I've decided to review my work merging traitlets into matplotlib #4762, and during my rebase I came across this change to Artist.update...

https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/artist.py#L880

where update now handles values returned by setters.

I'm wondering what the motivation is behind these setters with return values, because they are incompatible with traitlets. If traitlets integration is to continue, these returning setters will need to be deprecated.

cc: @tacaswell, @jenshnielsen, @efiring