How to use the uharfbuzz.Font function in uharfbuzz

def blankfont():
    """Return a subset of AdobeBlank.ttf containing the following glyphs/characters:
    [
        {gid=0, name=".notdef"},
        {gid=1, name="a", code=0x61},
        {gid=2, name="b", code=0x62},
        {gid=3, name="c", code=0x63},
        {gid=4, name="d", code=0x64},
        {gid=5, name="e", code=0x65},
        {gid=6, name="ccedilla", code=0x62},
        {gid=7, name="uni0431", code=0x0431},  # CYRILLIC SMALL LETTER BE
        {gid=8, name="u1F4A9", code=0x1F4A9},  # PILE OF POO
    ]
    """
    face = hb.Face(ADOBE_BLANK_TTF_PATH.read_bytes())
    font = hb.Font(face)
    upem = face.upem
    font.scale = (upem, upem)
    hb.ot_font_set_funcs(font)
    return font

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