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Calling the unicode function on a proxy object, doesn't use the proxi-ed
object's __unicode__ method, but its __str__ method.
>>> class A(object):
... def __str__(self):
... return "str" 
... def __unicode__(self):
... return "unicode" 
... 
>>> a = A()
>>> unicode(a)
u'unicode'
>>> import weakref
>>> b = weakref.proxy(a)
>>> unicode(b)
u'str'
I expected the last result to be u'unicode'. I did get u'unicode' in
Python 2.5 (but not in 2.6).

Same results on trunk.

This was broken by r64791. The problem is that PyObject_Unicode now uses
_PyType_Lookup instead of PyObject_GetItem, so that the proxy's custom
__getattr__ implementation is bypassed.

weakref.proxy needs to be fixed to delegate the unicode slot correctly.
>>> from weakref import proxy
>>> "__unicode__" in dir(type(proxy(Exception)))
False
That predicate must return true in order for the delegation to do the
right thing (this is actually the case for all of the slots and
pseudo-slots that can bypass __getattribute__ on the instance object -
it's just that most of them are already handled correctly).
This need to explicitly delegate all supported slots is the reason why
weakref proxy instances add so many magic method stubs when compared to
the actual interface of the underlying class:
>>> len(set(dir(type(proxy(Exception)))) - set(dir(Exception)))
53

Here's a patch.

Patch looks good to me.

Fixed in r76395.