Why would you think that branch prediction wouldn’t be? Do you think the interpreter is adding too much false sharing to the branch predictor to render it worthless?
if x > 128:
r = not r
That’s why I asked about false sharing specifically as that’s the main way I can think of that Python code wouldn’t be able to observe the branch predictor performance - because the interpreter has so many branches internally that it dominates any branches that might be caused by the Python code itself.