soap xml with namespaces using lxml and python

soap_envelope.py

from lxml import etree
from lxml.builder import ElementMaker

class NewRpcMessageEncoder(object):
    
    def _envelope(self, request_id, hold_requests):
        SOAP_ENV = "http://schemas.xmlsoap.org/soap/envelope/"
        SOAP_ENC = "http://schemas.xmlsoap.org/soap/encoding/"
        XSD = "http://www.w3.org/2001/XMLSchema"
        XSI = "http://www.w3.org/2001/XMLSchema-instance"
        CWMP = "urn:dslforum-org:cwmp-1-2"

        soap_enc = ElementMaker(namespace=SOAP_ENC)
        xsd = ElementMaker(namespace=XSD)
        xsi = ElementMaker(namespace=XSI)
        cwmp = ElementMaker(namespace=CWMP)

        soap = ElementMaker(namespace=SOAP_ENV,
                nsmap={
                       'xsi' : XSI,
                       'soap' : SOAP_ENV,
                       'soap-enc' : SOAP_ENC,
                       'xsd' : XSD,
                       'cwmp' : CWMP})

        SOAP_MUST_UNDERSTAND = '{{{0}}}mustUnderstand'.format(SOAP_ENV)
        MUST_UNDERSTAND = '1'

        body=()
        xml = soap.Envelope(
                soap.Header(
                    cwmp.ID(str(request_id), {SOAP_MUST_UNDERSTAND : MUST_UNDERSTAND}),
                    cwmp.HoldRequests('0', {SOAP_MUST_UNDERSTAND : MUST_UNDERSTAND})
                    ),
                soap.Body(body)
                )

    def __str__(self):
        xml_declaration = """<?xml version="1.0" encoding="utf-8"?>\n""" 
        # xml_declaration add's back slash to version an encoding quotes
        return xml_declaration + etree.tostring(xml, encoding='utf-8' , xml_declaration=False, pretty_print=True)