code: def parse(self, argument):
      
      if not isinstance(argument, six.string_types):
        raise TypeError('flag value must be a string, found "{}"'.format(
            type(argument)))
      return argument
docs: Parses the string argument and returns the native value.
  
      By default it returns its argument unmodified.
  
      Args:
        argument: string argument passed in the commandline.
  
      Raises:
        ValueError: Raised when it fails to parse the argument.
        TypeError: Raised when the argument has the wrong type.
  
      Returns:
        The parsed value in native type.