Return the layout placeholder this slide placeholder inherits from.
        Not to be confused with an instance of |BasePlaceholder|
        (necessarily).

def _base_placeholder(self):
        """
        Return the layout placeholder this slide placeholder inherits from.
        Not to be confused with an instance of |BasePlaceholder|
        (necessarily).
        """
        layout, idx = self.part.slide_layout, self._element.ph_idx
        return layout.placeholders.get(idx=idx)