使用 while 循环不断询问他们的输入，直到您收到您认为有效的内容:

Use a while loop to keep asking them for input until you receive something you consider valid:

shift = 0
while 1 > shift or 26 < shift:
    try:
        # Swap raw_input for input in Python 3.x
        shift = int(raw_input("Please enter your shift (1 - 26) : "))
    except ValueError:
        # Remember, print is a function in 3.x
        print "That wasn't an integer :("

您还需要在 int() 调用周围有一个 try-except 块，以防您收到 ValueError (例如，如果他们输入 a).

You'll also want to have a try-except block around the int() call, in case you get a ValueError (if they type a for example).

请注意，如果您使用 Python 2.x，您需要使用 raw_input() 而不是 input().后者将尝试将输入解释为 Python 代码 - 这可能非常糟糕.

Note if you use Python 2.x, you'll want to use raw_input() instead of input(). The latter will attempt to interpret the input as Python code - that can potentially be very bad.