[designspaceLib] Multiple <mappings> elements in .designspace format, without support in Python API?

[justvanrossum]

Re:
#3435
#3437

Do I understand correctly that multiple axis mappings groups are "flattened" into one group in the DesignSpaceDocument?

The "read" code seems to flatten all groups into one group:

fonttools/Lib/fontTools/designspaceLib/__init__.py

Lines 2117 to 2139 in 430e364

	for mappingsElement in self.root.findall(".axes/mappings"):
	groupDescription = mappingsElement.attrib.get("description")
	for mappingElement in mappingsElement.findall("mapping"):
	description = mappingElement.attrib.get("description")
	inputElement = mappingElement.find("input")
	outputElement = mappingElement.find("output")
	inputLoc = {}
	outputLoc = {}
	for dimElement in inputElement.findall(".dimension"):
	name = dimElement.attrib["name"]
	value = float(dimElement.attrib["xvalue"])
	inputLoc[name] = value
	for dimElement in outputElement.findall(".dimension"):
	name = dimElement.attrib["name"]
	value = float(dimElement.attrib["xvalue"])
	outputLoc[name] = value
	axisMappingObject = self.axisMappingDescriptorClass(
	inputLocation=inputLoc,
	outputLocation=outputLoc,
	description=description,
	groupDescription=groupDescription,
	)
	self.documentObject.axisMappings.append(axisMappingObject)

The "write" code only ever seems to output a single <mappings> element:

fonttools/Lib/fontTools/designspaceLib/__init__.py

Lines 1444 to 1457 in 430e364

	if self.documentObject.axisMappings:
	mappingsElement = None
	lastGroup = object()
	for mappingObject in self.documentObject.axisMappings:
	if getattr(mappingObject, "groupDescription", None) != lastGroup:
	if mappingsElement is not None:
	self.root.findall(".axes")[0].append(mappingsElement)
	lastGroup = getattr(mappingObject, "groupDescription", None)
	mappingsElement = ET.Element("mappings")
	if lastGroup is not None:
	mappingsElement.attrib["description"] = lastGroup
	self._addAxisMapping(mappingsElement, mappingObject)
	if mappingsElement is not None:
	self.root.findall(".axes")[0].append(mappingsElement)

I don't think it is a good idea to add features to the format without having the reader/writer support them in a round-trippable way.

This is really problematic when the .designspace file isn't edited in a text editor, like Fontra does, or RoboFont's DesignSpace editor.

Maybe it has not been spelled out, but to me designspaceLib has always promised "readfile, write it back and get the same document" (ignoring comments or formatting differences).

We should either change the API to properly support multiple mappings groups, or allow only one <mappings> element.

[justvanrossum]

@behdad @Lorp

[behdad]

It should work just fine if napping have descriptions. The groupDescription takes care of that.

Is that still problematic in your opinion? I can synthesize group descriptions or something to round-trip.

[justvanrossum]

Ah, I see I misunderstood the code, thanks for pointing that out.

So the code does output multiple <mappings> elements, based on the groupDescription attribute of individual mapping objects.

I now understand how this works for round tripping. But only as long as the group description is unique, though.

I find this an unfortunate mismatch between format and API, and feels like a hack.

[behdad]

We can definitely change it. No one replies on it yet... What do you suggest?

[justvanrossum]

My current thought is: allowing multiple <mappings> elements overcomplicates things, and the original use case could be done without that change (or any change), by using a prefix naming convention on the <mapping> description attribute.

[behdad]

I'm find that. I'll come up with a PR.

[Lorp]

I’m fine going back to a single <mappings>. Hacking the description attribute feels odd though. Maybe class or name?

[behdad]

I haven't forgotten this. I'll try to find the time to revert the change and just use description.