def _auth(self, username, password) :
        """ Authentificate user and store token """
        self.user_credentials = self.call('auth', {'username': username, 'password': password})
        
        if 'error' in self.user_credentials:
            raise T411Exception('Error while fetching authentication token: %s'\
                    % self.user_credentials['error'])
        # Create or update user file
        user_data = dumps({'uid': '%s' % self.user_credentials['uid'], 'token': '%s' % self.user_credentials['token']})
        with open(USER_CREDENTIALS_FILE, 'w') as user_cred_file:
            user_cred_file.write(user_data)
        return True