把一段代码从python3能用的改为2.7.8能用的

User={}
List=open("USER.txt","r")
for _list in List:
_List=_list.split()
User[_List[0]]=_list[2]
List.close()
List1=open("USER.txt","a")
print ("Welcome")
print ("是否需要注册？（Y/N）:")
a1=True
while a1:
_a=input("请输入选择：")
if _a not in ["Y","y","N","n"]:
print ("WARRING:请输入Y或者N进行选择")
continue
else:
a1=False
a1=True
if _a in ["Y","y"]:
print ("----------------注册界面----------------")
while a1:
a_id=input("请输入ID：")
if a_id in User:
print ("ID被占用，请重新输入！")
continue
else:
while a1:
a_pass=input("请输入密码：")
a_pass1=input("请再次输入密码：")
if a_pass!=a_pass1:
print ("两次密码不正确！重新输入")
continue
else:

List1.write(a_id)
List1.write("\t")
List1.write(a_pass)
List1.write("\n")
print ("注册完毕，将转入登录界面！")
只能用2.7.8系统