def METHOD_NAME(self):
    expected_params = {
        'PurchaseRequests': [
            {'PurchaseToken': 'foo', 'InstanceCount': 123}
        ],
    }
    self.stubber.add_response(
        self.function_name, self.service_response, expected_params
    )
    with self.stubber:
        self.client.purchase_scheduled_instances(
            PurchaseRequests=[
                {'PurchaseToken': 'foo', 'InstanceCount': 123}
            ]
        )
        self.assertIn('ClientToken', self.params_seen)