while 条件:
pass
实例：
#!/usr/bin/env python
#coding:utf-8
while True:
username = raw_input('请输入用户名：')
passwd = raw_input('请输入密码：')
if username == "gaojunsheng" and passwd == "sheng*1994":
print "登陆成功，欢迎使用"
break
else:
print "密码错误，请重新输入"
