def get_fitting_data(bs, spin, band_id, kpoint_id, num_sample_points=3):
    """Extract fitting data for band extrema based on spin, kpoint and band.

    Searches forward and backward from the extrema point, but will only sample
    there data if there are enough points in that direction.

    Args:
        bs (:obj:`~pymatgen.electronic_structure.bandstructure.BandStructureSymmLine`):
            The band structure.
        spin (:obj:`~pymatgen.electronic_structure.core.Spin`): Which spin
            channel to sample.
        band_id (int): Index of the band to sample.
        kpoint_id (int): Index of the kpoint to sample.

    Returns:
        list: The data necessary to calculate the effective mass, along with
        some metadata. Formatted as a :obj:`list` of :obj:`dict`, each with the
        keys:

        'energies' (:obj:`numpy.ndarray`)
            Band eigenvalues in eV.

        'distances' (:obj:`numpy.ndarray`)
            Distances of the k-points in reciprocal space.

        'band_id' (:obj:`int`)
            The index of the band,

        'spin' (:obj:`~pymatgen.electronic_structure.core.Spin`)
            The spin channel

        'start_kpoint' (:obj:`int`)
            The index of the k-point at which the band extrema occurs

        'end_kpoint' (:obj:`int`)
            The k-point towards which the data has been sampled.
    """
    # branch data provides data about the start and end points
    # of specific band paths
    branch_data = [b for b in bs.get_branch(kpoint_id)
                   if b['index'] == kpoint_id][0]
    start_kpoint = bs.kpoints[kpoint_id]

    fitting_data = []

    # check to see if there are enough points to sample from first
    # check in the forward direction
    if kpoint_id + num_sample_points <= branch_data['end_index']:

        # calculate sampling limits
        start_id = kpoint_id
        end_id = kpoint_id + num_sample_points + 1

        energies = np.array(bs.bands[spin][band_id][start_id:end_id].copy())
        dists = np.array(bs.distance[start_id:end_id].copy())

        # normalise eigenvalues and distances to starting point
        energies -= bs.bands[spin][band_id][kpoint_id]
        dists -= bs.distance[kpoint_id]

        # symmetrise the data to make fitting more reliable
        energies = np.concatenate([energies[::-1], energies[1:]])
        dists = np.concatenate([-dists[::-1], dists[1:]])

        end_kpoint = bs.kpoints[branch_data['end_index']]
        data = {'energies': energies, 'distances': dists, 'band_id': band_id,
                'spin': spin, 'start_kpoint': start_kpoint,
                'end_kpoint': end_kpoint}
        fitting_data.append(data)

    # check in the backward direction
    if kpoint_id - num_sample_points >= branch_data['start_index']:

        # calculate sampling limits
        start_id = kpoint_id - num_sample_points
        end_id = kpoint_id + 1

        energies = bs.bands[spin][band_id][start_id:end_id].copy()
        dists = bs.distance[start_id:end_id].copy()

        # normalise eigenvalues and distances to starting point
        energies -= bs.bands[spin][band_id][kpoint_id]
        dists -= bs.distance[kpoint_id]

        # symmetrise the data to make fitting more reliable
        energies = np.concatenate([energies[:-1], energies[::-1]])
        dists = np.concatenate([dists[:-1], -dists[::-1]])

        end_kpoint = bs.kpoints[branch_data['start_index']]
        data = {'energies': energies, 'distances': dists, 'band_id': band_id,
                'spin': spin, 'start_kpoint': start_kpoint,
                'end_kpoint': end_kpoint}
        fitting_data.append(data)

    return fitting_data
Extract fitting data for band extrema based on spin, kpoint and band.

    Searches forward and backward from the extrema point, but will only sample
    there data if there are enough points in that direction.

    Args:
        bs (:obj:`~pymatgen.electronic_structure.bandstructure.BandStructureSymmLine`):
            The band structure.
        spin (:obj:`~pymatgen.electronic_structure.core.Spin`): Which spin
            channel to sample.
        band_id (int): Index of the band to sample.
        kpoint_id (int): Index of the kpoint to sample.

    Returns:
        list: The data necessary to calculate the effective mass, along with
        some metadata. Formatted as a :obj:`list` of :obj:`dict`, each with the
        keys:

        'energies' (:obj:`numpy.ndarray`)
            Band eigenvalues in eV.

        'distances' (:obj:`numpy.ndarray`)
            Distances of the k-points in reciprocal space.

        'band_id' (:obj:`int`)
            The index of the band,

        'spin' (:obj:`~pymatgen.electronic_structure.core.Spin`)
            The spin channel

        'start_kpoint' (:obj:`int`)
            The index of the k-point at which the band extrema occurs

        'end_kpoint' (:obj:`int`)
            The k-point towards which the data has been sampled.