code: def _read_arg(arg):
      
  
      # If arg is None, just return it back
      if arg is None:
          arg_out = arg
  
      else:
          # If len(arg) == 1 and arg[0] is a valid filepath, read it as a grp file
          if len(arg) == 1 and os.path.exists(arg[0]):
              arg_out = grp.read(arg[0])
          else:
              arg_out = arg
  
          # Make sure that arg_out is a list of strings
          assert isinstance(arg_out, list), "arg_out must be a list."
          assert type(arg_out[0]) == str, "arg_out must be a list of strings."
  
      return arg_out
docs: If arg is a list with 1 element that corresponds to a valid file path, use
      set_io.grp to read the grp file. Otherwise, check that arg is a list of strings.
  
      Args:
          arg (list or None)
  
      Returns:
          arg_out (list or None)