Internal method to check if assigned value is None while it is required.
        Exception is thrown if ``True``

def _check_required(self, value):
        '''
        Internal method to check if assigned value is None while it is required.
        Exception is thrown if ``True``
        '''
        if value is None and self._required:
            err_msg = self._errors['required'].format(self.__class__.__name__, self.name)
            if self.container_model:
                err_msg += self._errors['required_extra'].format(self.container_model.__name__)
            raise ValueError(err_msg)