code: def export_csv(self, path, idx=None, header=None, formatted=False,
                     sort_idx=True, fmt='%.18e'):
          if not idx:
              idx = self._idx
          if not header:
              header = self.get_header(idx, formatted=formatted)
          assert len(idx) == len(header), \
              "Idx length does not match header length"
          body = self.get_values(idx)
          with open(path, 'w') as fd:
              fd.write(','.join(header) + '\n')
              np.savetxt(fd, body, fmt=fmt, delimiter=',')