This is a bummer. It was one of the few demonstrable benefits of subinterpreters. It seems that we're left with only the most ancient and awkward forms of communcation, shared file descriptors and pipes. Almost nothing in CSP can be readily expressed with those low level tools.

Side note: The PEP seems significantly understate the impact of isolating modules, "This situation is limited to modules that use C globals (or use libraries that use C globals) to store internal state.". As Erland Aasland sweeps through standard library making edits, the cost is becoming clear. Even simple stateless modules like bisect get significant churn. More interesting but stateless modules like itertools have about half the lines in the module changed (see gh-101277 for just one-fourth of the edits). The impact is massive. Along the way, the edits negatively impact performance which is annoying because almost the entire point of writing C accelerator code is to fix performance issues on critical code paths. We're losing freelists, ability of a type to reference its own methods, the _Py_IDENTIFIER() optimization, singletons, etc.

So far, this massive effort seems like all cost and no benefit. Gaining Go-like or CSP-like channels would have been the one win. Otherwise, all we left with is the questionable benefit of "looking like one process to the O/S". It isn't clear at all that that is something we want to pay for.

The cost/benefit proposition of PEP 554 has changed considerably since the beginning. The code churn is huge and is taking many man-months of time. Meanwhile the proposed benefits are down to "looks like a single process" and "ability to experiment with new concurrency models".