def METHOD_NAME(self):
    """
    This method was overridden because the AUTH_LDAP_USER_SEARCH
    configuration in the settings.py file
    is configured with a `lambda` problem value
    """
    user_search_union = [
        LDAPSearch(
            USER_SEARCH, ldap.SCOPE_SUBTREE,
            settings.AUTH_LDAP_SEARCH_FILTER
        )
        for USER_SEARCH in str(settings.AUTH_LDAP_SEARCH_OU).split("|")
    ]
    search = LDAPSearchUnion(*user_search_union)
    if search is None:
        raise ImproperlyConfigured(
            'AUTH_LDAP_USER_SEARCH must be an LDAPSearch instance.'
        )
    results = search.execute(self.connection, {'user': self._username})
    if results is not None and len(results) == 1:
        (user_dn, self._user_attrs) = next(iter(results))
    else:
        # 解决直接配置DC域，用户认证失败的问题(库不能从整棵树中搜索)
        user_dn = self._search_for_user_dn_from_ldap_util()
        if user_dn is None:
            self._user_dn = None
            self._user_attrs = None
        else:
            self._user_dn = user_dn
            self._user_attrs = self._load_user_attrs()
    return user_dn