abc don't play well with private method

[mouadino]

i have a problem here :

i want to make a plugin architecture using abstract base class , this
is my base class :

# -*- coding: utf-8 -*-

import abc


class BASE_Connector:
    """
    Mount point for plugins which refer to actions that can be
performed.

    Plugins implementing this reference should provide the following
methods:

    ========  ========================================================
    connect     the connection method that will initialize a transport
layer

    __login     the login  method to use to login in to the server
                this one should be called from connect directly

    get         this method is use for getting the files from the
remote server

    close       this method is called for closing the connection
suddenly

    quit        this method is more polite to close the connection
    ========  ========================================================

    """
    __metaclass__ = abc.ABCMeta


    @abc.abstractmethod
    def connect(self, host, user="", passwd="", port="",
private_key_file="", timeout=100):
        """ connect to a remote server this function call the __login
           function after the connection is established


        -- host : remote server name
        -- user : the user name in the remote server
               Default : empty string for server that don't need
                     authentication or use private key instead
        -- passwd : password of the user given
               Default : empty string for server that don't need
                     authentication or use private key instead
        -- port : the connection port
               Default : it's known from the protocol
        -- private_key_file : the file containing the private
                           key used special for ssh connection
                 Default : empty string
        -- timeout : the timeout of the connection
                 Default : empty string

        """
        pass

    @abc.abstractmethod
    def __login(self):
        """ private method to login to the remote server .
            it use the user name and the password or the private key
            for that

        """

    @abc.abstractmethod
    def get(self, remote_file_path, local_path="/tmp/
housead_matcher"):
        """ method to download the file from the remote server

        -- remote_file_path : the path of the file in the remote
                          server
        -- local_path : the local path where the file will be
                          save
        """
        pass

    @abc.abstractmethod
    def close(self):
        """ Close the connection unilaterally(from this host)
        """

    @abc.abstractmethod
    def quit(self):
        """ close the connection. This is the “polite” way to close a
          connection
        """


and i define my plugins in a differents module like this :

class FTP_Connector(FTP, Base_Connector) :
    """ this class is for the connection to a ftp server

    """


    def __init__(self):

        FTP.__init__()

    def connect(self, host, port, user, passwd, timeout=""):

        self.connect(host, port, timeout)
        self.___login(user, passwd)


    def ___login(self, username="", password=""):

        self.login(username, password)


    def get(self,  remote_file_path, local_path):

        remote_file_path = os.path.expanduser(remote_file_path)
        local_path =  os.path.expanduser(local_path)

        remote_directory, remote_file_name =
os.path.split(remote_file_path)
        local_file = os.path.join(local_path, remote_file_name)

        self.cwd(remote_directory)
        self.retrlines("RETR " + remote_file_name, lambda s,
w=local_file.write: w(s+"\n"))

    def close(self):

        self.close()

    def quit(self):

        self.quit()



and when i test my code i get this error :
Can't instantiate abstract class FTP_Connector with abstract methods
_BASE_Connector__login

i can understand why. but my problem is why why ? i can't put
"private" method in the ABC class ???