def fixed_len_str(length, converter):
    """Make converter that pads a string to the given ``length`` or ``None``.

    ``converter`` is called to further converti non-``None`` values.
    """

    def fixed_len_str_converter(value):
        if value is None:
            return None
        return converter('{value:{length}}'.format(value=value, length=length))

    return fixed_len_str_converter