Thanks for the review - shall action it all as it seems all good improvements.

Two key notes: the use of an exception triple is useful to ease backports of this: a primary goal for me is being able to use the locals stuff in unittest for existing production code bases like OpenStack: there's a balance to be struck between new shiny features like __traceback__ and backporting. I'll consider the idea of value-or-triple; if all the consuming code is going to be using the triple interface I'm not sure the value one would, drumroll please, have value.

The locals stuff is as you note not complete or exposed yet. I wanted it in the design to be able to vet that it will actually be feasible without a later API break. I'm not -quite- there but I do also agree that it doesn't need to be in the output layer yet (in fact there's a different ticket for that anyway).