How to use the signxml.methods.detached function in signxml

    @staticmethod
    def calculate_signature(signed_object_xml):
        """
            Calculate a digital signature for the SignedObject to be sent to the VTN.

        @param signed_object_xml: (xml string) A SignedObject.
        @return: (lxml) A Signature and a SignedObject.
        """
        signed_object_lxml = etree_.fromstring(signed_object_xml)
        signed_object_lxml.set('Id', 'signedObject')
        # Use XMLSigner to create a Signature.
        # Use "detached method": the signature lives alonside the signed object in the XML element tree.
        # Use c14n "exclusive canonicalization": the signature is independent of namespace inclusion/exclusion.
        signer = signxml.XMLSigner(method=signxml.methods.detached,
                                   c14n_algorithm='http://www.w3.org/2001/10/xml-exc-c14n#')
        signature_lxml = signer.sign(signed_object_lxml,
                                     key=open(KEY_FILENAME, 'rb').read(),
                                     cert=open(CERT_FILENAME, 'rb').read(),
                                     key_name='123')
        # This generated Signature lacks the ReplayProtect property described in OpenADR profile spec section 10.6.3.
        return signature_lxml, signed_object_lxml