code: def prepare_handler(cfg):
      
      positions, velocities, box = None, None, None
      _path = cfg.get('_path', './')
      forcefield = cfg.pop('forcefield', None)
      topology_args = sanitize_args_for_file(cfg.pop('topology'), _path)
  
      if 'checkpoint' in cfg:
          restart_args = sanitize_args_for_file(cfg.pop('checkpoint'), _path)
          restart = Restart.load(*restart_args)
          positions = restart.positions
          velocities = restart.velocities
          box = restart.box
  
      if 'positions' in cfg:
          positions_args = sanitize_args_for_file(cfg.pop('positions'), _path)
          positions = Positions.load(*positions_args)
          box = BoxVectors.load(*positions_args)
  
      if 'velocities' in cfg:
          velocities_args = sanitize_args_for_file(cfg.pop('velocities'), _path)
          velocities = Velocities.load(*velocities_args)
  
      if 'box' in cfg:
          box_args = sanitize_args_for_file(cfg.pop('box'), _path)
          box = BoxVectors.load(*box_args)
  
      options = {}
      for key in 'positions velocities box forcefield'.split():
          value = locals()[key]
          if value is not None:
              options[key] = value
  
      return SystemHandler.load(*topology_args, **options)
docs: Load all files into single object.