'Preview update for existing stack with a new template/parameters.'

@util.identified_stack
def preview_update(self, req, identity, body):

    data = InstantiationData(body)
    args = self.prepare_args(data)
    show_nested = self._param_show_nested(req)
    if (show_nested is not None):
        args[rpc_api.PARAM_SHOW_NESTED] = show_nested
    changes = self.rpc_client.preview_update_stack(req.context, identity, data.template(), data.environment(), data.files(), args, environment_files=data.environment_files())
    return {'resource_changes': changes}
