> And yet, it appears that *something* changed between Python 2 and Python 3 such that it *is* atomic:
I haven't looked, but something to check in the source is opcode prediction. It's possible that after the BINARY_OP executes, opcode prediction jumps straight to the STORE_FAST opcode, avoiding the transfer to the top of the virtual machine loop. That would (I think) avoid checks related to GIL release and thread switches. I don't guarantee that's what's going on, and even if I'm correct, I don't think you can rely on it. Skip -- https://mail.python.org/mailman/listinfo/python-list
