code: def _log_effective_mass_data(data, is_spin_polarized, mass_type='m_e'):
      
      s = ' ({})'.format(data['spin'].name) if is_spin_polarized else ''
  
      # add 1 to band id to be consistent with VASP
      band_str = 'band {}{}'.format(data['band_id'] + 1, s)
  
      start_kpoint = data['start_kpoint']
      end_kpoint = data['end_kpoint']
      eff_mass = data['effective_mass']
  
      kpoint_str = kpt_str.format(k=start_kpoint.frac_coords)
      if start_kpoint.label:
          kpoint_str += ' ({})'.format(start_kpoint.label)
      kpoint_str += ' -> '
      kpoint_str += kpt_str.format(k=end_kpoint.frac_coords)
      if end_kpoint.label:
          kpoint_str += ' ({})'.format(end_kpoint.label)
  
      logging.info('  {}: {:.3f} | {} | {}'.format(mass_type, eff_mass,
                                                   band_str, kpoint_str))
docs: Log data about the effective masses and their directions.
  
      Args:
          data (dict): The effective mass data. Formatted as a :obj:`dict` with
              the keys:
  
              'effective_mass' (:obj:`float`)
                  The effective mass in units of electron rest mass, :math:`m_0`.
  
              'energies' (:obj:`numpy.ndarray`)
                  Band eigenvalues in eV.
  
              '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.
  
          is_spin_polarized (bool): Whether the system is spin polarized.