<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sign(self, response):
 """Sign a response. I take a L{OpenIDResponse}, create a signature for everything in its L{signed<OpenIDResponse.signed>} list, and return a new copy of the response object with that signature included. @param response: A response to sign. @type response: L{OpenIDResponse} @returns: A signed copy of the response. @returntype: L{OpenIDResponse} """

        signed_response = deepcopy(response)
        assoc_handle = response.request.assoc_handle
        if assoc_handle:
            # normal mode
            # disabling expiration check because even if the association
            # is expired, we still need to know some properties of the
            # association so that we may preserve those properties when
            # creating the fallback association.
            assoc = self.getAssociation(assoc_handle, dumb=False,
                                        checkExpiration=False)

            if not assoc or assoc.expiresIn <= 0:
                # fall back to dumb mode
                signed_response.fields.setArg(
                    OPENID_NS, 'invalidate_handle', assoc_handle)
                assoc_type = assoc and assoc.assoc_type or 'HMAC-SHA1'
                if assoc and assoc.expiresIn <= 0:
                    # now do the clean-up that the disabled checkExpiration
                    # code didn't get to do.
                    self.invalidate(assoc_handle, dumb=False)
                assoc = self.createAssociation(dumb=True, assoc_type=assoc_type)
        else:
            # dumb mode.
            assoc = self.createAssociation(dumb=True)

        try:
            signed_response.fields = assoc.signMessage(signed_response.fields)
        except kvform.KVFormError, err:
            raise EncodingError(response, explanation=str(err))
        return signed_response