Sure, but: a) All reference counts after the fork are going to cause a copy of memory, so any memory access (even a read) can trigger a copy. b) Even to send

Sure, but: a) All reference counts after the fork are going to cause a copy of memory, so any memory access (even a read) can trigger a copy. b) Even to send 'xs' over you must serialize it via pickle, and then the callee must deserialize it That may be fine for your use case but it's strictly worse than just sharing a reference across a thread.