Pure uninformed speculation follows ... I suspect sub-interpreters are a punt and a feint. My guess is that there will likely be exactly 2 sub-interpreters in

Pure uninformed speculation follows ... I suspect sub-interpreters are a punt and a feint. My guess is that there will likely be exactly 2 sub-interpreters in most Python code. One which talks to an old C API with a GIL and one which talks to a new C API without a GIL. It's going to be a lot easier to manage handing objects between two Python sub-interpreters than to manage handing objects between two incompatible ABIs.