human
I wrote the below code

    def cosine(x, amplitude=1):
        return [(amplitude * np.cos(x))]
    @custom_model(fit_deriv=cosine)
    def sine(x, amplitude=1):
        return (amplitude * np.sin(x))
    assert issubclass(sine, Model)
    s = sine(2)
    assert_allclose(s((np.pi / 2)), 2)
    assert_allclose(s.fit_deriv(0, 2), 2)

What's a good function header?
assistant
#Here's a good function header.

def test_custom_model_parametrized_decorator():