Eric Snow added the comment:

Just to be clear, I'm still -1 on any of this.  On the one hand, there's a risk 
of backward-compatibility breakage (just as much a corner-case as the need 
expressed in this issue).  On the other hand, I'd actually push for 
_PyObject_LookupSpecial to be fixed to chain AttributeError coming from a 
descriptor into a TypeError.

Allowing instances to determine the capability of a class feels wrong and 
potentially broken.  Furthermore, doing so via AttributeError is problematic 
since it may mask an AttributeError that bubbles up (which is very confusing 
and hard to debug).  I've been bitten by this with pickle.

Still, it may be a good idea to expose _PyObject_LookupSpecial via the inspect 
module, but that should be addressed in a separate issue.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue23990>
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