Generate instances from a designspace or superpolator document. This will work in RF3 and RF4. Note that if you have complex substitutions as Superpolator allowed, these can not always be converted variable font style rules.

generate_instances.py

import ufoProcessor
from mojo.UI import PutFile, GetFile
import os

"""
    Open a .sp3 or .designspace document and generate the instances.
    No reporting, no editing, no debugging. 
"""

def convertSuperpolatorToDesignSpace(path):
    # this a superpolator path and needs to be converted first
    newPathName = path.replace(".sp3", "_converted.designspace")
    newPathName = PutFile(message="Save this Superpolator document as Designspace:", fileName=os.path.basename(newPathName))
    if newPathName:
        ufoProcessor.sp3.sp3_to_designspace(path, newPathName)
    return newPathName

def generateInstances():
    path = GetFile("Open a Desingspace or Superpolator file to generate:", fileTypes=['sp3', 'designspace'])
    if path is None:
        return
    if os.path.splitext(path)[-1] == ".sp3":
        path = convertSuperpolatorToDesignSpace(path)
        if path is None:
            return
    ufoProcessor.build(path)
    
generateInstances()
print('done')