code: def _add_none_handler(validation_callable,  # type: Callable
                        none_policy           # type: int
                        ):
      # type: (...) -> Callable
      
      if none_policy is NonePolicy.SKIP:
          return _none_accepter(validation_callable)  # accept all None values
  
      elif none_policy is NonePolicy.FAIL:
          return _none_rejecter(validation_callable)  # reject all None values
  
      elif none_policy is NonePolicy.VALIDATE:
          return validation_callable                  # do not handle None specifically, do not wrap
  
      else:
          raise ValueError('Invalid none_policy : ' + str(none_policy))
docs: Adds a wrapper or nothing around the provided validation_callable, depending on the selected policy
  
      :param validation_callable:
      :param none_policy: an int representing the None policy, see NonePolicy
      :return: