code: def existing_path(string):
      '''"Convert" a string to a string that is a path to an existing file.'''
      if not os.path.exists(string):
          msg = 'path {0!r} does not exist'.format(string)
          raise argparse.ArgumentTypeError(msg)
      return strinf existing_path(string):
      '''"Convert" a string to a string that is a path to an existing file.'''
      if not os.path.exists(string):
          msg = 'path {0!r} does not exist'.format(string)
          raise argparse.ArgumentTypeError(msg)
      return string
docs: "Convert" a string to a string that is a path to an existing file.