How to get font variations for `add_font_instance` on Mac?

[kaiwk]

I'm little confused about parameter font variations of add_font_instance, and in almost all examples, we just pass an empty vector to it.

Recently, I found create CTFont will lose font variation, and as @jrmuizel said, we create native font with postscript name and create data font with font descriptor(and font variation).

I thought we can get font variations only if we got CTFont/CGFont, right?