Return the attribute value, e.g. 'width' of the base placeholder this
        placeholder inherits from.

def _inherited_value(self, attr_name):
        """
        Return the attribute value, e.g. 'width' of the base placeholder this
        placeholder inherits from.
        """
        base_placeholder = self._base_placeholder
        if base_placeholder is None:
            return None
        inherited_value = getattr(base_placeholder, attr_name)
        return inherited_value