Python 如何检查字符串是否不等于某个值？

这是我到目前为止问卷的编码。在第6行，如果用户输入的字符集不是1234，我如何使其打印无效

 print ("Welcome, Please enter your Canadian Armed Forces Identification Number")
 answer = raw_input()
 if answer == ("1234"):
     print " Please identify yourself "
 elif answer == "":
     print " Invalid ID "
     print " 3 More attempts before termination of program "
 name= raw_input()

 if answer == ("Silva"):
     print " Welcome, Silva "

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我怎样才能让它重复，直到正确的数字是正确的？如果用户再次输入错误3次，我将如何使程序终止？这是两个不同的问题。我建议你选一本教你所有基本知识的书。好的，非常感谢你

if answer == ("1234"):
    print " Please identify yourself "
else:
    print " Invalid ID "
    print " 3 More attempts before termination of program " name= raw_input()