It's a font-object related function:
SetClassFlags(classIndex,isFirst,isSecond)
You need to iterate through classes, and depending on whether the class is 1st/left or 2nd/right (e.g. check if a tag is found in class names), set the according flag(s):
isFIRST_TAG = "_1ST"
isSECOND_TAG = "_2ND"
f = fl.font
classes = f.classes
for cIdx in range(len(classes)):
cName = classes[cIdx].split(":",1)[0]
if cName.startswith("_"):
if isFIRST_TAG in cName:
f.SetClassFlags(cIdx, 1, 0)
elif isSECOND_TAG in cName:
f.SetClassFlags(cIdx, 0, 1)
else:
f.SetClassFlags(cIdx, 1, 1)
Then update font.