Add registry for custom table packer/unpacker classes (#2055)

* Add custom table packer/unpacker registry

* fix documentation for getTableClass(): it will never return None

* add unregisterCustomTableClass(), mostly for testing

Lib/fontTools/ttLib/ttFont.py

@@ -830,10 +830,48 @@ def getTableModule(tag):
 		return getattr(tables, pyTag)
 
 
-def getTableClass(tag):
-	"""Fetch the packer/unpacker class for a table.
-	Return None when no class is found.
+# Registry for custom table packer/unpacker classes. Keys are table
+# tags, values are (moduleName, className) tuples.
+# See registerCustomTableClass() and getCustomTableClass()
+_customTableRegistry = {}
+
+
+def registerCustomTableClass(tag, moduleName, className=None):
+	"""Register a custom packer/unpacker class for a table.
+	The 'moduleName' must be an importable module. If no 'className'
+	is given, it is derived from the tag, for example it will be
+	table_C_U_S_T_ for a 'CUST' tag.
+
+	The registered table class should be a subclass of
+	fontTools.ttLib.tables.DefaultTable.DefaultTable
 	"""
+	if className is None:
+		className = "table_" + tagToIdentifier(tag)
+	_customTableRegistry[tag] = (moduleName, className)
+
+
+def unregisterCustomTableClass(tag):
+	"""Unregister the custom packer/unpacker class for a table."""
+	del _customTableRegistry[tag]
+
+
+def getCustomTableClass(tag):
+	"""Return the custom table class for tag, if one has been registered
+	with 'registerCustomTableClass()'. Else return None.
+	"""
+	if tag not in _customTableRegistry:
+		return None
+	import importlib
+	moduleName, className = _customTableRegistry[tag]
+	module = importlib.import_module(moduleName)
+	return getattr(module, className)
+
+
+def getTableClass(tag):
+	"""Fetch the packer/unpacker class for a table."""
+	tableClass = getCustomTableClass(tag)
+	if tableClass is not None:
+		return tableClass
 	module = getTableModule(tag)
 	if module is None:
 		from .tables.DefaultTable import DefaultTable

Tests/ttLib/ttFont_test.py

@@ -0,0 +1,48 @@
+import io
+from fontTools.ttLib import TTFont, newTable, registerCustomTableClass, unregisterCustomTableClass
+from fontTools.ttLib.tables.DefaultTable import DefaultTable
+
+
+class CustomTableClass(DefaultTable):
+
+    def decompile(self, data, ttFont):
+        self.numbers = list(data)
+
+    def compile(self, ttFont):
+        return bytes(self.numbers)
+
+    # not testing XML read/write
+
+
+table_C_U_S_T_ = CustomTableClass  # alias for testing
+
+
+TABLETAG = "CUST"
+
+
+def test_registerCustomTableClass():
+    font = TTFont()
+    font[TABLETAG] = newTable(TABLETAG)
+    font[TABLETAG].data = b"\x00\x01\xff"
+    f = io.BytesIO()
+    font.save(f)
+    f.seek(0)
+    assert font[TABLETAG].data == b"\x00\x01\xff"
+    registerCustomTableClass(TABLETAG, "ttFont_test", "CustomTableClass")
+    try:
+        font = TTFont(f)
+        assert font[TABLETAG].numbers == [0, 1, 255]
+        assert font[TABLETAG].compile(font) == b"\x00\x01\xff"
+    finally:
+        unregisterCustomTableClass(TABLETAG)
+
+
+def test_registerCustomTableClassStandardName():
+    registerCustomTableClass(TABLETAG, "ttFont_test")
+    try:
+        font = TTFont()
+        font[TABLETAG] = newTable(TABLETAG)
+        font[TABLETAG].numbers = [4, 5, 6]
+        assert font[TABLETAG].compile(font) == b"\x04\x05\x06"
+    finally:
+        unregisterCustomTableClass(TABLETAG)