The implementation of weakref.proxy's methods call back into the Python API using a "borrowed reference" of the weakly referenced object (acquired via PyWeakref_GET_OBJECT). This API call may delete the last reference to the object (either directly or via GC), leaving a dangling pointer, which can be subsequently dereferenced.Tested with Python 3.8.0b4 (debug build)The following code crashes with a debug build of Python 3.8.0b4 on Linux.import weakrefobj = Noneclass MyObj:    def __iter__(self):        global obj        del obj        return NotImplementedobj = MyObj()p = weakref.proxy(obj)print(5 in p)This particular test case does not crash with a release build (on Linux).The implementation of `in` on a proxy object calls proxy_contains:   return PySequence_Contains(PyWeakref_GET_OBJECT(proxy), value);https://github.com/python/cpython/blob/v3.8.0b4/Objects/weakrefobject.c#L556This eventually calls _PySequence_IterSearch. The call to PyObject_GetIter can call arbitrary code, which can lead to seq (the proxy's referent) being deleted. The subsequent call to type_error dereferences a dead object.     it = PyObject_GetIter(seq);    if (it == NULL) {        type_error("argument of type '%.200s' is not iterable", seq);        return -1;    }https://github.com/python/cpython/blob/v3.8.0b4/Objects/abstract.c#L2003-L2007I believe some functions, like proxy_length, may be immune to this problem because they do not access the borrowed referent after calling into user code. However, this is hard to verify from reading the code and may be fragile -- small changes to PyObject_Length/Size, for example, might .See alsohttps://bugs.python.org/issue16602

I'm marking this as release blocker for the 3.8 incoming release so we don't forget about fixing this.

Note I'm going to ignore this for the purposes of 2.7.17 because it doesn't look like a new regression.

New changeset10cd00a9e3c22af37c748ea5a417f6fb66601e21 by Pablo Galindo in branch 'master':bpo-38395: Fix ownership in weakref.proxy methods (GH-16632)https://github.com/python/cpython/commit/10cd00a9e3c22af37c748ea5a417f6fb66601e21

New changeset526ef856dd598fd3cefdfadeb18ede7a8e57aa41 by Pablo Galindo in branch '3.8':[3.8]bpo-38395: Fix ownership in weakref.proxy methods (GH-16632) (GH-16662)https://github.com/python/cpython/commit/526ef856dd598fd3cefdfadeb18ede7a8e57aa41

New changeset193366e25c4f84a58d2f6c6c577fd9f0143dc6e1 by Miss Islington (bot) (Pablo Galindo) in branch '3.7':[3.7]bpo-38395: Fix ownership in weakref.proxy methods (GH-16632) (GH-16663)https://github.com/python/cpython/commit/193366e25c4f84a58d2f6c6c577fd9f0143dc6e1

All fixed now, right?