Class Cb1

tests/python/test_callback.py:397–400

                return False

        class Cb1(xgb.callback.TrainingCallback):
            def before_iteration(self, model, epoch: int, evals_log) -> bool:
                assert fst_call == 2
                return False

        callbacks = [Cb2(), Cb1()]
        xgb.train({}, dtrain=xgb.QuantileDMatrix(X, y, weight=w), callbacks=callbacks)