python怎么获取xml内容

python如何获取xml内容
s= " " " <?xml   version   =   "1.0 "?>
<book>
      <title> titlename </title>
      <date> one   date </date>
      <author>
            <firstname> aaaa </firstname>
            <lastname> bbbb </lastname>
      </author>
</book> " " "

如何获取aaaa和bbbb这两个字符串？

------解决方案--------------------
或者采用dom的方法
from xml.dom.minidom import parseString

dom = parseString(s)

def getElemValue(tagName):
for fn in dom.getElementsByTagName(tagName):
for node in fn.childNodes:
if node.nodeType == node.TEXT_NODE:
print tagName, node.data

getElemValue( "firstname ")
getElemValue( "lastname ")