How to join absolute and relative urls?

You should use urlparse.urljoin :

>>> import urlparse
>>> urlparse.urljoin(url1, url2)
'http://127.0.0.1/test1/test4/test6.xml'

With Python 3 (where urlparse is renamed to urllib.parse) you could use it as follow:

>>> import urllib.parse
>>> urllib.parse.urljoin(url1, url2)
'http://127.0.0.1/test1/test4/test6.xml'