This is an example Python script that showcases GeoNetwork authentication, session handling, and CSW transactions. 

#!/usr/bin/env python
# this is gn_csw_transaction_example.py

"""
Example CSW 2.0.2 transaction with authentication Python script for GeoNetwork
Tested with GeoNetwork 2.4.1 on Windows

Python 2.6
Wolfgang Grunberg
Arizona Geological Survey
10/20/2009
"""

# Library Imports
import urllib
import urllib2
import cookielib

# module globals and constants
__author__ = "Wolfgang Grunberg"
__credits__ = ["Wolfgang Grunberg", "the Internets"]
__email__ = "wgrunberg@azgs.az.gov"
__status__ = "Prototype"                  # "Prototype", "Development", or "Production"

# GeoNetwork constants
gn_username = "admin"
gn_password = "admin"
gn_baseURL = "http://localhost:8080"
gn_loginURI = "/geonetwork/srv/en/xml.user.login"
gn_logoutURI = "/geonetwork/srv/en/xml.user.logout"
gn_cswURI = "/geonetwork/srv/en/csw"


def gn_csw_transaction():
    """
    Execute GeoNetwork authentication and CSW transaction post
    """
    # HTTP header for authentication 
    header_urlencode = {"Content-type": "application/x-www-form-urlencoded", "Accept": "text/plain"}
    # HTTP header for CSW request
    header_xml = {"Content-type": "application/xml", "Accept": "text/plain"}
    # authentication Post parameters
    post_parameters = urllib.urlencode({"username": gn_username, "password": gn_password})
    
    # Sample CSW transactions
    xml_request = "\
            "
    
    # log in URL
    url_in = gn_baseURL+gn_loginURI
    # log out URL
    url_out = gn_baseURL+gn_logoutURI
    # CSW URL
    url_csw = gn_baseURL+gn_cswURI
    
    
    # first, always log out 
    request = urllib2.Request(url_out)
    response = urllib2.urlopen(request)
    #print response.read()       # debug
    
    # send authentication request
    request = urllib2.Request(url_in, post_parameters, header_urlencode)
    response = urllib2.urlopen(request)
    # a basic memory-only cookie jar instance
    cookies = cookielib.CookieJar()
    cookies.extract_cookies(response,request)
    cookie_handler= urllib2.HTTPCookieProcessor( cookies )
    # a redirect handler
    redirect_handler= urllib2.HTTPRedirectHandler()
    # save cookie and redirect handler for future HTTP Posts
    opener = urllib2.build_opener(redirect_handler,cookie_handler)
    
    # CSW request
    request = urllib2.Request(url_csw, xml_request2, header_xml)
    response = opener.open(request)
    # CSW respons
    xml_response = response.read()
    print xml_response  # debug
    
    # Do something with the response. For example:
    #xmldoc = minidom.parseString(xml_response)
    #for node in xmldoc.getElementsByTagName('ows:ExceptionText'):       # display  value(s)
    #    print "    EXCEPTION: "+node.firstChild.nodeValue
    #xmldoc.unlink()     # cleanup DOM for improved performance
    
    # more CSW requests if desired
    
    # Last, always log out
    request = urllib2.Request(url_out)
    response = opener.open(request)
    #print response.read()       # debug
    
    
if __name__=="__main__":
    gn_csw_transaction()

Files: 

gn_csw_transaction_example.py_.txt