Config now looks like this, already removed the post_modification  
Yang service X

key "name";
      leaf name{
        type string;
      }

Python Service X

class ServiceCallbacks(Service):

    @service.create
    def cb_create(self, tctx, root, service, proplist):
        self.log.info('Service create(service=', service._path, ')')

        template = ncs.template.Template(service)
        template_vars = ncs.template.Variables()
        root.services.Y.create(service.X)

Yang service Y

      key name ;
      leaf name {
                type leafref {
                  path "/ncs:services/X:X/X:name";
              }
      }