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

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)