[cu2qu/ufo] return set of modified glyph names from fonts_to_quadratic

In ufo2ft preProcessor, we need to know which specific glyphs were actually modified (converted to quadratic), in order to do other things while processing filters, not simply if the fonts were modified as a whole; thus, here I changed fonts_to_quadratic to return the set of modified glyph names instead of just True/False. The change is backward compatible because code that checks whether the returned value is True/False will continue to work since bool(set) is True for non-empty set, False for empty ones.

Lib/fontTools/cu2qu/ufo.py

@@ -250,7 +250,7 @@ def fonts_to_quadratic(
     compatibility. If this is not required, calling fonts_to_quadratic with one
     font at a time may yield slightly more optimized results.
 
-    Return True if fonts were modified, else return False.
+    Return the set of modified glyph names if any, else return an empty set.
 
     By default, cu2qu stores the curve type in the fonts' lib, under a private
     key "com.github.googlei18n.cu2qu.curve_type", and will not try to convert
@@ -296,7 +296,7 @@ def fonts_to_quadratic(
     elif max_err_em:
         max_errors = [f.info.unitsPerEm * max_err_em for f in fonts]
 
-    modified = False
+    modified = set()
     glyph_errors = {}
     for name in set().union(*(f.keys() for f in fonts)):
         glyphs = []
@@ -306,9 +306,10 @@ def fonts_to_quadratic(
                 glyphs.append(font[name])
                 cur_max_errors.append(error)
         try:
-            modified |= _glyphs_to_quadratic(
+            if _glyphs_to_quadratic(
                 glyphs, cur_max_errors, reverse_direction, stats, all_quadratic
-            )
+            ):
+                modified.add(name)
         except IncompatibleGlyphsError as exc:
             logger.error(exc)
             glyph_errors[name] = exc
@@ -329,7 +330,6 @@ def fonts_to_quadratic(
             new_curve_type = "quadratic" if all_quadratic else "mixed"
             if curve_type != new_curve_type:
                 font.lib[CURVE_TYPE_LIB_KEY] = new_curve_type
-                modified = True
     return modified
 
 
@@ -343,7 +343,7 @@ def glyph_to_quadratic(glyph, **kwargs):
 
 def font_to_quadratic(font, **kwargs):
     """Convenience wrapper around fonts_to_quadratic, for just one font.
-    Return True if the font was modified, else return False.
+    Return the set of modified glyph names if any, else return empty set.
     """
 
     return fonts_to_quadratic([font], **kwargs)

Tests/cu2qu/ufo_test.py

@@ -35,8 +35,14 @@ def fonts():
 
 class FontsToQuadraticTest(object):
     def test_modified(self, fonts):
+        # previously this method returned True/False, now it returns a set of modified
+        # glyph names.
         modified = fonts_to_quadratic(fonts)
+        # the first assertion continues to work whether the return value is a bool/set
+        # so the change is backward compatible
         assert modified
+        assert len(modified) > 0
+        assert "B" in modified
 
     def test_stats(self, fonts):
         stats = {}