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[Python-Dev] PEP 246, redux

Phillip J. Eby pje at telecommunity.com
Tue Jan 11 16:34:20 CET 2005 
At 12:41 PM 1/11/05 +0000, Armin Rigo wrote:
>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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