def optional(converter):
    """
    A converter that allows an attribute to be optional. An optional attribute
    is one which can be set to ``None``.

    Type annotations will be inferred from the wrapped converter's, if it
    has any.

    :param callable converter: the converter that is used for non-``None``
        values.

    .. versionadded:: 17.1.0
    """

    def optional_converter(val):
        if val is None:
            return None
        return converter(val)

    xtr = _AnnotationExtractor(converter)

    t = xtr.get_first_param_type()
    if t:
        optional_converter.__annotations__["val"] = typing.Optional[t]

    rt = xtr.get_return_type()
    if rt:
        optional_converter.__annotations__["return"] = typing.Optional[rt]

    return optional_converter

    A converter that allows an attribute to be optional. An optional attribute
    is one which can be set to ``None``.

    Type annotations will be inferred from the wrapped converter's, if it
    has any.

    :param callable converter: the converter that is used for non-``None``
        values.

    .. versionadded:: 17.1.0
    
product/gradle-plugin/src/main/python/attr/converters.py