Python 2.7避免循环导入

Question

我有两个文件：Server.py和DBManger.py 我通过启动server.py启动项目，并在get_answer函数中的客户端请求中，我在Login类中调用DBManager函数。
如果登录被批准，我想调用静态add_online_user函数 现在，当我尝试这样做时，会出现此错误：

Traceback (most recent call last):
  File "K:/Python/Spotify/Server/Server.py", line 6, in <module>
    import DBManager
  File "K:\Python\Spotify\Server\DBManager.py", line 4, in <module>
    import Server
  File "K:\Python\Spotify\Server\Server.py", line 156, in <module>
    db = DBManager.DataManager()
AttributeError: 'module' object has no attribute 'DataManager'

Server.py代码：

import DBManager

def get_answer(data, sock):
    global db, online_users
    actions = {"LG": lambda *x: db.login(sock, *x),
               }
    param = json.loads(data[3:])
    param = [str(x) if type(x) == unicode else x for x in param]
    return json.dumps(actions[data[:2]](*param))

@staticmethod
def add_online_user(socket, username):
    global online_users
    if socket not in online_users:
        online_users.update({socket: username})
        print str(socket.getsockname()) + "is now login as: " + username

db = DBManager.DataManager()

DBManager.py代码：

import Server

class DataManager:
   def login(self, sock, username, password):
        # doing someting...
        if approved:
                Server.add_online_user(sock, username)
                return True
        return False

问题：

如何避免此循环导入？我似乎无法单独解决它......