[otlLib] Return None for empty argument to buildAttachList()

This simplifies call sites when building GDEF tables.
Also, publicly expose the buildAttachPoint() function.

https://github.com/behdad/fonttools/issues/468#issuecomment-173006650

Lib/fontTools/feaLib/builder.py

@@ -162,9 +162,8 @@ class Builder(object):
     def buildGDEF(self):
         gdef = otTables.GDEF()
         gdef.GlyphClassDef = self.buildGDEFGlyphClassDef_()
-        gdef.AttachList = (
+        gdef.AttachList = \
             otl.buildAttachList(self.attachPoints_, self.glyphMap)
-            if self.attachPoints_ else None)
         gdef.LigCaretList = self.buildGDEFLigCaretList_()
         gdef.MarkAttachClassDef = self.buildGDEFMarkAttachClassDef_()
         gdef.MarkGlyphSetsDef = self.buildGDEFMarkGlyphSetsDef_()

Lib/fontTools/otlLib/builder.py

@@ -225,16 +225,18 @@ def buildValue(value):
 # GDEF
 
 def buildAttachList(attachPoints, glyphMap):
-    """{"glyphName": [4, 23]} --> otTables.AttachList"""
+    """{"glyphName": [4, 23]} --> otTables.AttachList, or None"""
+    if not attachPoints:
+        return None
     self = ot.AttachList()
     self.Coverage = buildCoverage(attachPoints.keys(), glyphMap)
-    self.AttachPoint = [_buildAttachPoint(attachPoints[g])
+    self.AttachPoint = [buildAttachPoint(attachPoints[g])
                         for g in self.Coverage.glyphs]
     self.GlyphCount = len(self.AttachPoint)
     return self
 
 
-def _buildAttachPoint(points):
+def buildAttachPoint(points):
     """[4, 23, 41] --> otTables.AttachPoint"""
     self = ot.AttachPoint()
     self.PointIndex = sorted(points)

Lib/fontTools/otlLib/builder_test.py

@@ -75,6 +75,18 @@ class BuilderTest(unittest.TestCase):
                          '  </AttachPoint>'
                          '</AttachList>')
 
+    def test_buildAttachList_empty(self):
+        self.assertIsNone(builder.buildAttachList({}, self.GLYPHMAP))
+
+    def test_buildAttachPoint(self):
+        attachPoint = builder.buildAttachPoint([7, 3])
+        self.assertEqual(getXML(attachPoint.toXML),
+                         '<AttachPoint>'
+                         '  <!-- PointCount=2 -->'
+                         '  <PointIndex index="0" value="3"/>'
+                         '  <PointIndex index="1" value="7"/>'
+                         '</AttachPoint>')
+
     def test_buildCoverage(self):
         cov = builder.buildCoverage({"two", "four"}, {"two": 2, "four": 4})
         self.assertEqual(getXML(cov.toXML),