Find ligature glyph names by parsing the GSUB using fonttools.feaLib

findLigaturesInGsub.py

from fontTools.feaLib.parser import Parser
from fontTools.feaLib.ast import *
from io import StringIO
"""
RoboFont Script
- Type: Mastering
- Purpose: To find ligature by parsing the features.
- Specifications:
	- Parse the substitutions in the current font features using fontTools.feaLib
	- Find lookup references in the provided feature tags
	- Find ligature statements only inside the the provided feature tags or lookup references
	- Collect name of ligature glyphs and return it as a set
- Publish Date: 10 August 2021
- Author: Bahman Eslami
- License: MIT
"""

def getFeatureFileFeaLibAstElementFromFont(font):
	featurefile = StringIO(font.features.text)
	featurefile.name = font.path
	parser = Parser(featurefile, font.keys())
	return parser.parse()

def getLigaturesInFeaLibAstElement(e, featureTags, lookupNames):
	ligatureNames = set()
	if type(e) is FeatureBlock:
		if e.name not in featureTags:
			return set()
	elif type(e) is LookupBlock:
		if e.name not in lookupNames:
			return set()
	if hasattr(e, 'statements'):
		for s in e.statements:
			ligatureNames.update(getLigaturesInFeaLibAstElement(s, featureTags, lookupNames))
	if type(e) is MultipleSubstStatement:
		ligatureNames.add(e.replacement)
	elif type(e) is LigatureSubstStatement:
		ligatureNames.add(e.replacement)
	return ligatureNames

def getLookupNamesInFeaLibAstElement(e, featureTags):
	lookupNames = set()
	if type(e) is FeatureBlock:
		if e.name not in featureTags:
			return set()
	if hasattr(e, 'statements'):
		for s in e.statements:
			lookupNames.update(getLookupNamesInFeaLibAstElement(s, featureTags))
	if type(e) is LookupReferenceStatement:
		lookupNames.add(e.lookup.name)
	elif type(e) is LookupBlock:
		lookupNames.add(e.name)
	return lookupNames

def getGlyphNamesFromListOfAstGlyphNames(astGlyphList):
	return set([g.glyph for g in astGlyphList if type(g) is GlyphName])

def getLigatureAlternates(e, ligatureNames):
	if hasattr(e, 'statements'):
		for s in e.statements:
			ligatureNames.update(getLigatureAlternates(s, ligatureNames))
	if type(e) is SingleSubstStatement:
		glyphs = getGlyphNamesFromListOfAstGlyphNames(e.glyphs)
		if glyphs - ligatureNames == set():
			ligatureNames.update(getGlyphNamesFromListOfAstGlyphNames(e.replacements))
	return ligatureNames

def getLigaturesInFont(font, featureTags):
	"""
	Returns a set of glyph names which are ligatures. The featureTags argument is an iterable
	which defines which features should be parsed.
	"""
	features = getFeatureFileFeaLibAstElementFromFont(font)
	lookupNames = getLookupNamesInFeaLibAstElement(features, featureTags)
	ligatureNames = getLigaturesInFeaLibAstElement(features, featureTags, lookupNames)
	while True:
		numLigas = len(ligatureNames)
		ligatureNames = getLigatureAlternates(features, ligatureNames)
		if len(ligatureNames) == numLigas:
			break
	return ligatureNames

if __name__ == "__main__":
	font = CurrentFont()
	featureTags = set(["liga", "dlig", "rlig"])
	ligas = getLigaturesInFont(font, featureTags)
	print(ligas)