<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def notes_master_part(self):
 """ Return the |NotesMasterPart| object for this presentation. If the presentation does not have a notes master, one is created from a default template. The same single instance is returned on each call. """

        try:
            return self.part_related_by(RT.NOTES_MASTER)
        except KeyError:
            notes_master_part = NotesMasterPart.create_default(self.package)
            self.relate_to(notes_master_part, RT.NOTES_MASTER)
            return notes_master_part