<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_by_slot(self, parent_object, slot):
 """ Return a placeholder by key. """

        placeholder = self.parent(parent_object).get(slot=slot)
        placeholder.parent = parent_object  # fill the reverse cache
        return placeholder