def test_diff_multiple_calls_to_fit(self):
        """
        Tests that `Diff` updates `start_vals` parameter when refitted to new data.
        """
        diff = Diff(lags=[2, 2], dropna=True)

        diff.fit(self.sine_series)
        startvals1 = deepcopy(diff._fitted_params)[0][0]

        diff.fit(self.sine_series + 1)
        startvals2 = deepcopy(diff._fitted_params)[0][0]

        assert not np.allclose(startvals1, startvals2)

        Tests that `Diff` updates `start_vals` parameter when refitted to new data.
        
darts/tests/dataprocessing/transformers/test_diff.py