def compute_rotational_transform(
    i_l,
    iota,
    data=None,
):
    """Compute rotational transform profile.

    Parameters
    ----------
    i_l : ndarray
        Spectral coefficients of iota(rho) -- rotational transform profile.
    iota : Profile
        Transforms i_l coefficients to real space.

    Returns
    -------
    data : dict
        Dictionary of ndarray, shape(num_nodes,) of rotational transform profile.
        Keys are of the form 'X_y' meaning the derivative of X wrt to y.

    """
    if data is None:
        data = {}

    data["iota"] = iota.compute(i_l, dr=0)
    data["iota_r"] = iota.compute(i_l, dr=1)
    data["iota_rr"] = iota.compute(i_l, dr=2)

    return data