code: def to_file(self, filename, format='shtools', header=None, **kwargs):
          
          if format is 'shtools':
              with open(filename, mode='w') as file:
                  if header is not None:
                      file.write(header + '\n')
                  for l in range(self.lmax+1):
                      for m in range(l+1):
                          file.write('{:d}, {:d}, {:.16e}, {:.16e}\n'
                                     .format(l, m, self.coeffs[0, l, m],
                                             self.coeffs[1, l, m]))
          elif format is 'npy':
              _np.save(filename, self.coeffs, **kwargs)
          else:
              raise NotImplementedError(
                  'format={:s} not implemented'.format(repr(format)))
docs: Save raw spherical harmonic coefficients to a file.
  
          Usage
          -----
          x.to_file(filename, [format='shtools', header])
          x.to_file(filename, [format='npy', **kwargs])
  
          Parameters
          ----------
          filename : str
              Name of the output file.
          format : str, optional, default = 'shtools'
              'shtools' or 'npy'. See method from_file() for more information.
          header : str, optional, default = None
              A header string written to an 'shtools'-formatted file directly
              before the spherical harmonic coefficients.
          **kwargs : keyword argument list, optional for format = 'npy'
              Keyword arguments of numpy.save().
  
          Description
          -----------
          If format='shtools', the coefficients will be written to an ascii
          formatted file. The first line of the file is an optional user provided
          header line, and the spherical harmonic coefficients are then listed,
          with increasing degree and order, with the format
  
          l, m, coeffs[0, l, m], coeffs[1, l, m]
  
          where l and m are the spherical harmonic degree and order,
          respectively.
  
          If format='npy', the spherical harmonic coefficients will be saved to
          a binary numpy 'npy' file using numpy.save().