def normalize_str_idx(idx, length, is_start=True):
    """
    Parameters
    ----------
    idx : int or None
        the index
    length : int
        the string length
    is_start : bool; optional with defaults to True
        Is it the *start* or the *stop* of the slice?

    Returns
    -------
    norm_idx : int
        normalized index
    """
    if idx is None:
        if is_start:
            return 0
        else:
            return length
    elif idx < 0:
        idx += length

    if idx < 0 or idx >= length:
        raise IndexError("string index out of range")

    return idx