代码的持续改进

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#工厂模式
def createDomTree(htmlStream,type='soup'):
    if type == "soup":
        return tnDomTreeWithSoup(htmlStream)
    
    if type == "lxml":
        return tnDomTreeWithlXml(htmlStream)
    
    return None
    
#外部依赖的接口，它屏蔽了我具体使用的第三方库
class tnDomTree:
    #public
    def __init__(self,htmlStream):
        self.htmlStream = htmlStream
    
    def getLinkList(self):
        return self._getElementByTagName('a')#提取子类共有行为到基类
    
    def getImageList(self):
        return self._getElementByTagName('img')#提取子类共有行为到基类
    
    def elementToString(self,element):
        pass
    
    def getAttrValueOfElement(self,element,attName):
        pass
    
    #private
    def _getElementByTagName(self,tagName):#子类实现该方法
        pass

#使用BeautifulSoup的类
from BeautifulSoup import BeautifulSoup
class tnDomTreeWithSoup(tnDomTree):
    def __init__(self,htmlStream):
        tnDomTree.__init__(self,htmlStream)
        self._tree = BeautifulSoup(self.htmlStream)
        
    def _getElementByTagName(self,tagName):
        return self._tree.findAll(tagName)
    
    def elementToString(self,element):
        return str(element)
    
    def getAttrValueOfElement(self,element,attName):
        if hasattr(element,attName):
            return str(element[attName])
        else:
            return ""

#使用lxml
from lxml.html import tostring
from lxml.html.soupparser import fromstring
class tnDomTreeWithlXml(tnDomTree):
    def __init__(self,htmlStream):
        tnDomTree.__init__(self,htmlStream)
        self._tree = fromstring(self.htmlStream)
        
    def _getElementByTagName(self,tagName):
        list = []
        for i in self._tree.iter():
            if i.tag == tagName:
                list.append(i)
        return list
    
    def elementToString(self,element):
        return tostring(element)
    
    def getAttrValueOfElement(self,element,attName):
        if attName in set(element.keys()):
            return str(element.attrib[attName])
        else:
            return ""

if __name__ == "__main__":
    s1 = '''
    <p>BEIAI</p> <p><img src="/attachment/28" alt=""></p>
    '''
    print s1
    domtree = createDomTree(s1,'soup')
    list = domtree.getImageList()
    for i in list:
        print domtree.getAttrValueOfElement(i,'src')
        print domtree.elementToString(i)

为什么能抽出一个公用的接口出来？本质上是因为这些东西有相似性，而具体到代码的设计，就应该设计成和其他类相似的API，否则类的设计就是不对的