builder/sources.py: only write locations along defined axes

[anthrotype]

similarly to what we already do for instance.location at

glyphsLib/Lib/glyphsLib/builder/instances.py

Lines 90 to 96 in e00221c

	designspace_axis_tags = {a.tag for a in self.designspace.axes}
	location = {}
	for axis_def in get_axis_definitions(self.font):
	# Only write locations along defined axes
	if axis_def.tag in designspace_axis_tags:
	location[axis_def.name] = axis_def.get_design_loc(instance)
	ufo_instance.location = location

Otherwise source.locations may contain a spurious third axis called "Custom" which may not alwyas be used (e.g. NotoSans only uses "Weight" and "Width".

We don't normally see this because fontTools.designspaceLib prunes those automatically upon writing the designspace file. But if one attempts to build a VF from the glyphsLib-generated designspace object (not the file path saved on disk), the extra undefined axis prompts an error while attempting to find the default location.

[anthrotype]

trivial change, merging.

[schriftgestalt]

Wouldn’t it be better to proper set font.axes on loading the file? Most .glyphs file should have a custom parameter (in glyph2) or the actual axes (for glyphs3).

If there us no data, Glyphs is checks all masters and builds axis ranges (for weight, width and Custom). Only if the range has nonzero length, it adds an axis. All other data (axis coordinates) is setup based on those axes.

[anthrotype]

I believe that's what we already do in glyphsLib.builder.axes. Only axes that do vary are written out to designspace axes. The problem here was that when setting the location of designspace <source> elements, we were always outputting a location along all three predefined axes (weight, width, custom) even when some of them actually did nothing. For designspace <instance> locations we were already pruning those (see above linked code)