In the past week or so I finished implementing the core functionality of my project, importing modules using isolated state. Loaders and importers now accept an optional engine parameter. If it's not supplied, they fall back on a GlobalImportEngine instance which uses global state.

I also discovered that some loaders (namely for builtin and extension modules) call functions from the implementation of import in C (imp.*) and those are hardcoded to inject modules into sys.modules. This is a problem, but thanks to limitations imposed on these kind of modules (one copy of the module per process), it should be fine to place them into modules dictionary in an ImportEngine instance after they are imported by the imp module.

Finally, I realised that the test structure is more complex because the global import state needs to be preserved. So there was a number of context managers and other tricks which I didn't change before but I think now I got it right.

As a side note, I had some personal trouble in the last few days but things should be back to normal now and I'll try to post more often here.