code: async def prepare_container(self, size, container, elem_type=None):
          
          if not self.writing:
              if container is None:
                  return gen_elem_array(size, elem_type)
  
              fvalue = get_elem(container)
              if fvalue is None:
                  fvalue = []
              fvalue += gen_elem_array(max(0, size - len(fvalue)), elem_type)
              set_elem(container, fvalue)
              return fvalue
docs: Prepares container for serialization
          :param size:
          :param container:
          :return: