Hello! I've found a problematic behaviour of pickle when it pickles
(direct or indirect) weak proxies to self. I suspect pickle cannot
detect reference loops with weak proxies. I managed to narrow the case
to the following example:

import pickle, weakref

class TestPickle(object):
    def __init__(self):
        self.recursive = weakref.proxy(self)

    def __getstate__(self):
        print "__getstate__", id(self)
        return self.__dict__.copy()

    def __setstate__(self, d):
        print "__setstate__", id(self)
        self.__dict__.update(d)

print "- 1 -"
test = TestPickle()
print "- 2 -"
data = pickle.dumps(test, pickle.HIGHEST_PROTOCOL)
print "- 3 -"
test2 = pickle.loads(data)
print "- 4 -"
print "Result:", id(test2)
print "- 5 -"

   It prints:

- 1 -
- 2 -
__getstate__ 3075348620
__getstate__ 3075348620
- 3 -
__setstate__ 3075348844
__setstate__ 3075349004
- 4 -
Result: 3075349004
- 5 -

   That is, __getstate__ is called twice for the same object. And what
is worse, __setstate__ is called twice for different objects. The
resulting unpickled object is the last one, but in the library that I
have been debugging creation of two different objects during unpickling
is a bug.

   I can fix it by avoiding pickling the proxy and recreating the proxy
on unpickling:

import pickle, weakref

class TestPickle(object):
    def __init__(self):
        self.recursive = weakref.proxy(self)

    def __getstate__(self):
        print "__getstate__", id(self)
        d = self.__dict__.copy()
        del d['recursive']
        return d

    def __setstate__(self, d):
        print "__setstate__", id(self)
        self.__dict__.update(d)
        self.recursive = weakref.proxy(self)

print "- 1 -"
test = TestPickle()
print "- 2 -"
data = pickle.dumps(test, pickle.HIGHEST_PROTOCOL)
print "- 3 -"
test2 = pickle.loads(data)
print "- 4 -"
print "Result:", id(test2)
print "- 5 -"

- 1 -
- 2 -
__getstate__ 3075070092
- 3 -
__setstate__ 3075070188
- 4 -
Result: 3075070188
- 5 -

   But I wonder if it's a bug that should be fixed? If it's an expected
behaviour it perhaps should be documented as a warning in docs for
pickle or weakref or both.

I don't think weakref objects are meant to be picklable at all. At least there's no support code for that, so pickling weak proxies only works by chance.

The weakref.proxy object delegates all attribute access to referred object, including special attributes used in pickling, like __class__ or __getstate__.

test.recursive is not test, but it looks as TestPickle instance for the pickler. It is pickled and unpickled as TestPickle instance.

>>> test.recursive.__class__
<class '__main__.TestPickle'>
>>> test.recursive.__getstate__
<bound method TestPickle.__getstate__ of <__main__.TestPickle object at 0xb7130bac>>
>>> test.recursive.__reduce_ex__(2)
__getstate__ 3071478700
(<function __newobj__ at 0xb74b76f4>, (<class '__main__.TestPickle'>,), {'recursive': <weakproxy at 0xb7127a04 to TestPickle at 0xb7130bac>}, None, None)
>>> pickle.loads(pickle.dumps(test.recursive, 2))
__getstate__ 3071478700
__setstate__ 3071525356
<__main__.TestPickle object at 0xb713c1ec>

Since test.recursive.recursive is test.recursive, this recursion is detected by pickler.

This issue is similar to issue6395, but since weakref.proxy is not subclassable, we can just implement correct __reduce__.