def photo_url(self):
        """获取用户头像图片地址.

        :return: 用户头像url
        :rtype: str
        """
        if self.url is not None:
            if self.soup is not None:
                img = self.soup.find('img', class_='Avatar Avatar--l')['src']
                return img.replace('_l', '_r')
            else:
                assert (self.card is not None)
                return PROTOCOL + self.card.img['src'].replace('_xs', '_r')
        else:
            return 'http://pic1.zhimg.com/da8e974dc_r.jpg'
获取用户头像图片地址.

        :return: 用户头像url
        :rtype: str