def METHOD_NAME(self, object, start: int = None, stop: int = None):
    if object not in self._map:
        raise ValueError("not in list")
    # get idx from _map directly
    idx = self._map[object]
    if start:
        if not stop:
            stop = len(self)
        if idx not in range(start, stop):
            raise ValueError("not in list")
    return idx