docs: def is_valid(self):
          # type: () -> Union[bool, None]
          """Determine if the value in the property is valid.
  
          If the value of the property is validated as 'valid', than returns a True, otherwise a False.
          When no validators are configured, returns a None. It checks against all configured validators
          and returns a single boolean outcome.
  
          :returns: True when the :ivar:`value` is valid
          :rtype: bool or None
          """
          if not hasattr(self, '_validators'):
              return None
          else:
              self.validate(reason=False)
              if all([vr is None for vr in self._validation_results]):
                  return None
              else:
                  return all(self._validation_results)