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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_grades(self):
 """Gets the grades in this system ranked from highest to lowest. return: (osid.grading.GradeList) - the list of grades raise: IllegalState - ``is_based_on_grades()`` is ``false`` raise: OperationFailed - unable to complete request *compliance: mandatory -- This method must be implemented.* """

        # Implemented from template for osid.repository.Asset.get_asset_contents_template
        return GradeList(
            self._my_map['grades'],
            runtime=self._runtime,
            proxy=self._proxy)