def _ParseArgument(argument):
  """Parses a CGI argument name.

  Args:
    argument: CGI argument name of the form [model.]attribute[.operator]
        (str or unicode)

  Returns:
    model (str), attribute (str), operator name (str)
  """
  tokens = str(argument).split('.', 2)
  if len(tokens) == 1:
    # If only one token, assume it is the attribute.
    return None, tokens[0], None
  elif len(tokens) == 2:
    # No operator specified.
    return tokens + [None]
  else:
    return tokens