[issue643841] New class special method lookup change

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

And (mainly for Barry's benefit) a quick recap of why I think this is
necessary for Python 3.0:

For performance or correctness reasons, the interpreter is permitted to
bypass the normal __getattribute__ when looking up special methods such
as __print__ or __index__. Whether or not the normal attribute lookup
machinery is bypassed for a specific special method is an application
independent decision.

In CPython's case, this bypassing can occur either because there is a
tp_* slot dedicated to the method, or because the interpreter uses
Py_TYPE(obj) and _PyType_Lookup instead of PyObject_GetAttr to find the
method implementation (or type(obj).meth instead of obj.meth for special
method lookups implemented in Python code).

This behaviour creates a problem for value-based delegation such as that
provided by weakref.proxy: unlike overriding __getattr__ on a classic
class, merely overriding __getattribute__ on a new-style class instance
is insufficient to be able to correctly delegate all of the special methods.

The intent of providing a typetools.ProxyMixin (or alternatively a
types.ProxyMixin class) is to allow fairly simply conversion of classic
classes that implement value-based delegation to new-style classes by
inheriting from ProxyMixin rather than inheriting from object directly.

Given the close proximity of the beta perhaps I should PEP'ify this to
get a formal yea or nay from Guido? I haven't managed to get much
response to previous python-dev posts about it.

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