code: def _arrange_fields(self, channels, sampfrom=0, expanded=False):
          
  
          # Rearrange signal specification fields
          for field in _header.SIGNAL_SPECS.index:
              item = getattr(self, field)
              setattr(self, field, [item[c] for c in channels])
  
          # Expanded signals - multiple samples per frame.
          if expanded:
              # Checksum and init_value to be updated if present
              # unless the whole signal length was input
              if self.sig_len != int(len(self.e_d_signal[0]) / self.samps_per_frame[0]):
                  self.checksum = self.calc_checksum(expanded)
                  self.init_value = [s[0] for s in self.e_d_signal]
  
              self.n_sig = len(channels)
              self.sig_len = int(len(self.e_d_signal[0]) / self.samps_per_frame[0])
  
          # MxN numpy array d_signal
          else:
              # Checksum and init_value to be updated if present
              # unless the whole signal length was input
              if self.sig_len != self.d_signal.shape[0]:
  
                  if self.checksum is not None:
                      self.checksum = self.calc_checksum()
                  if self.init_value is not None:
                      ival = list(self.d_signal[0, :])
                      self.init_value = [int(i) for i in ival]
  
              # Update record specification parameters
              # Important that these get updated after^^
              self.n_sig = len(channels)
              self.sig_len = self.d_signal.shape[0]
  
          # Adjust date and time if necessary
          self._adjust_datetime(sampfrom=sampfrom)
docs: Arrange/edit object fields to reflect user channel and/or signal
          range input.
  
          Parameters
          ----------
          channels : list
              List of channel numbers specified.
          sampfrom : int, optional
              Starting sample number read.
          expanded : bool, optional
              Whether the record was read in expanded mode.