code: def arg_file_is_new(parser, arg, mode='w'):
      
      if os.path.exists(arg):
          parser.error("\nThe file \"%s\"\nalready exists and "
                       "cannot be overwritten!" % arg)
      else:
          # return an open file handle
          handler = open(arg, mode=mode)
          return handler
docs: Auxiliary function to give an error if the file already exists.
  
      Parameters
      ----------
      parser : parser object
          Instance of argparse.ArgumentParser()
      arg : string
          File name.
      mode : string
          Optional string that specifies the mode in which the file is
          opened.
  
      Returns
      -------
      handler : file object
          Open file handle.