break:跳出循环 ，continue:停止当前循环，进入下一次循环，但为跳出循环。
passwdList=["123","456"]
valid = False
count = 3
while count > 0:
input = raw_input("enter password: ")
for eachPasswd in passwdList:
if input == eachPasswd:
valid = True
break #跳出for循环，继续while循环
if not valid: # (or valid == 0)
print "invalid input"
count -= 1
continue #进入下一次while循环
else:
break #跳出while循环
