How to use the specutils.cextinction function in specutils

if not HAS_SCIPY:
            raise ImportError('To use this function scipy needs to be installed')

        from scipy.interpolate import spleval

        wave_shape = wave.shape
        wave = _process_wave(wave)

        _check_wave(wave, 909.091* u.angstrom, 6. * u.micron)

        res = np.empty_like(wave.__array__(), dtype=np.float64)

        # Analytic function in the UV.
        uvmask = wave < (2700. * u.angstrom)
        if np.any(uvmask):
            res[uvmask] = cextinction.f99uv(wave[uvmask].value, self.a_v, self.r_v) * wave.unit

        # Spline in the Optical/IR
        oirmask = ~uvmask
        if np.any(oirmask):
            k = spleval(self._spline, 1. / wave[oirmask].to('micron'))
            res[oirmask] = self.a_v / self.r_v * (k + self.r_v)

        return res.reshape(wave_shape)

not connect smoothly with longer and shorter wavelengths, it is
       not included here.

    References
    ----------
    .. [1] Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245
    .. [2] O'Donnell, J. E. 1994, ApJ, 422, 158O 
    .. [3] Savage & Mathis 1979, ARA&A, 17, 73
    .. [4] Longo et al. 1989, ApJ, 339,474
    .. [5] Valencic et al. 2004, ApJ, 616, 912
    """


    _check_wave(wave, 909.091 * u.angstrom, 33333.333 * u.angstrom)

    res = cextinction.od94(_process_wave(wave).value, a_v, r_v) * wave.unit

    return res.reshape(wave.shape)

References
    ----------
    .. [1] Fitzpatrick, E. L. 1999, PASP, 111, 63
    .. [2] Fitzpatrick, E. L. & Massa, D. 1990, ApJS, 72, 163
    """

    f = ExtinctionF99(a_v, r_v)
    return f(wave)


# fm07 knots for spline
_fm07_r_v = 3.1
_fm07_xknots = np.array([0., 0.25, 0.50, 0.75, 1., 1.e4/5530., 1.e4/4000.,
                        1.e4/3300., 1.e4/2700., 1.e4/2600.])
_fm07_kknots = cextinction.fm07kknots(_fm07_xknots)
try:
    from scipy.interpolate import splmake
    _fm07_spline = splmake(_fm07_xknots, _fm07_kknots, order=3)
except ImportError:
    pass


def extinction_fm07(wave, a_v):
    """Fitzpatrick & Massa (2007) extinction model for R_V = 3.1.

    The Fitzpatrick & Massa (2007) [1]_ model, which has a slightly
    different functional form from that of Fitzpatrick (1999) [3]_
    (`extinction_f99`). Fitzpatrick & Massa (2007) claim it is
    preferable, although it is unclear if signficantly so (Gordon et
    al. 2009 [2]_). Defined from 910 A to 6 microns.

def __init__(self, a_v, r_v=3.1):
        if not HAS_SCIPY:
            raise ImportError('To use this function scipy needs to be installed')

        from scipy.interpolate import splmake

        self.a_v = a_v
        self.r_v = r_v

        kknots = cextinction.f99kknots(_f99_xknots, self.r_v)

        self._spline = splmake(_f99_xknots, kknots, order=3)

.. math::

       A(\lambda) = A_V (a(x) + b(x) / R_V)

    where A_V can either be specified directly or via E(B-V)
    (by defintion, A_V = R_V * E(B-V)).

    References
    ----------
    .. [1] Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245
    """

    _check_wave(wave, 909.091 * u.angstrom, 33333.333 * u.angstrom)

    res = cextinction.ccm89(_process_wave(wave).value, a_v, r_v) * wave.unit

    return res.reshape(wave.shape)