fleas.

到目前为止，我有这个并且我不断收到无效的语法。我对如何在输出中输入答案感到困惑。它应该是尝试/除一个while循环，因为它是一个学校作业。

while True: 
    try: 
        num = int(raw_input('Enter 1 or 2:'))
        if num in [1,2]:
            break
        print "You have to enter 1 or 2, try again"

    if (num == 1):
        num = int(raw_input('Enter a number:'))
        num = int(raw_input('encipher'))
        print "Enter text to encipher"
        print "Enter the number of characters you want to shift" 


    elif (num == 2):
        num = int(raw_input('Enter a number:'))
        num = int(raw_input('decipher'))
        print "Enter text to decipher"
        print "Enter the number of characters you want to shift"

Answer 1

您没有正确使用try-except。 这个想法是尝试一段代码；但如果遇到错误/异常，请执行其他操作。

while True: 
    try: 
        num = int(raw_input('Enter 1 or 2:'))
        if num in [1,2]:
            break
        print "You have to enter 1 or 2, try again"

在您的脚本中，您尝试接收一个数组中的 int，并且您正在处理它不在您预定义的选项列表中的场景。但是，如果它们是该代码块的异常，则您不会做任何事情。

要使用try-except，请执行以下操作：

while True: 
    try: 
        num = int(raw_input('Enter 1 or 2:'))
        if num in [1,2]:
            break
        print "You have to enter 1 or 2, try again"
    except Exception, e:
        print e

Answer 2

您不一定需要使用 try/except，但如果您想尝试立即转换为整数，则可以。主要问题是您在任何地方都没有 except 块。

while True:
    try:
        num = int(raw_input('Enter 1 or 2:'))
        if num in [1,2]:
            break
    except ValueError as e:
        print "You didn't enter a number.  Try again"