[filters] Add loadFilterFromString()

[moyogo]

This add the ufo2ft.filters.loadFilterFromString() function to be used in fontmake.
See:

• Open Corners allowed in Glyphs source?  fontmake#288 (comment)
• Check for nested components fonttools/fontbakery#2961 (comment)

BaseFilter only takes kwargs instead of args and kwargs. None of the ufo2ft filters use the args.
Edit: The args can be given with kwargs key args.

[anthrotype]

Thanks Denis for working on this.
I see this resembles very much the way we treat loading featureWriters from a string. I wonder if we can find a way to try reuse the shared logic in both instead of duplicating it? Like a generic _loadPluginFromString or something

[moyogo]

I see this resembles very much the way we treat loading featureWriters from a string. I wonder if we can find a way to try reuse the shared logic in both instead of duplicating it? Like a generic _loadPluginFromString or something

I added _loadPluginFromString().

[anthrotype]

this posargs handling could be moved to the base classes constructors, so they can also be initialized using keywords.. But then you'd have to duplicate in both BaseFeatureWriter and BaseFilter __init__.. I guess it's ok like that

[anthrotype]

actually, only the filters can have positional arguments at all (have that _args attribute that you're checking in here). Yeah, i think it makes more sense to move this logic to base filter class init. Do you mind doing that?

[moyogo]

BaseFeatureWriter and BaseFilter should probably inherit from a common BasePlugin class. Should that be in the PR or another?

[anthrotype]

yeah, that'd be nice too. I'd say do it in same PR

[anthrotype]

if you can't reconcile the differences between the two base classes, then just move this pos-to-kwargs logic to the base filter init

[moyogo]

OK. I removed posargs logic from _loadPluginFromString and changed BaseFilter. That's much nicer.

[anthrotype]

it seems you're only handling the case where all the positionals are specified as keywords, see my other comment #466 (comment)

[moyogo]

why only 0? One could potentially want to construct a filter with some strictly positional and some positional-as-keywords.

This sounds like a bad idea. Now I have to check for duplicated args as both positional and positional-as-keywords.

[anthrotype]

This sounds like a bad idea.

but that's how python args and kwargs work, would be weird to diverge from that. Unless a parameter is marked as positional-only, one can always use the equivalent keyword to pass the argument