code: def _setDriftList(self, drift_length):
          
          if isinstance(drift_length, tuple) or isinstance(drift_length, list):
              if len(drift_length) == 1:
                  self.dflist = drift_length * 3
              elif len(drift_length) == 2:
                  self.dflist = []
                  self.dflist.extend(drift_length)
                  self.dflist.append(drift_length[0])
              elif len(drift_length) >= 3:
                  self.dflist = drift_length[0:3]
                  if self.dflist[0] != self.dflist[-1]:
                      print("warning: chicane is not symmetric.")
              else:
                  print("drift_length is not a valid list or tuple.")
                  self.mflag = False
          else:
              self.dflist = []
              self.dflist.extend([drift_length, drift_length, drift_length])
docs: set drift length list of three elements
  
          :param drift_length: input drift_length in [m], single float, or list/tuple of float numbers