The point about tp_xxx slots is that when implemented in C with slots, you get the latter (correct) effect for free. This is how metaconfusion is avoided in post-2.2 Python. Using getattr() for that is essentially broken. Trying to call the method and catching TypeErrors seems pretty fragile -- e.g. if you are calling a __conform__() which is implemented in C you won't get a Python frame in the traceback either.
An excellent point. The issue hasn't come up before now, though, because there aren't any __conform__ methods written in C in the field that I know of. Presumably, if there are any added to CPython in future, it will be because there's a tp_conform slot and it's needed for built-in types, in which case the problem is again moot for the implementation.
(FYI, C methods implemented in Pyrex add a dummy frame to the traceback such that you see the file and line number of the original Pyrex source code. Very handy for debugging.)
Anyway, I agree that your version of the code should be used to form the reference implementation, since the purpose of the reference implementation is to show the complete required semantics.
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