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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_first_part_id(self, assessment_id):
 """This session implemenation assumes all items are assigned to the first assessment part"""

        if assessment_id not in self._first_part_index:
            self._first_part_index[assessment_id] = get_first_part_id_for_assessment(
                assessment_id,
                runtime=self._runtime,
                proxy=self._proxy,
                create=True,
                bank_id=self._catalog_id)
        return self._first_part_index[assessment_id]