尝试/例外错误Python？

Question

The output of my code is supposed to look like:

Encipher:

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.

Decipher:

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.

So far I have this and I keep getting invalid syntax. I am confused on how to be able to type an answer in the output. It's supposed to be try/except with a while loop because it is a school assignment.

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

You don't absolutely need to use a try/except, but if you want to try to do the conversion to integer right then and there, you could. The main problem is that you don't have an except block anywhere.

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"

Answer 2

You're not using a try-except correctly. The idea is to try a chunk of code; but if it runs into an error/exception, do something else.

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"

In your script you are trying to take in an int that is in an array and you are handling a scenario in which it is not in your pre-defined list of options. However, you are not doing anything if their is an exception to that chunk of code.

To utilize the try-except do the following:

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