gistfile1.py

import urllib
from lxml import etree

parl_members_xml = etree.parse(urllib.urlopen('http://data.parliament.uk/membersdataplatform/services/mnis/members/query/House=Commons%7CIsEligible=true/'))
parl_members = parl_members_xml.getroot()

print etree.tostring(parl_members.find('Member[@Member_Id="638"]'))

for member in parl_members:
    if member.attrib['Member_Id'] == '638':
         print etree.tostring(member)