一种相当常见的方法是使用而True 循环将无限期运行， break 输入有效时退出循环的语句：

A fairly common way to do this is to use a while True loop that will run indefinitely, with break statements to exit the loop when the input is valid:

print "pick a number, 1 or 2"
while True:
    a = int(raw_input("> ")
    if a == 1:
        print "this"
        break
    if a == 2:
        print "that"
        break
    print "you have made an invalid choice, try again."

此处还有一个很好的方法来限制重试，例如：

There is also a nice way here to restrict the number of retries, for example:

print "pick a number, 1 or 2"
for retry in range(5):
    a = int(raw_input("> ")
    if a == 1:
        print "this"
        break
    if a == 2:
        print "that"
        break
    print "you have made an invalid choice, try again."
else:
    print "you keep making invalid choices, exiting."
    sys.exit(1)