code: def zLoadFile(self, fileName, append=None):
          
          reply = None
          if append:
              cmd = "LoadFile,{},{}".format(fileName, append)
          else:
              cmd = "LoadFile,{}".format(fileName)
          reply = self._sendDDEcommand(cmd)
          if reply:
              return int(reply) #Note: Zemax returns -999 if update fails.
          else:
              return -998
docs: Loads a zmx file into the DDE server