How to use the uharfbuzz.ot_font_set_funcs function in uharfbuzz

def opensans():
    """Return a subset of OpenSans.ttf containing the following glyphs/characters:
    [
        {gid=0, name=".notdef"},
        {gid=1, name="A", code=0x41},
    ]
    """
    face = hb.Face(OPEN_SANS_TTF_PATH.read_bytes())
    font = hb.Font(face)
    upem = face.upem
    font.scale = (upem, upem)
    hb.ot_font_set_funcs(font)
    return font

def shapeHB(text, font_name, font_size, features: Dict[str, bool] = None):
    font = pdfmetrics.getFont(font_name)
    if not isinstance(font, TTFont):
        # TODO make valid for all types of fonts
        raise RLKerningError("Not a TTF font")

    fontdata = font.face._ttf_data
    face = hb.Face(fontdata)
    font = hb.Font(face)

    # HB scales to integers in offset and advance so very big scale
    # will divide by SCALE_MULT to get the actual size in fractional points
    font.scale = (font_size * SCALE_MULT, font_size * SCALE_MULT)
    hb.ot_font_set_funcs(font)
    buf = hb.Buffer()
    buf.add_str(text)
    buf.guess_segment_properties()
    hb.shape(font, buf, features)

    return buf

# label image
        ctx.set_font_size(30)
        ctx.set_source_rgb(0.5, 0.5, 0.5)
        ctx.move_to(x_tab, 50)
        ctx.show_text("{}: {}".format(self.table_name, len(self._data)))
        ctx.move_to(x_tab, 100)
        if font_position:
            ctx.show_text("Font Set: {}".format(font_position))
        if len(self._data) > limit:
            ctx.set_font_size(20)
            ctx.move_to(x_tab, 150)
            ctx.show_text("Warning: {} different items. Only showing most serious {}".format(
                len(self._data), limit)
            )

        hb.ot_font_set_funcs(font.hbfont)

        # Draw glyphs
        x, y, baseline = x_tab, 200, 0
        x_pos = x_tab
        y_pos = 200
        for idx, row in enumerate(self._data[:limit]):
            string = "{}{}{}".format(
                prefix_characters,
                row['string'],
                suffix_characters)
            buf = self._shape_string(font, string, row['features'])
            char_info = buf.glyph_infos
            char_pos = buf.glyph_positions
            for info, pos in zip(char_info, char_pos):
                gid = info.codepoint
                font.ftfont.load_glyph(gid, flags=6)

if len(diff_table) > item_limit:
        ctx.set_font_size(20)
        ctx.move_to(20, 150)
        ctx.show_text("Warning: {} different items. Only showing most serious {}".format(
            len(diff_table), item_limit)
        )

    # HB font
    hb_face = hb.Face.create(open(font.path, 'rb').read())
    hb_font = hb.Font.create(hb_face)
    hb_upem = upm_size

    hb_font.scale = (hb_upem, hb_upem)
    if font.is_variable:
        hb_font.set_variations(font.axis_locations)
    hb.ot_font_set_funcs(hb_font)

    # Draw glyphs
    x, y, baseline = 20, 200, 0
    x_pos = 20
    y_pos = 200
    for row in diff_table[:item_limit]:

        buf = hb.Buffer.create()
        buf.add_str(row['string'])

        buf.guess_segment_properties()
        try:
            features = {f: True for f in row['features']}
            hb.shape(hb_font, buf, features)
        except KeyError:
            hb.shape(hb_font, buf)