[Samuele Pedroni]
... The point is that jython implements the letter of the manual, and even extend the caching opt to some others __magic__ methods. I wanted to know the intended behaviour in order to fix that in jython.
You got that one right the first time: this requires BDFL pronouncement! As semantically significant optimizations (the only reason for caching __getattr__, e.g.) creep into the code but the docs lag behind, it gets more and more unclear what's mandatory behavior and what's implementation-defined. This came up a couple weeks ago again in the context of what, exactly, rich comparisons are supposed to do in all cases. After poking holes in everything Guido wrote, he turned it around and told me to write up what I think it should say (which I have yet to do, as it's time-consuming and it appears some of the current CPython behavior is at least partly accidental -- but unclear exactly which parts). So don't be surprised if the same trick gets played on you ...