Add FVAR Instance

addFVARInstance.py

from fontTools.ttLib.tables._f_v_a_r import table__f_v_a_r, Axis, NamedInstance
from fontTools.ttLib.tables._n_a_m_e import table__n_a_m_e, NameRecord
from fontTools.ttLib import TTFont

def AddName(font, name):
    nameTable = font.get("name")
    if nameTable is None:
        nameTable = font["name"] = table__n_a_m_e()
        nameTable.names = []
    namerec = NameRecord()
    namerec.nameID = 1 + max([n.nameID for n in nameTable.names] + [256])
    namerec.string = name.encode('mac_roman')
    namerec.platformID, namerec.platEncID, namerec.langID = (1, 0, 0)
    nameTable.names.append(namerec)
    return namerec

fontPath = "path/to/font.ttf"
font = TTFont(fontPath)

instances = [("Light", 300),("ExtraSuperBold", 1000)]
fvarTable = font["fvar"]
for name, axisLocation in instances:
    inst = NamedInstance()
    inst.subfamilyNameID = AddName(font, name).nameID
    inst.coordinates = {"wght": weight}
    fvarTable.instances.append(inst)

font.save(fontPath.replace(".ttf","-newInstance.ttf"))