odoo开发中，如何从一个module导入另一个module的类

通过扩展 odoo/addons/base/ir 中的ir.qweb类，来扩展实现自定义功能，复制类结构如下：

# -*- coding: utf-8 -*-
import ast
from urlparse import urlparse
from lxml import html

from .qweb import QWeb, Contextifier
from .assetsbundle import AssetsBundle
from lxml import etree
from collections import OrderedDict

from odoo import api, models, tools
from odoo.tools.safe_eval import assert_valid_codeobj, _BUILTINS, _SAFE_OPCODES
from odoo.http import request
from odoo.modules.module import get_resource_path
import json
from time import time

import logging
_logger = logging.getLogger(__name__)


class IrQWebCF(models.AbstractModel, QWeb):
    '''
    继承ir.qweb类，实现自定义属性的渲染输出
    '''
    _name = 'ir.qweb'
    _inherit = 'ir.qweb'

    def _get_field(self, record, field_name, expression, tagName, field_options, options, values):
        field = record._fields[field_name]

        field_options['tagName'] = tagName
        field_options['expression'] = expression
        field_options['type'] = field_options.get('widget', field.type)
        inherit_branding = options.get('inherit_branding',
                                       options.get('inherit_branding_auto') and record.check_access_rights('write', False))
        field_options['inherit_branding'] = inherit_branding
        translate = options.get('edit_translations') and options.get('translatable') and field.translate
        field_options['translate'] = translate

        # field converter
        model = 'ir.qweb.field.' + field_options['type']
        converter = self.env[model] if model in self.env else self.env['ir.qweb.field']

        # get content
        content = converter.record_to_html(record, field_name, field_options)
        attributes = converter.attributes(record, field_name, field_options, values)

        return (attributes, content, inherit_branding or translate)

 运行时会报QWeb、Contextifier、AssetsBundle无法import，只要改成下面这样就可以了

# -*- coding: utf-8 -*-
import ast
from urlparse import urlparse
from lxml import html

#### 下面两行
#from .qweb import QWeb, Contextifier
#from .assetsbundle import AssetsBundle
### 改成
from odoo.addons.base.ir.ir_qweb.qweb import QWeb, Contextifier
from odoo.addons.base.ir.ir_qweb.assetsbundle import AssetsBundle