How to use the ufo2ft.compileInterpolatableTTFsFromDS function in ufo2ft

def empty_varfont(designspace_path):
    designspace = fontTools.designspaceLib.DesignSpaceDocument.fromfile(
        designspace_path
    )
    for source in designspace.sources:
        source.font = testutil.empty_UFO(source.styleName)
    ufo2ft.compileInterpolatableTTFsFromDS(designspace, inplace=True)
    varfont, _, _ = fontTools.varLib.build(designspace)
    return varfont

def generate_variable_font(
    designspace_path: Path,
    stylespace_path: Path,
    additional_locations: Optional[Mapping[str, float]] = None,
) -> fontTools.ttLib.TTFont:
    designspace = fontTools.designspaceLib.DesignSpaceDocument.fromfile(
        designspace_path
    )
    for source in designspace.sources:
        source.font = empty_UFO(source.styleName)
    ufo2ft.compileInterpolatableTTFsFromDS(designspace, inplace=True)
    varfont, _, _ = fontTools.varLib.build(designspace)

    stylespace = statmake.classes.Stylespace.from_file(stylespace_path)
    if additional_locations is None:
        additional_locations = designspace.lib.get(
            "org.statmake.additionalLocations", {}
        )
    statmake.lib.apply_stylespace_to_variable_font(
        stylespace, varfont, additional_locations
    )
    return reload_font(varfont)

def _build_interpolatable_masters(
        self,
        designspace,
        ttf,
        use_production_names=None,
        reverse_direction=True,
        conversion_error=None,
        feature_writers=None,
        cff_round_tolerance=None,
        debug_feature_file=None,
        **kwargs,
    ):
        designspace = self._load_designspace_sources(designspace)

        if ttf:
            return ufo2ft.compileInterpolatableTTFsFromDS(
                designspace,
                useProductionNames=use_production_names,
                reverseDirection=reverse_direction,
                cubicConversionError=conversion_error,
                featureWriters=feature_writers,
                debugFeatureFile=debug_feature_file,
                inplace=True,
            )
        else:
            return ufo2ft.compileInterpolatableOTFsFromDS(
                designspace,
                useProductionNames=use_production_names,
                roundTolerance=cff_round_tolerance,
                featureWriters=feature_writers,
                debugFeatureFile=debug_feature_file,
                inplace=True,