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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_assessment_part(self, part_id=None):
 """Gets an AssessmentPart given a part_id. Returns this Section's own part if part_id is None. Make this a private part, so that it doesn't collide with the AssessmentPart.get_assessment_part """

        if part_id is None:
            return self._assessment_part
        if part_id not in self._assessment_parts:
            lookup_session = self._get_assessment_part_lookup_session()
            self._assessment_parts[part_id] = lookup_session.get_assessment_part(part_id)
        return self._assessment_parts[part_id]