Add extra tests for weak references, and attempt a slightly-more-graceful...

Add extra tests for weak references, and attempt a slightly-more-graceful failure mode than AttributeError when cyclic garbage collection is clearing gevent objects.

See issue #1961

docs/changes/1961.bugfix

+Work around an ``AttributeError`` during cyclic garbage collection
+when Python finalizers (``__del__`` and the like) attempt to use
+gevent APIs. This is not a recommended practice, and it is unclear if
+catching this ``AttributeError`` will fix any problems or just shift
+them. (If we could determine the root situation that results in this
+cycle, we might be able to solve it.)

src/gevent/_gevent_c_hub_local.pxd

@@ -8,8 +8,7 @@ cpdef set_hub(SwitchOutGreenletWithLoop hub)
 cpdef get_loop()
 cpdef set_loop(loop)
 
-# We can't cdef this, it won't do varargs.
-# cpdef WaitOperationsGreenlet get_hub(*args, **kwargs)
+cpdef SwitchOutGreenletWithLoop get_hub()
 
 # XXX: TODO: Move the definition of TrackedRawGreenlet
 # into a file that can be cython compiled so get_hub can

src/gevent/_hub_local.py

@@ -9,7 +9,7 @@ from __future__ import division
 from __future__ import print_function
 
 
-from gevent._compat import thread_mod_name
+import _thread
 
 __all__ = [
     'get_hub',
@@ -21,13 +21,34 @@ __all__ = [
 # not monkey-patched.
 # We are imported early enough (by gevent/__init__) that
 # we can rely on not being monkey-patched in any way yet.
-class _Threadlocal(__import__(thread_mod_name)._local):
+assert 'gevent' not in str(_thread._local)
+class _Threadlocal(_thread._local):
 
     def __init__(self):
         # Use a class with an initializer so that we can test
         # for 'is None' instead of catching AttributeError, making
         # the code cleaner and possibly solving some corner cases
-        # (like #687)
+        # (like #687).
+        #
+        # However, under some weird circumstances, it _seems_ like the
+        # __init__ method doesn't get called properly ("seems" is the
+        # keyword). We've seen at least one instance
+        # (https://github.com/gevent/gevent/issues/1961) of
+        # ``AttributeError: '_Threadlocal' object has no attribute # 'hub'``
+        # which should be impossible unless:
+        #
+        # - Someone manually deletes the attribute
+        # - The _threadlocal object itself is in the process of being
+        #   deleted. The C ``tp_clear`` slot for it deletes the ``__dict__``
+        #   of each instance in each thread (and/or the ``tp_clear`` of ``dict`` itself
+        #   clears the instance). Now, how we could be getting
+        #   cleared while still being used is unclear, but clearing is part of
+        #   circular garbage collection, and in the bug report it looks like we're inside a
+        #   weakref finalizer or ``__del__`` method, which could suggest that
+        #   garbage collection is happening.
+        #
+        # See https://github.com/gevent/gevent/issues/1961
+        # and ``get_hub_if_exists()``
         super(_Threadlocal, self).__init__()
         self.Hub = None
         self.loop = None
@@ -51,7 +72,7 @@ def set_default_hub_class(hubtype):
     global Hub
     Hub = hubtype
 
-def get_hub(*args, **kwargs): # pylint:disable=unused-argument
+def get_hub():
     """
     Return the hub for the current thread.
 
@@ -66,26 +87,54 @@ def get_hub(*args, **kwargs): # pylint:disable=unused-argument
 
     .. versionchanged:: 1.5a3
        The *args* and *kwargs* arguments are now completely ignored.
-    """
 
-    return get_hub_noargs()
+    .. versionchanged:: NEXT
+       The long-deprecated ``args`` and ``kwargs`` parameters are no
+       longer accepted.
+    """
+    # See get_hub_if_exists
+    try:
+        hub = _threadlocal.hub
+    except AttributeError:
+        hub = None
+    if hub is None:
+        hubtype = get_hub_class()
+        hub = _threadlocal.hub = hubtype()
+    return hub
 
+# For Cython purposes, we need to duplicate get_hub into this function so it
+# can be directly called.
 def get_hub_noargs():
-    # Just like get_hub, but cheaper to call because it
-    # takes no arguments or kwargs. See also a copy in
-    # gevent/greenlet.py
-    hub = _threadlocal.hub
+    # See get_hub_if_exists
+    try:
+        hub = _threadlocal.hub
+    except AttributeError:
+        hub = None
     if hub is None:
         hubtype = get_hub_class()
         hub = _threadlocal.hub = hubtype()
     return hub
 
 def get_hub_if_exists():
-    """Return the hub for the current thread.
+    """
+    Return the hub for the current thread.
 
     Return ``None`` if no hub has been created yet.
     """
-    return _threadlocal.hub
+    # Attempt a band-aid for the poorly-understood behaviour
+    # seen in https://github.com/gevent/gevent/issues/1961
+    # where the ``hub`` attribute has gone missing.
+    try:
+        return _threadlocal.hub
+    except AttributeError:
+        # XXX: I'd really like to report this, but I'm not sure how
+        # that can be done safely (because I don't know how we get
+        # here in the first place). We may be in a place where imports
+        # are unsafe, or the interpreter is shutting down, or the
+        # thread is exiting, or...
+        return None
+
+
 
 
 def set_hub(hub):

src/gevent/tests/test__destroy.py

@@ -32,7 +32,8 @@ class TestDestroyHub(unittest.TestCase):
         hub.destroy(destroy_loop=True)
 
         # Create new hub and explicitly request creation of a new default loop.
-        hub = gevent.get_hub(default=True)
+        # (using default=True, but that's no longer possible.)
+        hub = gevent.get_hub()
         self.assertTrue(hub.loop.default)
 
         # `gevent.core.loop` objects as well as libev loop pointers must differ.

src/greentest/3.12/test_weakref.py

+import gc
+import sys
+import doctest
+import unittest
+import collections
+import weakref
+import operator
+import contextlib
+import copy
+import threading
+import time
+import random
+
+from test import support
+from test.support import script_helper, ALWAYS_EQ
+from test.support import gc_collect
+from test.support import threading_helper
+
+# Used in ReferencesTestCase.test_ref_created_during_del() .
+ref_from_del = None
+
+# Used by FinalizeTestCase as a global that may be replaced by None
+# when the interpreter shuts down.
+_global_var = 'foobar'
+
+class C:
+    def method(self):
+        pass
+
+
+class Callable:
+    bar = None
+
+    def __call__(self, x):
+        self.bar = x
+
+
+def create_function():
+    def f(): pass
+    return f
+
+def create_bound_method():
+    return C().method
+
+
+class Object:
+    def __init__(self, arg):
+        self.arg = arg
+    def __repr__(self):
+        return "<Object %r>" % self.arg
+    def __eq__(self, other):
+        if isinstance(other, Object):
+            return self.arg == other.arg
+        return NotImplemented
+    def __lt__(self, other):
+        if isinstance(other, Object):
+            return self.arg < other.arg
+        return NotImplemented
+    def __hash__(self):
+        return hash(self.arg)
+    def some_method(self):
+        return 4
+    def other_method(self):
+        return 5
+
+
+class RefCycle:
+    def __init__(self):
+        self.cycle = self
+
+
+class TestBase(unittest.TestCase):
+
+    def setUp(self):
+        self.cbcalled = 0
+
+    def callback(self, ref):
+        self.cbcalled += 1
+
+
+@contextlib.contextmanager
+def collect_in_thread(period=0.0001):
+    """
+    Ensure GC collections happen in a different thread, at a high frequency.
+    """
+    please_stop = False
+
+    def collect():
+        while not please_stop:
+            time.sleep(period)
+            gc.collect()
+
+    with support.disable_gc():
+        t = threading.Thread(target=collect)
+        t.start()
+        try:
+            yield
+        finally:
+            please_stop = True
+            t.join()
+
+
+class ReferencesTestCase(TestBase):
+
+    def test_basic_ref(self):
+        self.check_basic_ref(C)
+        self.check_basic_ref(create_function)
+        self.check_basic_ref(create_bound_method)
+
+        # Just make sure the tp_repr handler doesn't raise an exception.
+        # Live reference:
+        o = C()
+        wr = weakref.ref(o)
+        repr(wr)
+        # Dead reference:
+        del o
+        repr(wr)
+
+    def test_repr_failure_gh99184(self):
+        class MyConfig(dict):
+            def __getattr__(self, x):
+                return self[x]
+
+        obj = MyConfig(offset=5)
+        obj_weakref = weakref.ref(obj)
+
+        self.assertIn('MyConfig', repr(obj_weakref))
+        self.assertIn('MyConfig', str(obj_weakref))
+
+    def test_basic_callback(self):
+        self.check_basic_callback(C)
+        self.check_basic_callback(create_function)
+        self.check_basic_callback(create_bound_method)
+
+    @support.cpython_only
+    def test_cfunction(self):
+        import _testcapi
+        create_cfunction = _testcapi.create_cfunction
+        f = create_cfunction()
+        wr = weakref.ref(f)
+        self.assertIs(wr(), f)
+        del f
+        self.assertIsNone(wr())
+        self.check_basic_ref(create_cfunction)
+        self.check_basic_callback(create_cfunction)
+
+    def test_multiple_callbacks(self):
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del o
+        gc_collect()  # For PyPy or other GCs.
+        self.assertIsNone(ref1(), "expected reference to be invalidated")
+        self.assertIsNone(ref2(), "expected reference to be invalidated")
+        self.assertEqual(self.cbcalled, 2,
+                     "callback not called the right number of times")
+
+    def test_multiple_selfref_callbacks(self):
+        # Make sure all references are invalidated before callbacks are called
+        #
+        # What's important here is that we're using the first
+        # reference in the callback invoked on the second reference
+        # (the most recently created ref is cleaned up first).  This
+        # tests that all references to the object are invalidated
+        # before any of the callbacks are invoked, so that we only
+        # have one invocation of _weakref.c:cleanup_helper() active
+        # for a particular object at a time.
+        #
+        def callback(object, self=self):
+            self.ref()
+        c = C()
+        self.ref = weakref.ref(c, callback)
+        ref1 = weakref.ref(c, callback)
+        del c
+
+    def test_constructor_kwargs(self):
+        c = C()
+        self.assertRaises(TypeError, weakref.ref, c, callback=None)
+
+    def test_proxy_ref(self):
+        o = C()
+        o.bar = 1
+        ref1 = weakref.proxy(o, self.callback)
+        ref2 = weakref.proxy(o, self.callback)
+        del o
+        gc_collect()  # For PyPy or other GCs.
+
+        def check(proxy):
+            proxy.bar
+
+        self.assertRaises(ReferenceError, check, ref1)
+        self.assertRaises(ReferenceError, check, ref2)
+        ref3 = weakref.proxy(C())
+        gc_collect()  # For PyPy or other GCs.
+        self.assertRaises(ReferenceError, bool, ref3)
+        self.assertEqual(self.cbcalled, 2)
+
+    def check_basic_ref(self, factory):
+        o = factory()
+        ref = weakref.ref(o)
+        self.assertIsNotNone(ref(),
+                     "weak reference to live object should be live")
+        o2 = ref()
+        self.assertIs(o, o2,
+                     "<ref>() should return original object if live")
+
+    def check_basic_callback(self, factory):
+        self.cbcalled = 0
+        o = factory()
+        ref = weakref.ref(o, self.callback)
+        del o
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(self.cbcalled, 1,
+                     "callback did not properly set 'cbcalled'")
+        self.assertIsNone(ref(),
+                     "ref2 should be dead after deleting object reference")
+
+    def test_ref_reuse(self):
+        o = C()
+        ref1 = weakref.ref(o)
+        # create a proxy to make sure that there's an intervening creation
+        # between these two; it should make no difference
+        proxy = weakref.proxy(o)
+        ref2 = weakref.ref(o)
+        self.assertIs(ref1, ref2,
+                     "reference object w/out callback should be re-used")
+
+        o = C()
+        proxy = weakref.proxy(o)
+        ref1 = weakref.ref(o)
+        ref2 = weakref.ref(o)
+        self.assertIs(ref1, ref2,
+                     "reference object w/out callback should be re-used")
+        self.assertEqual(weakref.getweakrefcount(o), 2,
+                     "wrong weak ref count for object")
+        del proxy
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(weakref.getweakrefcount(o), 1,
+                     "wrong weak ref count for object after deleting proxy")
+
+    def test_proxy_reuse(self):
+        o = C()
+        proxy1 = weakref.proxy(o)
+        ref = weakref.ref(o)
+        proxy2 = weakref.proxy(o)
+        self.assertIs(proxy1, proxy2,
+                     "proxy object w/out callback should have been re-used")
+
+    def test_basic_proxy(self):
+        o = C()
+        self.check_proxy(o, weakref.proxy(o))
+
+        L = collections.UserList()
+        p = weakref.proxy(L)
+        self.assertFalse(p, "proxy for empty UserList should be false")
+        p.append(12)
+        self.assertEqual(len(L), 1)
+        self.assertTrue(p, "proxy for non-empty UserList should be true")
+        p[:] = [2, 3]
+        self.assertEqual(len(L), 2)
+        self.assertEqual(len(p), 2)
+        self.assertIn(3, p, "proxy didn't support __contains__() properly")
+        p[1] = 5
+        self.assertEqual(L[1], 5)
+        self.assertEqual(p[1], 5)
+        L2 = collections.UserList(L)
+        p2 = weakref.proxy(L2)
+        self.assertEqual(p, p2)
+        ## self.assertEqual(repr(L2), repr(p2))
+        L3 = collections.UserList(range(10))
+        p3 = weakref.proxy(L3)
+        self.assertEqual(L3[:], p3[:])
+        self.assertEqual(L3[5:], p3[5:])
+        self.assertEqual(L3[:5], p3[:5])
+        self.assertEqual(L3[2:5], p3[2:5])
+
+    def test_proxy_unicode(self):
+        # See bug 5037
+        class C(object):
+            def __str__(self):
+                return "string"
+            def __bytes__(self):
+                return b"bytes"
+        instance = C()
+        self.assertIn("__bytes__", dir(weakref.proxy(instance)))
+        self.assertEqual(bytes(weakref.proxy(instance)), b"bytes")
+
+    def test_proxy_index(self):
+        class C:
+            def __index__(self):
+                return 10
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(operator.index(p), 10)
+
+    def test_proxy_div(self):
+        class C:
+            def __floordiv__(self, other):
+                return 42
+            def __ifloordiv__(self, other):
+                return 21
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(p // 5, 42)
+        p //= 5
+        self.assertEqual(p, 21)
+
+    def test_proxy_matmul(self):
+        class C:
+            def __matmul__(self, other):
+                return 1729
+            def __rmatmul__(self, other):
+                return -163
+            def __imatmul__(self, other):
+                return 561
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(p @ 5, 1729)
+        self.assertEqual(5 @ p, -163)
+        p @= 5
+        self.assertEqual(p, 561)
+
+    # The PyWeakref_* C API is documented as allowing either NULL or
+    # None as the value for the callback, where either means "no
+    # callback".  The "no callback" ref and proxy objects are supposed
+    # to be shared so long as they exist by all callers so long as
+    # they are active.  In Python 2.3.3 and earlier, this guarantee
+    # was not honored, and was broken in different ways for
+    # PyWeakref_NewRef() and PyWeakref_NewProxy().  (Two tests.)
+
+    def test_shared_ref_without_callback(self):
+        self.check_shared_without_callback(weakref.ref)
+
+    def test_shared_proxy_without_callback(self):
+        self.check_shared_without_callback(weakref.proxy)
+
+    def check_shared_without_callback(self, makeref):
+        o = Object(1)
+        p1 = makeref(o, None)
+        p2 = makeref(o, None)
+        self.assertIs(p1, p2, "both callbacks were None in the C API")
+        del p1, p2
+        p1 = makeref(o)
+        p2 = makeref(o, None)
+        self.assertIs(p1, p2, "callbacks were NULL, None in the C API")
+        del p1, p2
+        p1 = makeref(o)
+        p2 = makeref(o)
+        self.assertIs(p1, p2, "both callbacks were NULL in the C API")
+        del p1, p2
+        p1 = makeref(o, None)
+        p2 = makeref(o)
+        self.assertIs(p1, p2, "callbacks were None, NULL in the C API")
+
+    def test_callable_proxy(self):
+        o = Callable()
+        ref1 = weakref.proxy(o)
+
+        self.check_proxy(o, ref1)
+
+        self.assertIs(type(ref1), weakref.CallableProxyType,
+                     "proxy is not of callable type")
+        ref1('twinkies!')
+        self.assertEqual(o.bar, 'twinkies!',
+                     "call through proxy not passed through to original")
+        ref1(x='Splat.')
+        self.assertEqual(o.bar, 'Splat.',
+                     "call through proxy not passed through to original")
+
+        # expect due to too few args
+        self.assertRaises(TypeError, ref1)
+
+        # expect due to too many args
+        self.assertRaises(TypeError, ref1, 1, 2, 3)
+
+    def check_proxy(self, o, proxy):
+        o.foo = 1
+        self.assertEqual(proxy.foo, 1,
+                     "proxy does not reflect attribute addition")
+        o.foo = 2
+        self.assertEqual(proxy.foo, 2,
+                     "proxy does not reflect attribute modification")
+        del o.foo
+        self.assertFalse(hasattr(proxy, 'foo'),
+                     "proxy does not reflect attribute removal")
+
+        proxy.foo = 1
+        self.assertEqual(o.foo, 1,
+                     "object does not reflect attribute addition via proxy")
+        proxy.foo = 2
+        self.assertEqual(o.foo, 2,
+            "object does not reflect attribute modification via proxy")
+        del proxy.foo
+        self.assertFalse(hasattr(o, 'foo'),
+                     "object does not reflect attribute removal via proxy")
+
+    def test_proxy_deletion(self):
+        # Test clearing of SF bug #762891
+        class Foo:
+            result = None
+            def __delitem__(self, accessor):
+                self.result = accessor
+        g = Foo()
+        f = weakref.proxy(g)
+        del f[0]
+        self.assertEqual(f.result, 0)
+
+    def test_proxy_bool(self):
+        # Test clearing of SF bug #1170766
+        class List(list): pass
+        lyst = List()
+        self.assertEqual(bool(weakref.proxy(lyst)), bool(lyst))
+
+    def test_proxy_iter(self):
+        # Test fails with a debug build of the interpreter
+        # (see bpo-38395).
+
+        obj = None
+
+        class MyObj:
+            def __iter__(self):
+                nonlocal obj
+                del obj
+                return NotImplemented
+
+        obj = MyObj()
+        p = weakref.proxy(obj)
+        with self.assertRaises(TypeError):
+            # "blech" in p calls MyObj.__iter__ through the proxy,
+            # without keeping a reference to the real object, so it
+            # can be killed in the middle of the call
+            "blech" in p
+
+    def test_proxy_next(self):
+        arr = [4, 5, 6]
+        def iterator_func():
+            yield from arr
+        it = iterator_func()
+
+        class IteratesWeakly:
+            def __iter__(self):
+                return weakref.proxy(it)
+
+        weak_it = IteratesWeakly()
+
+        # Calls proxy.__next__
+        self.assertEqual(list(weak_it), [4, 5, 6])
+
+    def test_proxy_bad_next(self):
+        # bpo-44720: PyIter_Next() shouldn't be called if the reference
+        # isn't an iterator.
+
+        not_an_iterator = lambda: 0
+
+        class A:
+            def __iter__(self):
+                return weakref.proxy(not_an_iterator)
+        a = A()
+
+        msg = "Weakref proxy referenced a non-iterator"
+        with self.assertRaisesRegex(TypeError, msg):
+            list(a)
+
+    def test_proxy_reversed(self):
+        class MyObj:
+            def __len__(self):
+                return 3
+            def __reversed__(self):
+                return iter('cba')
+
+        obj = MyObj()
+        self.assertEqual("".join(reversed(weakref.proxy(obj))), "cba")
+
+    def test_proxy_hash(self):
+        class MyObj:
+            def __hash__(self):
+                return 42
+
+        obj = MyObj()
+        with self.assertRaises(TypeError):
+            hash(weakref.proxy(obj))
+
+        class MyObj:
+            __hash__ = None
+
+        obj = MyObj()
+        with self.assertRaises(TypeError):
+            hash(weakref.proxy(obj))
+
+    def test_getweakrefcount(self):
+        o = C()
+        ref1 = weakref.ref(o)
+        ref2 = weakref.ref(o, self.callback)
+        self.assertEqual(weakref.getweakrefcount(o), 2,
+                     "got wrong number of weak reference objects")
+
+        proxy1 = weakref.proxy(o)
+        proxy2 = weakref.proxy(o, self.callback)
+        self.assertEqual(weakref.getweakrefcount(o), 4,
+                     "got wrong number of weak reference objects")
+
+        del ref1, ref2, proxy1, proxy2
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(weakref.getweakrefcount(o), 0,
+                     "weak reference objects not unlinked from"
+                     " referent when discarded.")
+
+        # assumes ints do not support weakrefs
+        self.assertEqual(weakref.getweakrefcount(1), 0,
+                     "got wrong number of weak reference objects for int")
+
+    def test_getweakrefs(self):
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del ref1
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(weakref.getweakrefs(o), [ref2],
+                     "list of refs does not match")
+
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del ref2
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(weakref.getweakrefs(o), [ref1],
+                     "list of refs does not match")
+
+        del ref1
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(weakref.getweakrefs(o), [],
+                     "list of refs not cleared")
+
+        # assumes ints do not support weakrefs
+        self.assertEqual(weakref.getweakrefs(1), [],
+                     "list of refs does not match for int")
+
+    def test_newstyle_number_ops(self):
+        class F(float):
+            pass
+        f = F(2.0)
+        p = weakref.proxy(f)
+        self.assertEqual(p + 1.0, 3.0)
+        self.assertEqual(1.0 + p, 3.0)  # this used to SEGV
+
+    def test_callbacks_protected(self):
+        # Callbacks protected from already-set exceptions?
+        # Regression test for SF bug #478534.
+        class BogusError(Exception):
+            pass
+        data = {}
+        def remove(k):
+            del data[k]
+        def encapsulate():
+            f = lambda : ()
+            data[weakref.ref(f, remove)] = None
+            raise BogusError
+        try:
+            encapsulate()
+        except BogusError:
+            pass
+        else:
+            self.fail("exception not properly restored")
+        try:
+            encapsulate()
+        except BogusError:
+            pass
+        else:
+            self.fail("exception not properly restored")
+
+    def test_sf_bug_840829(self):
+        # "weakref callbacks and gc corrupt memory"
+        # subtype_dealloc erroneously exposed a new-style instance
+        # already in the process of getting deallocated to gc,
+        # causing double-deallocation if the instance had a weakref
+        # callback that triggered gc.
+        # If the bug exists, there probably won't be an obvious symptom
+        # in a release build.  In a debug build, a segfault will occur
+        # when the second attempt to remove the instance from the "list
+        # of all objects" occurs.
+
+        import gc
+
+        class C(object):
+            pass
+
+        c = C()
+        wr = weakref.ref(c, lambda ignore: gc.collect())
+        del c
+
+        # There endeth the first part.  It gets worse.
+        del wr
+
+        c1 = C()
+        c1.i = C()
+        wr = weakref.ref(c1.i, lambda ignore: gc.collect())
+
+        c2 = C()
+        c2.c1 = c1
+        del c1  # still alive because c2 points to it
+
+        # Now when subtype_dealloc gets called on c2, it's not enough just
+        # that c2 is immune from gc while the weakref callbacks associated
+        # with c2 execute (there are none in this 2nd half of the test, btw).
+        # subtype_dealloc goes on to call the base classes' deallocs too,
+        # so any gc triggered by weakref callbacks associated with anything
+        # torn down by a base class dealloc can also trigger double
+        # deallocation of c2.
+        del c2
+
+    def test_callback_in_cycle(self):
+        import gc
+
+        class J(object):
+            pass
+
+        class II(object):
+            def acallback(self, ignore):
+                self.J
+
+        I = II()
+        I.J = J
+        I.wr = weakref.ref(J, I.acallback)
+
+        # Now J and II are each in a self-cycle (as all new-style class
+        # objects are, since their __mro__ points back to them).  I holds
+        # both a weak reference (I.wr) and a strong reference (I.J) to class
+        # J.  I is also in a cycle (I.wr points to a weakref that references
+        # I.acallback).  When we del these three, they all become trash, but
+        # the cycles prevent any of them from getting cleaned up immediately.
+        # Instead they have to wait for cyclic gc to deduce that they're
+        # trash.
+        #
+        # gc used to call tp_clear on all of them, and the order in which
+        # it does that is pretty accidental.  The exact order in which we
+        # built up these things manages to provoke gc into running tp_clear
+        # in just the right order (I last).  Calling tp_clear on II leaves
+        # behind an insane class object (its __mro__ becomes NULL).  Calling
+        # tp_clear on J breaks its self-cycle, but J doesn't get deleted
+        # just then because of the strong reference from I.J.  Calling
+        # tp_clear on I starts to clear I's __dict__, and just happens to
+        # clear I.J first -- I.wr is still intact.  That removes the last
+        # reference to J, which triggers the weakref callback.  The callback
+        # tries to do "self.J", and instances of new-style classes look up
+        # attributes ("J") in the class dict first.  The class (II) wants to
+        # search II.__mro__, but that's NULL.   The result was a segfault in
+        # a release build, and an assert failure in a debug build.
+        del I, J, II
+        gc.collect()
+
+    def test_callback_reachable_one_way(self):
+        import gc
+
+        # This one broke the first patch that fixed the previous test. In this case,
+        # the objects reachable from the callback aren't also reachable
+        # from the object (c1) *triggering* the callback:  you can get to
+        # c1 from c2, but not vice-versa.  The result was that c2's __dict__
+        # got tp_clear'ed by the time the c2.cb callback got invoked.
+
+        class C:
+            def cb(self, ignore):
+                self.me
+                self.c1
+                self.wr
+
+        c1, c2 = C(), C()
+
+        c2.me = c2
+        c2.c1 = c1
+        c2.wr = weakref.ref(c1, c2.cb)
+
+        del c1, c2
+        gc.collect()
+
+    def test_callback_different_classes(self):
+        import gc
+
+        # Like test_callback_reachable_one_way, except c2 and c1 have different
+        # classes.  c2's class (C) isn't reachable from c1 then, so protecting
+        # objects reachable from the dying object (c1) isn't enough to stop
+        # c2's class (C) from getting tp_clear'ed before c2.cb is invoked.
+        # The result was a segfault (C.__mro__ was NULL when the callback
+        # tried to look up self.me).
+
+        class C(object):
+            def cb(self, ignore):
+                self.me
+                self.c1
+                self.wr
+
+        class D:
+            pass
+
+        c1, c2 = D(), C()
+
+        c2.me = c2
+        c2.c1 = c1
+        c2.wr = weakref.ref(c1, c2.cb)
+
+        del c1, c2, C, D
+        gc.collect()
+
+    def test_callback_in_cycle_resurrection(self):
+        import gc
+
+        # Do something nasty in a weakref callback:  resurrect objects
+        # from dead cycles.  For this to be attempted, the weakref and
+        # its callback must also be part of the cyclic trash (else the
+        # objects reachable via the callback couldn't be in cyclic trash
+        # to begin with -- the callback would act like an external root).
+        # But gc clears trash weakrefs with callbacks early now, which
+        # disables the callbacks, so the callbacks shouldn't get called
+        # at all (and so nothing actually gets resurrected).
+
+        alist = []
+        class C(object):
+            def __init__(self, value):
+                self.attribute = value
+
+            def acallback(self, ignore):
+                alist.append(self.c)
+
+        c1, c2 = C(1), C(2)
+        c1.c = c2
+        c2.c = c1
+        c1.wr = weakref.ref(c2, c1.acallback)
+        c2.wr = weakref.ref(c1, c2.acallback)
+
+        def C_went_away(ignore):
+            alist.append("C went away")
+        wr = weakref.ref(C, C_went_away)
+
+        del c1, c2, C   # make them all trash
+        self.assertEqual(alist, [])  # del isn't enough to reclaim anything
+
+        gc.collect()
+        # c1.wr and c2.wr were part of the cyclic trash, so should have
+        # been cleared without their callbacks executing.  OTOH, the weakref
+        # to C is bound to a function local (wr), and wasn't trash, so that
+        # callback should have been invoked when C went away.
+        self.assertEqual(alist, ["C went away"])
+        # The remaining weakref should be dead now (its callback ran).
+        self.assertEqual(wr(), None)
+
+        del alist[:]
+        gc.collect()
+        self.assertEqual(alist, [])
+
+    def test_callbacks_on_callback(self):
+        import gc
+
+        # Set up weakref callbacks *on* weakref callbacks.
+        alist = []
+        def safe_callback(ignore):
+            alist.append("safe_callback called")
+
+        class C(object):
+            def cb(self, ignore):
+                alist.append("cb called")
+
+        c, d = C(), C()
+        c.other = d
+        d.other = c
+        callback = c.cb
+        c.wr = weakref.ref(d, callback)     # this won't trigger
+        d.wr = weakref.ref(callback, d.cb)  # ditto
+        external_wr = weakref.ref(callback, safe_callback)  # but this will
+        self.assertIs(external_wr(), callback)
+
+        # The weakrefs attached to c and d should get cleared, so that
+        # C.cb is never called.  But external_wr isn't part of the cyclic
+        # trash, and no cyclic trash is reachable from it, so safe_callback
+        # should get invoked when the bound method object callback (c.cb)
+        # -- which is itself a callback, and also part of the cyclic trash --
+        # gets reclaimed at the end of gc.
+
+        del callback, c, d, C
+        self.assertEqual(alist, [])  # del isn't enough to clean up cycles
+        gc.collect()
+        self.assertEqual(alist, ["safe_callback called"])
+        self.assertEqual(external_wr(), None)
+
+        del alist[:]
+        gc.collect()
+        self.assertEqual(alist, [])
+
+    def test_gc_during_ref_creation(self):
+        self.check_gc_during_creation(weakref.ref)
+
+    def test_gc_during_proxy_creation(self):
+        self.check_gc_during_creation(weakref.proxy)
+
+    def check_gc_during_creation(self, makeref):
+        thresholds = gc.get_threshold()
+        gc.set_threshold(1, 1, 1)
+        gc.collect()
+        class A:
+            pass
+
+        def callback(*args):
+            pass
+
+        referenced = A()
+
+        a = A()
+        a.a = a
+        a.wr = makeref(referenced)
+
+        try:
+            # now make sure the object and the ref get labeled as
+            # cyclic trash:
+            a = A()
+            weakref.ref(referenced, callback)
+
+        finally:
+            gc.set_threshold(*thresholds)
+
+    def test_ref_created_during_del(self):
+        # Bug #1377858
+        # A weakref created in an object's __del__() would crash the
+        # interpreter when the weakref was cleaned up since it would refer to
+        # non-existent memory.  This test should not segfault the interpreter.
+        class Target(object):
+            def __del__(self):
+                global ref_from_del
+                ref_from_del = weakref.ref(self)
+
+        w = Target()
+
+    def test_init(self):
+        # Issue 3634
+        # <weakref to class>.__init__() doesn't check errors correctly
+        r = weakref.ref(Exception)
+        self.assertRaises(TypeError, r.__init__, 0, 0, 0, 0, 0)
+        # No exception should be raised here
+        gc.collect()
+
+    def test_classes(self):
+        # Check that classes are weakrefable.
+        class A(object):
+            pass
+        l = []
+        weakref.ref(int)
+        a = weakref.ref(A, l.append)
+        A = None
+        gc.collect()
+        self.assertEqual(a(), None)
+        self.assertEqual(l, [a])
+
+    def test_equality(self):
+        # Alive weakrefs defer equality testing to their underlying object.
+        x = Object(1)
+        y = Object(1)
+        z = Object(2)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        c = weakref.ref(z)
+        d = weakref.ref(x)
+        # Note how we directly test the operators here, to stress both
+        # __eq__ and __ne__.
+        self.assertTrue(a == b)
+        self.assertFalse(a != b)
+        self.assertFalse(a == c)
+        self.assertTrue(a != c)
+        self.assertTrue(a == d)
+        self.assertFalse(a != d)
+        self.assertFalse(a == x)
+        self.assertTrue(a != x)
+        self.assertTrue(a == ALWAYS_EQ)
+        self.assertFalse(a != ALWAYS_EQ)
+        del x, y, z
+        gc.collect()
+        for r in a, b, c:
+            # Sanity check
+            self.assertIs(r(), None)
+        # Dead weakrefs compare by identity: whether `a` and `d` are the
+        # same weakref object is an implementation detail, since they pointed
+        # to the same original object and didn't have a callback.
+        # (see issue #16453).
+        self.assertFalse(a == b)
+        self.assertTrue(a != b)
+        self.assertFalse(a == c)
+        self.assertTrue(a != c)
+        self.assertEqual(a == d, a is d)
+        self.assertEqual(a != d, a is not d)
+
+    def test_ordering(self):
+        # weakrefs cannot be ordered, even if the underlying objects can.
+        ops = [operator.lt, operator.gt, operator.le, operator.ge]
+        x = Object(1)
+        y = Object(1)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        for op in ops:
+            self.assertRaises(TypeError, op, a, b)
+        # Same when dead.
+        del x, y
+        gc.collect()
+        for op in ops:
+            self.assertRaises(TypeError, op, a, b)
+
+    def test_hashing(self):
+        # Alive weakrefs hash the same as the underlying object
+        x = Object(42)
+        y = Object(42)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        self.assertEqual(hash(a), hash(42))
+        del x, y
+        gc.collect()
+        # Dead weakrefs:
+        # - retain their hash is they were hashed when alive;
+        # - otherwise, cannot be hashed.
+        self.assertEqual(hash(a), hash(42))
+        self.assertRaises(TypeError, hash, b)
+
+    def test_trashcan_16602(self):
+        # Issue #16602: when a weakref's target was part of a long
+        # deallocation chain, the trashcan mechanism could delay clearing
+        # of the weakref and make the target object visible from outside
+        # code even though its refcount had dropped to 0.  A crash ensued.
+        class C:
+            def __init__(self, parent):
+                if not parent:
+                    return
+                wself = weakref.ref(self)
+                def cb(wparent):
+                    o = wself()
+                self.wparent = weakref.ref(parent, cb)
+
+        d = weakref.WeakKeyDictionary()
+        root = c = C(None)
+        for n in range(100):
+            d[c] = c = C(c)
+        del root
+        gc.collect()
+
+    def test_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        self.assertIs(ref1.__callback__, callback)
+
+        ref2 = weakref.ref(x)
+        self.assertIsNone(ref2.__callback__)
+
+    def test_callback_attribute_after_deletion(self):
+        x = Object(1)
+        ref = weakref.ref(x, self.callback)
+        self.assertIsNotNone(ref.__callback__)
+        del x
+        support.gc_collect()
+        self.assertIsNone(ref.__callback__)
+
+    def test_set_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        with self.assertRaises(AttributeError):
+            ref1.__callback__ = lambda ref: None
+
+    def test_callback_gcs(self):
+        class ObjectWithDel(Object):
+            def __del__(self): pass
+        x = ObjectWithDel(1)
+        ref1 = weakref.ref(x, lambda ref: support.gc_collect())
+        del x
+        support.gc_collect()
+
+
+class SubclassableWeakrefTestCase(TestBase):
+
+    def test_subclass_refs(self):
+        class MyRef(weakref.ref):
+            def __init__(self, ob, callback=None, value=42):
+                self.value = value
+                super().__init__(ob, callback)
+            def __call__(self):
+                self.called = True
+                return super().__call__()
+        o = Object("foo")
+        mr = MyRef(o, value=24)
+        self.assertIs(mr(), o)
+        self.assertTrue(mr.called)
+        self.assertEqual(mr.value, 24)
+        del o
+        gc_collect()  # For PyPy or other GCs.
+        self.assertIsNone(mr())
+        self.assertTrue(mr.called)
+
+    def test_subclass_refs_dont_replace_standard_refs(self):
+        class MyRef(weakref.ref):
+            pass
+        o = Object(42)
+        r1 = MyRef(o)
+        r2 = weakref.ref(o)
+        self.assertIsNot(r1, r2)
+        self.assertEqual(weakref.getweakrefs(o), [r2, r1])
+        self.assertEqual(weakref.getweakrefcount(o), 2)
+        r3 = MyRef(o)
+        self.assertEqual(weakref.getweakrefcount(o), 3)
+        refs = weakref.getweakrefs(o)
+        self.assertEqual(len(refs), 3)
+        self.assertIs(r2, refs[0])
+        self.assertIn(r1, refs[1:])
+        self.assertIn(r3, refs[1:])
+
+    def test_subclass_refs_dont_conflate_callbacks(self):
+        class MyRef(weakref.ref):
+            pass
+        o = Object(42)
+        r1 = MyRef(o, id)
+        r2 = MyRef(o, str)
+        self.assertIsNot(r1, r2)
+        refs = weakref.getweakrefs(o)
+        self.assertIn(r1, refs)
+        self.assertIn(r2, refs)
+
+    def test_subclass_refs_with_slots(self):
+        class MyRef(weakref.ref):
+            __slots__ = "slot1", "slot2"
+            def __new__(type, ob, callback, slot1, slot2):
+                return weakref.ref.__new__(type, ob, callback)
+            def __init__(self, ob, callback, slot1, slot2):
+                self.slot1 = slot1
+                self.slot2 = slot2
+            def meth(self):
+                return self.slot1 + self.slot2
+        o = Object(42)
+        r = MyRef(o, None, "abc", "def")
+        self.assertEqual(r.slot1, "abc")
+        self.assertEqual(r.slot2, "def")
+        self.assertEqual(r.meth(), "abcdef")
+        self.assertFalse(hasattr(r, "__dict__"))
+
+    def test_subclass_refs_with_cycle(self):
+        """Confirm https://bugs.python.org/issue3100 is fixed."""
+        # An instance of a weakref subclass can have attributes.
+        # If such a weakref holds the only strong reference to the object,
+        # deleting the weakref will delete the object. In this case,
+        # the callback must not be called, because the ref object is
+        # being deleted.
+        class MyRef(weakref.ref):
+            pass
+
+        # Use a local callback, for "regrtest -R::"
+        # to detect refcounting problems
+        def callback(w):
+            self.cbcalled += 1
+
+        o = C()
+        r1 = MyRef(o, callback)
+        r1.o = o
+        del o
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+        # Same test, with two weakrefs to the same object
+        # (since code paths are different)
+        o = C()
+        r1 = MyRef(o, callback)
+        r2 = MyRef(o, callback)
+        r1.r = r2
+        r2.o = o
+        del o
+        del r2
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+
+class WeakMethodTestCase(unittest.TestCase):
+
+    def _subclass(self):
+        """Return an Object subclass overriding `some_method`."""
+        class C(Object):
+            def some_method(self):
+                return 6
+        return C
+
+    def test_alive(self):
+        o = Object(1)
+        r = weakref.WeakMethod(o.some_method)
+        self.assertIsInstance(r, weakref.ReferenceType)
+        self.assertIsInstance(r(), type(o.some_method))
+        self.assertIs(r().__self__, o)
+        self.assertIs(r().__func__, o.some_method.__func__)
+        self.assertEqual(r()(), 4)
+
+    def test_object_dead(self):
+        o = Object(1)
+        r = weakref.WeakMethod(o.some_method)
+        del o
+        gc.collect()
+        self.assertIs(r(), None)
+
+    def test_method_dead(self):
+        C = self._subclass()
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method)
+        del C.some_method
+        gc.collect()
+        self.assertIs(r(), None)
+
+    def test_callback_when_object_dead(self):
+        # Test callback behaviour when object dies first.
+        C = self._subclass()
+        calls = []
+        def cb(arg):
+            calls.append(arg)
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method, cb)
+        del o
+        gc.collect()
+        self.assertEqual(calls, [r])
+        # Callback is only called once.
+        C.some_method = Object.some_method
+        gc.collect()
+        self.assertEqual(calls, [r])
+
+    def test_callback_when_method_dead(self):
+        # Test callback behaviour when method dies first.
+        C = self._subclass()
+        calls = []
+        def cb(arg):
+            calls.append(arg)
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method, cb)
+        del C.some_method
+        gc.collect()
+        self.assertEqual(calls, [r])
+        # Callback is only called once.
+        del o
+        gc.collect()
+        self.assertEqual(calls, [r])
+
+    @support.cpython_only
+    def test_no_cycles(self):
+        # A WeakMethod doesn't create any reference cycle to itself.
+        o = Object(1)
+        def cb(_):
+            pass
+        r = weakref.WeakMethod(o.some_method, cb)
+        wr = weakref.ref(r)
+        del r
+        self.assertIs(wr(), None)
+
+    def test_equality(self):
+        def _eq(a, b):
+            self.assertTrue(a == b)
+            self.assertFalse(a != b)
+        def _ne(a, b):
+            self.assertTrue(a != b)
+            self.assertFalse(a == b)
+        x = Object(1)
+        y = Object(1)
+        a = weakref.WeakMethod(x.some_method)
+        b = weakref.WeakMethod(y.some_method)
+        c = weakref.WeakMethod(x.other_method)
+        d = weakref.WeakMethod(y.other_method)
+        # Objects equal, same method
+        _eq(a, b)
+        _eq(c, d)
+        # Objects equal, different method
+        _ne(a, c)
+        _ne(a, d)
+        _ne(b, c)
+        _ne(b, d)
+        # Objects unequal, same or different method
+        z = Object(2)
+        e = weakref.WeakMethod(z.some_method)
+        f = weakref.WeakMethod(z.other_method)
+        _ne(a, e)
+        _ne(a, f)
+        _ne(b, e)
+        _ne(b, f)
+        # Compare with different types
+        _ne(a, x.some_method)
+        _eq(a, ALWAYS_EQ)
+        del x, y, z
+        gc.collect()
+        # Dead WeakMethods compare by identity
+        refs = a, b, c, d, e, f
+        for q in refs:
+            for r in refs:
+                self.assertEqual(q == r, q is r)
+                self.assertEqual(q != r, q is not r)
+
+    def test_hashing(self):
+        # Alive WeakMethods are hashable if the underlying object is
+        # hashable.
+        x = Object(1)
+        y = Object(1)
+        a = weakref.WeakMethod(x.some_method)
+        b = weakref.WeakMethod(y.some_method)
+        c = weakref.WeakMethod(y.other_method)
+        # Since WeakMethod objects are equal, the hashes should be equal.
+        self.assertEqual(hash(a), hash(b))
+        ha = hash(a)
+        # Dead WeakMethods retain their old hash value
+        del x, y
+        gc.collect()
+        self.assertEqual(hash(a), ha)
+        self.assertEqual(hash(b), ha)
+        # If it wasn't hashed when alive, a dead WeakMethod cannot be hashed.
+        self.assertRaises(TypeError, hash, c)
+
+
+class MappingTestCase(TestBase):
+
+    COUNT = 10
+
+    def check_len_cycles(self, dict_type, cons):
+        N = 20
+        items = [RefCycle() for i in range(N)]
+        dct = dict_type(cons(o) for o in items)
+        # Keep an iterator alive
+        it = dct.items()
+        try:
+            next(it)
+        except StopIteration:
+            pass
+        del items
+        gc.collect()
+        n1 = len(dct)
+        del it
+        gc.collect()
+        n2 = len(dct)
+        # one item may be kept alive inside the iterator
+        self.assertIn(n1, (0, 1))
+        self.assertEqual(n2, 0)
+
+    def test_weak_keyed_len_cycles(self):
+        self.check_len_cycles(weakref.WeakKeyDictionary, lambda k: (k, 1))
+
+    def test_weak_valued_len_cycles(self):
+        self.check_len_cycles(weakref.WeakValueDictionary, lambda k: (1, k))
+
+    def check_len_race(self, dict_type, cons):
+        # Extended sanity checks for len() in the face of cyclic collection
+        self.addCleanup(gc.set_threshold, *gc.get_threshold())
+        for th in range(1, 100):
+            N = 20
+            gc.collect(0)
+            gc.set_threshold(th, th, th)
+            items = [RefCycle() for i in range(N)]
+            dct = dict_type(cons(o) for o in items)
+            del items
+            # All items will be collected at next garbage collection pass
+            it = dct.items()
+            try:
+                next(it)
+            except StopIteration:
+                pass
+            n1 = len(dct)
+            del it
+            n2 = len(dct)
+            self.assertGreaterEqual(n1, 0)
+            self.assertLessEqual(n1, N)
+            self.assertGreaterEqual(n2, 0)
+            self.assertLessEqual(n2, n1)
+
+    def test_weak_keyed_len_race(self):
+        self.check_len_race(weakref.WeakKeyDictionary, lambda k: (k, 1))
+
+    def test_weak_valued_len_race(self):
+        self.check_len_race(weakref.WeakValueDictionary, lambda k: (1, k))
+
+    def test_weak_values(self):
+        #
+        #  This exercises d.copy(), d.items(), d[], del d[], len(d).
+        #
+        dict, objects = self.make_weak_valued_dict()
+        for o in objects:
+            self.assertEqual(weakref.getweakrefcount(o), 1)
+            self.assertIs(o, dict[o.arg],
+                         "wrong object returned by weak dict!")
+        items1 = list(dict.items())
+        items2 = list(dict.copy().items())
+        items1.sort()
+        items2.sort()
+        self.assertEqual(items1, items2,
+                     "cloning of weak-valued dictionary did not work!")
+        del items1, items2
+        self.assertEqual(len(dict), self.COUNT)
+        del objects[0]
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(len(dict), self.COUNT - 1,
+                     "deleting object did not cause dictionary update")
+        del objects, o
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(len(dict), 0,
+                     "deleting the values did not clear the dictionary")
+        # regression on SF bug #447152:
+        dict = weakref.WeakValueDictionary()
+        self.assertRaises(KeyError, dict.__getitem__, 1)
+        dict[2] = C()
+        gc_collect()  # For PyPy or other GCs.
+        self.assertRaises(KeyError, dict.__getitem__, 2)
+
+    def test_weak_keys(self):
+        #
+        #  This exercises d.copy(), d.items(), d[] = v, d[], del d[],
+        #  len(d), k in d.
+        #
+        dict, objects = self.make_weak_keyed_dict()
+        for o in objects:
+            self.assertEqual(weakref.getweakrefcount(o), 1,
+                         "wrong number of weak references to %r!" % o)
+            self.assertIs(o.arg, dict[o],
+                         "wrong object returned by weak dict!")
+        items1 = dict.items()
+        items2 = dict.copy().items()
+        self.assertEqual(set(items1), set(items2),
+                     "cloning of weak-keyed dictionary did not work!")
+        del items1, items2
+        self.assertEqual(len(dict), self.COUNT)
+        del objects[0]
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(len(dict), (self.COUNT - 1),
+                     "deleting object did not cause dictionary update")
+        del objects, o
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(len(dict), 0,
+                     "deleting the keys did not clear the dictionary")
+        o = Object(42)
+        dict[o] = "What is the meaning of the universe?"
+        self.assertIn(o, dict)
+        self.assertNotIn(34, dict)
+
+    def test_weak_keyed_iters(self):
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_iters(dict)
+
+        # Test keyrefs()
+        refs = dict.keyrefs()
+        self.assertEqual(len(refs), len(objects))
+        objects2 = list(objects)
+        for wr in refs:
+            ob = wr()
+            self.assertIn(ob, dict)
+            self.assertIn(ob, dict)
+            self.assertEqual(ob.arg, dict[ob])
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+        # Test iterkeyrefs()
+        objects2 = list(objects)
+        self.assertEqual(len(list(dict.keyrefs())), len(objects))
+        for wr in dict.keyrefs():
+            ob = wr()
+            self.assertIn(ob, dict)
+            self.assertIn(ob, dict)
+            self.assertEqual(ob.arg, dict[ob])
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+    def test_weak_valued_iters(self):
+        dict, objects = self.make_weak_valued_dict()
+        self.check_iters(dict)
+
+        # Test valuerefs()
+        refs = dict.valuerefs()
+        self.assertEqual(len(refs), len(objects))
+        objects2 = list(objects)
+        for wr in refs:
+            ob = wr()
+            self.assertEqual(ob, dict[ob.arg])
+            self.assertEqual(ob.arg, dict[ob.arg].arg)
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+        # Test itervaluerefs()
+        objects2 = list(objects)
+        self.assertEqual(len(list(dict.itervaluerefs())), len(objects))
+        for wr in dict.itervaluerefs():
+            ob = wr()
+            self.assertEqual(ob, dict[ob.arg])
+            self.assertEqual(ob.arg, dict[ob.arg].arg)
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+    def check_iters(self, dict):
+        # item iterator:
+        items = list(dict.items())
+        for item in dict.items():
+            items.remove(item)
+        self.assertFalse(items, "items() did not touch all items")
+
+        # key iterator, via __iter__():
+        keys = list(dict.keys())
+        for k in dict:
+            keys.remove(k)
+        self.assertFalse(keys, "__iter__() did not touch all keys")
+
+        # key iterator, via iterkeys():
+        keys = list(dict.keys())
+        for k in dict.keys():
+            keys.remove(k)
+        self.assertFalse(keys, "iterkeys() did not touch all keys")
+
+        # value iterator:
+        values = list(dict.values())
+        for v in dict.values():
+            values.remove(v)
+        self.assertFalse(values,
+                     "itervalues() did not touch all values")
+
+    def check_weak_destroy_while_iterating(self, dict, objects, iter_name):
+        n = len(dict)
+        it = iter(getattr(dict, iter_name)())
+        next(it)             # Trigger internal iteration
+        # Destroy an object
+        del objects[-1]
+        gc.collect()    # just in case
+        # We have removed either the first consumed object, or another one
+        self.assertIn(len(list(it)), [len(objects), len(objects) - 1])
+        del it
+        # The removal has been committed
+        self.assertEqual(len(dict), n - 1)
+
+    def check_weak_destroy_and_mutate_while_iterating(self, dict, testcontext):
+        # Check that we can explicitly mutate the weak dict without
+        # interfering with delayed removal.
+        # `testcontext` should create an iterator, destroy one of the
+        # weakref'ed objects and then return a new key/value pair corresponding
+        # to the destroyed object.
+        with testcontext() as (k, v):
+            self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.__delitem__, k)
+        self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.pop, k)
+        self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            dict[k] = v
+        self.assertEqual(dict[k], v)
+        ddict = copy.copy(dict)
+        with testcontext() as (k, v):
+            dict.update(ddict)
+        self.assertEqual(dict, ddict)
+        with testcontext() as (k, v):
+            dict.clear()
+        self.assertEqual(len(dict), 0)
+
+    def check_weak_del_and_len_while_iterating(self, dict, testcontext):
+        # Check that len() works when both iterating and removing keys
+        # explicitly through various means (.pop(), .clear()...), while
+        # implicit mutation is deferred because an iterator is alive.
+        # (each call to testcontext() should schedule one item for removal
+        #  for this test to work properly)
+        o = Object(123456)
+        with testcontext():
+            n = len(dict)
+            # Since underlying dict is ordered, first item is popped
+            dict.pop(next(dict.keys()))
+            self.assertEqual(len(dict), n - 1)
+            dict[o] = o
+            self.assertEqual(len(dict), n)
+        # last item in objects is removed from dict in context shutdown
+        with testcontext():
+            self.assertEqual(len(dict), n - 1)
+            # Then, (o, o) is popped
+            dict.popitem()
+            self.assertEqual(len(dict), n - 2)
+        with testcontext():
+            self.assertEqual(len(dict), n - 3)
+            del dict[next(dict.keys())]
+            self.assertEqual(len(dict), n - 4)
+        with testcontext():
+            self.assertEqual(len(dict), n - 5)
+            dict.popitem()
+            self.assertEqual(len(dict), n - 6)
+        with testcontext():
+            dict.clear()
+            self.assertEqual(len(dict), 0)
+        self.assertEqual(len(dict), 0)
+
+    def test_weak_keys_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'keyrefs')
+        dict, objects = self.make_weak_keyed_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                v = objects.pop().arg
+                gc.collect()      # just in case
+                yield Object(v), v
+            finally:
+                it = None           # should commit all removals
+                gc.collect()
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+        # Issue #21173: len() fragile when keys are both implicitly and
+        # explicitly removed.
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_weak_del_and_len_while_iterating(dict, testcontext)
+
+    def test_weak_values_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_valued_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'itervaluerefs')
+        self.check_weak_destroy_while_iterating(dict, objects, 'valuerefs')
+        dict, objects = self.make_weak_valued_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                k = objects.pop().arg
+                gc.collect()      # just in case
+                yield k, Object(k)
+            finally:
+                it = None           # should commit all removals
+                gc.collect()
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+        dict, objects = self.make_weak_valued_dict()
+        self.check_weak_del_and_len_while_iterating(dict, testcontext)
+
+    def test_make_weak_keyed_dict_from_dict(self):
+        o = Object(3)
+        dict = weakref.WeakKeyDictionary({o:364})
+        self.assertEqual(dict[o], 364)
+
+    def test_make_weak_keyed_dict_from_weak_keyed_dict(self):
+        o = Object(3)
+        dict = weakref.WeakKeyDictionary({o:364})
+        dict2 = weakref.WeakKeyDictionary(dict)
+        self.assertEqual(dict[o], 364)
+
+    def make_weak_keyed_dict(self):
+        dict = weakref.WeakKeyDictionary()
+        objects = list(map(Object, range(self.COUNT)))
+        for o in objects:
+            dict[o] = o.arg
+        return dict, objects
+
+    def test_make_weak_valued_dict_from_dict(self):
+        o = Object(3)
+        dict = weakref.WeakValueDictionary({364:o})
+        self.assertEqual(dict[364], o)
+
+    def test_make_weak_valued_dict_from_weak_valued_dict(self):
+        o = Object(3)
+        dict = weakref.WeakValueDictionary({364:o})
+        dict2 = weakref.WeakValueDictionary(dict)
+        self.assertEqual(dict[364], o)
+
+    def test_make_weak_valued_dict_misc(self):
+        # errors
+        self.assertRaises(TypeError, weakref.WeakValueDictionary.__init__)
+        self.assertRaises(TypeError, weakref.WeakValueDictionary, {}, {})
+        self.assertRaises(TypeError, weakref.WeakValueDictionary, (), ())
+        # special keyword arguments
+        o = Object(3)
+        for kw in 'self', 'dict', 'other', 'iterable':
+            d = weakref.WeakValueDictionary(**{kw: o})
+            self.assertEqual(list(d.keys()), [kw])
+            self.assertEqual(d[kw], o)
+
+    def make_weak_valued_dict(self):
+        dict = weakref.WeakValueDictionary()
+        objects = list(map(Object, range(self.COUNT)))
+        for o in objects:
+            dict[o.arg] = o
+        return dict, objects
+
+    def check_popitem(self, klass, key1, value1, key2, value2):
+        weakdict = klass()
+        weakdict[key1] = value1
+        weakdict[key2] = value2
+        self.assertEqual(len(weakdict), 2)
+        k, v = weakdict.popitem()
+        self.assertEqual(len(weakdict), 1)
+        if k is key1:
+            self.assertIs(v, value1)
+        else:
+            self.assertIs(v, value2)
+        k, v = weakdict.popitem()
+        self.assertEqual(len(weakdict), 0)
+        if k is key1:
+            self.assertIs(v, value1)
+        else:
+            self.assertIs(v, value2)
+
+    def test_weak_valued_dict_popitem(self):
+        self.check_popitem(weakref.WeakValueDictionary,
+                           "key1", C(), "key2", C())
+
+    def test_weak_keyed_dict_popitem(self):
+        self.check_popitem(weakref.WeakKeyDictionary,
+                           C(), "value 1", C(), "value 2")
+
+    def check_setdefault(self, klass, key, value1, value2):
+        self.assertIsNot(value1, value2,
+                     "invalid test"
+                     " -- value parameters must be distinct objects")
+        weakdict = klass()
+        o = weakdict.setdefault(key, value1)
+        self.assertIs(o, value1)
+        self.assertIn(key, weakdict)
+        self.assertIs(weakdict.get(key), value1)
+        self.assertIs(weakdict[key], value1)
+
+        o = weakdict.setdefault(key, value2)
+        self.assertIs(o, value1)
+        self.assertIn(key, weakdict)
+        self.assertIs(weakdict.get(key), value1)
+        self.assertIs(weakdict[key], value1)
+
+    def test_weak_valued_dict_setdefault(self):
+        self.check_setdefault(weakref.WeakValueDictionary,
+                              "key", C(), C())
+
+    def test_weak_keyed_dict_setdefault(self):
+        self.check_setdefault(weakref.WeakKeyDictionary,
+                              C(), "value 1", "value 2")
+
+    def check_update(self, klass, dict):
+        #
+        #  This exercises d.update(), len(d), d.keys(), k in d,
+        #  d.get(), d[].
+        #
+        weakdict = klass()
+        weakdict.update(dict)
+        self.assertEqual(len(weakdict), len(dict))
+        for k in weakdict.keys():
+            self.assertIn(k, dict, "mysterious new key appeared in weak dict")
+            v = dict.get(k)
+            self.assertIs(v, weakdict[k])
+            self.assertIs(v, weakdict.get(k))
+        for k in dict.keys():
+            self.assertIn(k, weakdict, "original key disappeared in weak dict")
+            v = dict[k]
+            self.assertIs(v, weakdict[k])
+            self.assertIs(v, weakdict.get(k))
+
+    def test_weak_valued_dict_update(self):
+        self.check_update(weakref.WeakValueDictionary,
+                          {1: C(), 'a': C(), C(): C()})
+        # errors
+        self.assertRaises(TypeError, weakref.WeakValueDictionary.update)
+        d = weakref.WeakValueDictionary()
+        self.assertRaises(TypeError, d.update, {}, {})
+        self.assertRaises(TypeError, d.update, (), ())
+        self.assertEqual(list(d.keys()), [])
+        # special keyword arguments
+        o = Object(3)
+        for kw in 'self', 'dict', 'other', 'iterable':
+            d = weakref.WeakValueDictionary()
+            d.update(**{kw: o})
+            self.assertEqual(list(d.keys()), [kw])
+            self.assertEqual(d[kw], o)
+
+    def test_weak_valued_union_operators(self):
+        a = C()
+        b = C()
+        c = C()
+        wvd1 = weakref.WeakValueDictionary({1: a})
+        wvd2 = weakref.WeakValueDictionary({1: b, 2: a})
+        wvd3 = wvd1.copy()
+        d1 = {1: c, 3: b}
+        pairs = [(5, c), (6, b)]
+
+        tmp1 = wvd1 | wvd2 # Between two WeakValueDictionaries
+        self.assertEqual(dict(tmp1), dict(wvd1) | dict(wvd2))
+        self.assertIs(type(tmp1), weakref.WeakValueDictionary)
+        wvd1 |= wvd2
+        self.assertEqual(wvd1, tmp1)
+
+        tmp2 = wvd2 | d1 # Between WeakValueDictionary and mapping
+        self.assertEqual(dict(tmp2), dict(wvd2) | d1)
+        self.assertIs(type(tmp2), weakref.WeakValueDictionary)
+        wvd2 |= d1
+        self.assertEqual(wvd2, tmp2)
+
+        tmp3 = wvd3.copy() # Between WeakValueDictionary and iterable key, value
+        tmp3 |= pairs
+        self.assertEqual(dict(tmp3), dict(wvd3) | dict(pairs))
+        self.assertIs(type(tmp3), weakref.WeakValueDictionary)
+
+        tmp4 = d1 | wvd3 # Testing .__ror__
+        self.assertEqual(dict(tmp4), d1 | dict(wvd3))
+        self.assertIs(type(tmp4), weakref.WeakValueDictionary)
+
+        del a
+        self.assertNotIn(2, tmp1)
+        self.assertNotIn(2, tmp2)
+        self.assertNotIn(1, tmp3)
+        self.assertNotIn(1, tmp4)
+
+    def test_weak_keyed_dict_update(self):
+        self.check_update(weakref.WeakKeyDictionary,
+                          {C(): 1, C(): 2, C(): 3})
+
+    def test_weak_keyed_delitem(self):
+        d = weakref.WeakKeyDictionary()
+        o1 = Object('1')
+        o2 = Object('2')
+        d[o1] = 'something'
+        d[o2] = 'something'
+        self.assertEqual(len(d), 2)
+        del d[o1]
+        self.assertEqual(len(d), 1)
+        self.assertEqual(list(d.keys()), [o2])
+
+    def test_weak_keyed_union_operators(self):
+        o1 = C()
+        o2 = C()
+        o3 = C()
+        wkd1 = weakref.WeakKeyDictionary({o1: 1, o2: 2})
+        wkd2 = weakref.WeakKeyDictionary({o3: 3, o1: 4})
+        wkd3 = wkd1.copy()
+        d1 = {o2: '5', o3: '6'}
+        pairs = [(o2, 7), (o3, 8)]
+
+        tmp1 = wkd1 | wkd2 # Between two WeakKeyDictionaries
+        self.assertEqual(dict(tmp1), dict(wkd1) | dict(wkd2))
+        self.assertIs(type(tmp1), weakref.WeakKeyDictionary)
+        wkd1 |= wkd2
+        self.assertEqual(wkd1, tmp1)
+
+        tmp2 = wkd2 | d1 # Between WeakKeyDictionary and mapping
+        self.assertEqual(dict(tmp2), dict(wkd2) | d1)
+        self.assertIs(type(tmp2), weakref.WeakKeyDictionary)
+        wkd2 |= d1
+        self.assertEqual(wkd2, tmp2)
+
+        tmp3 = wkd3.copy() # Between WeakKeyDictionary and iterable key, value
+        tmp3 |= pairs
+        self.assertEqual(dict(tmp3), dict(wkd3) | dict(pairs))
+        self.assertIs(type(tmp3), weakref.WeakKeyDictionary)
+
+        tmp4 = d1 | wkd3 # Testing .__ror__
+        self.assertEqual(dict(tmp4), d1 | dict(wkd3))
+        self.assertIs(type(tmp4), weakref.WeakKeyDictionary)
+
+        del o1
+        self.assertNotIn(4, tmp1.values())
+        self.assertNotIn(4, tmp2.values())
+        self.assertNotIn(1, tmp3.values())
+        self.assertNotIn(1, tmp4.values())
+
+    def test_weak_valued_delitem(self):
+        d = weakref.WeakValueDictionary()
+        o1 = Object('1')
+        o2 = Object('2')
+        d['something'] = o1
+        d['something else'] = o2
+        self.assertEqual(len(d), 2)
+        del d['something']
+        self.assertEqual(len(d), 1)
+        self.assertEqual(list(d.items()), [('something else', o2)])
+
+    def test_weak_keyed_bad_delitem(self):
+        d = weakref.WeakKeyDictionary()
+        o = Object('1')
+        # An attempt to delete an object that isn't there should raise
+        # KeyError.  It didn't before 2.3.
+        self.assertRaises(KeyError, d.__delitem__, o)
+        self.assertRaises(KeyError, d.__getitem__, o)
+
+        # If a key isn't of a weakly referencable type, __getitem__ and
+        # __setitem__ raise TypeError.  __delitem__ should too.
+        self.assertRaises(TypeError, d.__delitem__,  13)
+        self.assertRaises(TypeError, d.__getitem__,  13)
+        self.assertRaises(TypeError, d.__setitem__,  13, 13)
+
+    def test_weak_keyed_cascading_deletes(self):
+        # SF bug 742860.  For some reason, before 2.3 __delitem__ iterated
+        # over the keys via self.data.iterkeys().  If things vanished from
+        # the dict during this (or got added), that caused a RuntimeError.
+
+        d = weakref.WeakKeyDictionary()
+        mutate = False
+
+        class C(object):
+            def __init__(self, i):
+                self.value = i
+            def __hash__(self):
+                return hash(self.value)
+            def __eq__(self, other):
+                if mutate:
+                    # Side effect that mutates the dict, by removing the
+                    # last strong reference to a key.
+                    del objs[-1]
+                return self.value == other.value
+
+        objs = [C(i) for i in range(4)]
+        for o in objs:
+            d[o] = o.value
+        del o   # now the only strong references to keys are in objs
+        # Find the order in which iterkeys sees the keys.
+        objs = list(d.keys())
+        # Reverse it, so that the iteration implementation of __delitem__
+        # has to keep looping to find the first object we delete.
+        objs.reverse()
+
+        # Turn on mutation in C.__eq__.  The first time through the loop,
+        # under the iterkeys() business the first comparison will delete
+        # the last item iterkeys() would see, and that causes a
+        #     RuntimeError: dictionary changed size during iteration
+        # when the iterkeys() loop goes around to try comparing the next
+        # key.  After this was fixed, it just deletes the last object *our*
+        # "for o in obj" loop would have gotten to.
+        mutate = True
+        count = 0
+        for o in objs:
+            count += 1
+            del d[o]
+        gc_collect()  # For PyPy or other GCs.
+        self.assertEqual(len(d), 0)
+        self.assertEqual(count, 2)
+
+    def test_make_weak_valued_dict_repr(self):
+        dict = weakref.WeakValueDictionary()
+        self.assertRegex(repr(dict), '<WeakValueDictionary at 0x.*>')
+
+    def test_make_weak_keyed_dict_repr(self):
+        dict = weakref.WeakKeyDictionary()
+        self.assertRegex(repr(dict), '<WeakKeyDictionary at 0x.*>')
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_valued_setdefault(self):
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(100000):
+                x = d.setdefault(10, RefCycle())
+                self.assertIsNot(x, None)  # we never put None in there!
+                del x
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_valued_pop(self):
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(100000):
+                d[10] = RefCycle()
+                x = d.pop(10, 10)
+                self.assertIsNot(x, None)  # we never put None in there!
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_valued_consistency(self):
+        # Issue #28427: old keys should not remove new values from
+        # WeakValueDictionary when collecting from another thread.
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(200000):
+                o = RefCycle()
+                d[10] = o
+                # o is still alive, so the dict can't be empty
+                self.assertEqual(len(d), 1)
+                o = None  # lose ref
+
+    def check_threaded_weak_dict_copy(self, type_, deepcopy):
+        # `type_` should be either WeakKeyDictionary or WeakValueDictionary.
+        # `deepcopy` should be either True or False.
+        exc = []
+
+        class DummyKey:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        class DummyValue:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        def dict_copy(d, exc):
+            try:
+                if deepcopy is True:
+                    _ = copy.deepcopy(d)
+                else:
+                    _ = d.copy()
+            except Exception as ex:
+                exc.append(ex)
+
+        def pop_and_collect(lst):
+            gc_ctr = 0
+            while lst:
+                i = random.randint(0, len(lst) - 1)
+                gc_ctr += 1
+                lst.pop(i)
+                if gc_ctr % 10000 == 0:
+                    gc.collect()  # just in case
+
+        self.assertIn(type_, (weakref.WeakKeyDictionary, weakref.WeakValueDictionary))
+
+        d = type_()
+        keys = []
+        values = []
+        # Initialize d with many entries
+        for i in range(70000):
+            k, v = DummyKey(i), DummyValue(i)
+            keys.append(k)
+            values.append(v)
+            d[k] = v
+            del k
+            del v
+
+        t_copy = threading.Thread(target=dict_copy, args=(d, exc,))
+        if type_ is weakref.WeakKeyDictionary:
+            t_collect = threading.Thread(target=pop_and_collect, args=(keys,))
+        else:  # weakref.WeakValueDictionary
+            t_collect = threading.Thread(target=pop_and_collect, args=(values,))
+
+        t_copy.start()
+        t_collect.start()
+
+        t_copy.join()
+        t_collect.join()
+
+        # Test exceptions
+        if exc:
+            raise exc[0]
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_key_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, False)
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_key_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, True)
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_value_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, False)
+
+    @threading_helper.requires_working_threading()
+    def test_threaded_weak_value_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, True)
+
+    @support.cpython_only
+    def test_remove_closure(self):
+        d = weakref.WeakValueDictionary()
+        self.assertIsNone(d._remove.__closure__)
+
+
+from test import mapping_tests
+
+class WeakValueDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
+    """Check that WeakValueDictionary conforms to the mapping protocol"""
+    __ref = {"key1":Object(1), "key2":Object(2), "key3":Object(3)}
+    type2test = weakref.WeakValueDictionary
+    def _reference(self):
+        return self.__ref.copy()
+
+class WeakKeyDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
+    """Check that WeakKeyDictionary conforms to the mapping protocol"""
+    __ref = {Object("key1"):1, Object("key2"):2, Object("key3"):3}
+    type2test = weakref.WeakKeyDictionary
+    def _reference(self):
+        return self.__ref.copy()
+
+
+class FinalizeTestCase(unittest.TestCase):
+
+    class A:
+        pass
+
+    def _collect_if_necessary(self):
+        # we create no ref-cycles so in CPython no gc should be needed
+        if sys.implementation.name != 'cpython':
+            support.gc_collect()
+
+    def test_finalize(self):
+        def add(x,y,z):
+            res.append(x + y + z)
+            return x + y + z
+
+        a = self.A()
+
+        res = []
+        f = weakref.finalize(a, add, 67, 43, z=89)
+        self.assertEqual(f.alive, True)
+        self.assertEqual(f.peek(), (a, add, (67,43), {'z':89}))
+        self.assertEqual(f(), 199)
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [199])
+
+        res = []
+        f = weakref.finalize(a, add, 67, 43, 89)
+        self.assertEqual(f.peek(), (a, add, (67,43,89), {}))
+        self.assertEqual(f.detach(), (a, add, (67,43,89), {}))
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [])
+
+        res = []
+        f = weakref.finalize(a, add, x=67, y=43, z=89)
+        del a
+        self._collect_if_necessary()
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [199])
+
+    def test_arg_errors(self):
+        def fin(*args, **kwargs):
+            res.append((args, kwargs))
+
+        a = self.A()
+
+        res = []
+        f = weakref.finalize(a, fin, 1, 2, func=3, obj=4)
+        self.assertEqual(f.peek(), (a, fin, (1, 2), {'func': 3, 'obj': 4}))
+        f()
+        self.assertEqual(res, [((1, 2), {'func': 3, 'obj': 4})])
+
+        with self.assertRaises(TypeError):
+            weakref.finalize(a, func=fin, arg=1)
+        with self.assertRaises(TypeError):
+            weakref.finalize(obj=a, func=fin, arg=1)
+        self.assertRaises(TypeError, weakref.finalize, a)
+        self.assertRaises(TypeError, weakref.finalize)
+
+    def test_order(self):
+        a = self.A()
+        res = []
+
+        f1 = weakref.finalize(a, res.append, 'f1')
+        f2 = weakref.finalize(a, res.append, 'f2')
+        f3 = weakref.finalize(a, res.append, 'f3')
+        f4 = weakref.finalize(a, res.append, 'f4')
+        f5 = weakref.finalize(a, res.append, 'f5')
+
+        # make sure finalizers can keep themselves alive
+        del f1, f4
+
+        self.assertTrue(f2.alive)
+        self.assertTrue(f3.alive)
+        self.assertTrue(f5.alive)
+
+        self.assertTrue(f5.detach())
+        self.assertFalse(f5.alive)
+
+        f5()                       # nothing because previously unregistered
+        res.append('A')
+        f3()                       # => res.append('f3')
+        self.assertFalse(f3.alive)
+        res.append('B')
+        f3()                       # nothing because previously called
+        res.append('C')
+        del a
+        self._collect_if_necessary()
+                                   # => res.append('f4')
+                                   # => res.append('f2')
+                                   # => res.append('f1')
+        self.assertFalse(f2.alive)
+        res.append('D')
+        f2()                       # nothing because previously called by gc
+
+        expected = ['A', 'f3', 'B', 'C', 'f4', 'f2', 'f1', 'D']
+        self.assertEqual(res, expected)
+
+    def test_all_freed(self):
+        # we want a weakrefable subclass of weakref.finalize
+        class MyFinalizer(weakref.finalize):
+            pass
+
+        a = self.A()
+        res = []
+        def callback():
+            res.append(123)
+        f = MyFinalizer(a, callback)
+
+        wr_callback = weakref.ref(callback)
+        wr_f = weakref.ref(f)
+        del callback, f
+
+        self.assertIsNotNone(wr_callback())
+        self.assertIsNotNone(wr_f())
+
+        del a
+        self._collect_if_necessary()
+
+        self.assertIsNone(wr_callback())
+        self.assertIsNone(wr_f())
+        self.assertEqual(res, [123])
+
+    @classmethod
+    def run_in_child(cls):
+        def error():
+            # Create an atexit finalizer from inside a finalizer called
+            # at exit.  This should be the next to be run.
+            g1 = weakref.finalize(cls, print, 'g1')
+            print('f3 error')
+            1/0
+
+        # cls should stay alive till atexit callbacks run
+        f1 = weakref.finalize(cls, print, 'f1', _global_var)
+        f2 = weakref.finalize(cls, print, 'f2', _global_var)
+        f3 = weakref.finalize(cls, error)
+        f4 = weakref.finalize(cls, print, 'f4', _global_var)
+
+        assert f1.atexit == True
+        f2.atexit = False
+        assert f3.atexit == True
+        assert f4.atexit == True
+
+    def test_atexit(self):
+        prog = ('from test.test_weakref import FinalizeTestCase;'+
+                'FinalizeTestCase.run_in_child()')
+        rc, out, err = script_helper.assert_python_ok('-c', prog)
+        out = out.decode('ascii').splitlines()
+        self.assertEqual(out, ['f4 foobar', 'f3 error', 'g1', 'f1 foobar'])
+        self.assertTrue(b'ZeroDivisionError' in err)
+
+
+class ModuleTestCase(unittest.TestCase):
+    def test_names(self):
+        for name in ('ReferenceType', 'ProxyType', 'CallableProxyType',
+                     'WeakMethod', 'WeakSet', 'WeakKeyDictionary', 'WeakValueDictionary'):
+            obj = getattr(weakref, name)
+            if name != 'WeakSet':
+                self.assertEqual(obj.__module__, 'weakref')
+            self.assertEqual(obj.__name__, name)
+            self.assertEqual(obj.__qualname__, name)
+
+
+libreftest = """ Doctest for examples in the library reference: weakref.rst
+
+>>> from test.support import gc_collect
+>>> import weakref
+>>> class Dict(dict):
+...     pass
+...
+>>> obj = Dict(red=1, green=2, blue=3)   # this object is weak referencable
+>>> r = weakref.ref(obj)
+>>> print(r() is obj)
+True
+
+>>> import weakref
+>>> class Object:
+...     pass
+...
+>>> o = Object()
+>>> r = weakref.ref(o)
+>>> o2 = r()
+>>> o is o2
+True
+>>> del o, o2
+>>> gc_collect()  # For PyPy or other GCs.
+>>> print(r())
+None
+
+>>> import weakref
+>>> class ExtendedRef(weakref.ref):
+...     def __init__(self, ob, callback=None, **annotations):
+...         super().__init__(ob, callback)
+...         self.__counter = 0
+...         for k, v in annotations.items():
+...             setattr(self, k, v)
+...     def __call__(self):
+...         '''Return a pair containing the referent and the number of
+...         times the reference has been called.
+...         '''
+...         ob = super().__call__()
+...         if ob is not None:
+...             self.__counter += 1
+...             ob = (ob, self.__counter)
+...         return ob
+...
+>>> class A:   # not in docs from here, just testing the ExtendedRef
+...     pass
+...
+>>> a = A()
+>>> r = ExtendedRef(a, foo=1, bar="baz")
+>>> r.foo
+1
+>>> r.bar
+'baz'
+>>> r()[1]
+1
+>>> r()[1]
+2
+>>> r()[0] is a
+True
+
+
+>>> import weakref
+>>> _id2obj_dict = weakref.WeakValueDictionary()
+>>> def remember(obj):
+...     oid = id(obj)
+...     _id2obj_dict[oid] = obj
+...     return oid
+...
+>>> def id2obj(oid):
+...     return _id2obj_dict[oid]
+...
+>>> a = A()             # from here, just testing
+>>> a_id = remember(a)
+>>> id2obj(a_id) is a
+True
+>>> del a
+>>> gc_collect()  # For PyPy or other GCs.
+>>> try:
+...     id2obj(a_id)
+... except KeyError:
+...     print('OK')
+... else:
+...     print('WeakValueDictionary error')
+OK
+
+"""
+
+__test__ = {'libreftest' : libreftest}
+
+def load_tests(loader, tests, pattern):
+    tests.addTest(doctest.DocTestSuite())
+    return tests
+
+
+if __name__ == "__main__":
+    unittest.main()

src/greentest/3.8/test_weakref.py

+import gc
+import sys
+import unittest
+import collections
+import weakref
+import operator
+import contextlib
+import copy
+import threading
+import time
+import random
+
+from test import support
+from test.support import script_helper
+
+# Used in ReferencesTestCase.test_ref_created_during_del() .
+ref_from_del = None
+
+# Used by FinalizeTestCase as a global that may be replaced by None
+# when the interpreter shuts down.
+_global_var = 'foobar'
+
+class C:
+    def method(self):
+        pass
+
+
+class Callable:
+    bar = None
+
+    def __call__(self, x):
+        self.bar = x
+
+
+def create_function():
+    def f(): pass
+    return f
+
+def create_bound_method():
+    return C().method
+
+
+class Object:
+    def __init__(self, arg):
+        self.arg = arg
+    def __repr__(self):
+        return "<Object %r>" % self.arg
+    def __eq__(self, other):
+        if isinstance(other, Object):
+            return self.arg == other.arg
+        return NotImplemented
+    def __lt__(self, other):
+        if isinstance(other, Object):
+            return self.arg < other.arg
+        return NotImplemented
+    def __hash__(self):
+        return hash(self.arg)
+    def some_method(self):
+        return 4
+    def other_method(self):
+        return 5
+
+
+class RefCycle:
+    def __init__(self):
+        self.cycle = self
+
+
+class TestBase(unittest.TestCase):
+
+    def setUp(self):
+        self.cbcalled = 0
+
+    def callback(self, ref):
+        self.cbcalled += 1
+
+
+@contextlib.contextmanager
+def collect_in_thread(period=0.0001):
+    """
+    Ensure GC collections happen in a different thread, at a high frequency.
+    """
+    please_stop = False
+
+    def collect():
+        while not please_stop:
+            time.sleep(period)
+            gc.collect()
+
+    with support.disable_gc():
+        t = threading.Thread(target=collect)
+        t.start()
+        try:
+            yield
+        finally:
+            please_stop = True
+            t.join()
+
+
+class ReferencesTestCase(TestBase):
+
+    def test_basic_ref(self):
+        self.check_basic_ref(C)
+        self.check_basic_ref(create_function)
+        self.check_basic_ref(create_bound_method)
+
+        # Just make sure the tp_repr handler doesn't raise an exception.
+        # Live reference:
+        o = C()
+        wr = weakref.ref(o)
+        repr(wr)
+        # Dead reference:
+        del o
+        repr(wr)
+
+    def test_basic_callback(self):
+        self.check_basic_callback(C)
+        self.check_basic_callback(create_function)
+        self.check_basic_callback(create_bound_method)
+
+    @support.cpython_only
+    def test_cfunction(self):
+        import _testcapi
+        create_cfunction = _testcapi.create_cfunction
+        f = create_cfunction()
+        wr = weakref.ref(f)
+        self.assertIs(wr(), f)
+        del f
+        self.assertIsNone(wr())
+        self.check_basic_ref(create_cfunction)
+        self.check_basic_callback(create_cfunction)
+
+    def test_multiple_callbacks(self):
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del o
+        self.assertIsNone(ref1(), "expected reference to be invalidated")
+        self.assertIsNone(ref2(), "expected reference to be invalidated")
+        self.assertEqual(self.cbcalled, 2,
+                     "callback not called the right number of times")
+
+    def test_multiple_selfref_callbacks(self):
+        # Make sure all references are invalidated before callbacks are called
+        #
+        # What's important here is that we're using the first
+        # reference in the callback invoked on the second reference
+        # (the most recently created ref is cleaned up first).  This
+        # tests that all references to the object are invalidated
+        # before any of the callbacks are invoked, so that we only
+        # have one invocation of _weakref.c:cleanup_helper() active
+        # for a particular object at a time.
+        #
+        def callback(object, self=self):
+            self.ref()
+        c = C()
+        self.ref = weakref.ref(c, callback)
+        ref1 = weakref.ref(c, callback)
+        del c
+
+    def test_constructor_kwargs(self):
+        c = C()
+        self.assertRaises(TypeError, weakref.ref, c, callback=None)
+
+    def test_proxy_ref(self):
+        o = C()
+        o.bar = 1
+        ref1 = weakref.proxy(o, self.callback)
+        ref2 = weakref.proxy(o, self.callback)
+        del o
+
+        def check(proxy):
+            proxy.bar
+
+        self.assertRaises(ReferenceError, check, ref1)
+        self.assertRaises(ReferenceError, check, ref2)
+        self.assertRaises(ReferenceError, bool, weakref.proxy(C()))
+        self.assertEqual(self.cbcalled, 2)
+
+    def check_basic_ref(self, factory):
+        o = factory()
+        ref = weakref.ref(o)
+        self.assertIsNotNone(ref(),
+                     "weak reference to live object should be live")
+        o2 = ref()
+        self.assertIs(o, o2,
+                     "<ref>() should return original object if live")
+
+    def check_basic_callback(self, factory):
+        self.cbcalled = 0
+        o = factory()
+        ref = weakref.ref(o, self.callback)
+        del o
+        self.assertEqual(self.cbcalled, 1,
+                     "callback did not properly set 'cbcalled'")
+        self.assertIsNone(ref(),
+                     "ref2 should be dead after deleting object reference")
+
+    def test_ref_reuse(self):
+        o = C()
+        ref1 = weakref.ref(o)
+        # create a proxy to make sure that there's an intervening creation
+        # between these two; it should make no difference
+        proxy = weakref.proxy(o)
+        ref2 = weakref.ref(o)
+        self.assertIs(ref1, ref2,
+                     "reference object w/out callback should be re-used")
+
+        o = C()
+        proxy = weakref.proxy(o)
+        ref1 = weakref.ref(o)
+        ref2 = weakref.ref(o)
+        self.assertIs(ref1, ref2,
+                     "reference object w/out callback should be re-used")
+        self.assertEqual(weakref.getweakrefcount(o), 2,
+                     "wrong weak ref count for object")
+        del proxy
+        self.assertEqual(weakref.getweakrefcount(o), 1,
+                     "wrong weak ref count for object after deleting proxy")
+
+    def test_proxy_reuse(self):
+        o = C()
+        proxy1 = weakref.proxy(o)
+        ref = weakref.ref(o)
+        proxy2 = weakref.proxy(o)
+        self.assertIs(proxy1, proxy2,
+                     "proxy object w/out callback should have been re-used")
+
+    def test_basic_proxy(self):
+        o = C()
+        self.check_proxy(o, weakref.proxy(o))
+
+        L = collections.UserList()
+        p = weakref.proxy(L)
+        self.assertFalse(p, "proxy for empty UserList should be false")
+        p.append(12)
+        self.assertEqual(len(L), 1)
+        self.assertTrue(p, "proxy for non-empty UserList should be true")
+        p[:] = [2, 3]
+        self.assertEqual(len(L), 2)
+        self.assertEqual(len(p), 2)
+        self.assertIn(3, p, "proxy didn't support __contains__() properly")
+        p[1] = 5
+        self.assertEqual(L[1], 5)
+        self.assertEqual(p[1], 5)
+        L2 = collections.UserList(L)
+        p2 = weakref.proxy(L2)
+        self.assertEqual(p, p2)
+        ## self.assertEqual(repr(L2), repr(p2))
+        L3 = collections.UserList(range(10))
+        p3 = weakref.proxy(L3)
+        self.assertEqual(L3[:], p3[:])
+        self.assertEqual(L3[5:], p3[5:])
+        self.assertEqual(L3[:5], p3[:5])
+        self.assertEqual(L3[2:5], p3[2:5])
+
+    def test_proxy_unicode(self):
+        # See bug 5037
+        class C(object):
+            def __str__(self):
+                return "string"
+            def __bytes__(self):
+                return b"bytes"
+        instance = C()
+        self.assertIn("__bytes__", dir(weakref.proxy(instance)))
+        self.assertEqual(bytes(weakref.proxy(instance)), b"bytes")
+
+    def test_proxy_index(self):
+        class C:
+            def __index__(self):
+                return 10
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(operator.index(p), 10)
+
+    def test_proxy_div(self):
+        class C:
+            def __floordiv__(self, other):
+                return 42
+            def __ifloordiv__(self, other):
+                return 21
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(p // 5, 42)
+        p //= 5
+        self.assertEqual(p, 21)
+
+    def test_proxy_matmul(self):
+        class C:
+            def __matmul__(self, other):
+                return 1729
+            def __rmatmul__(self, other):
+                return -163
+            def __imatmul__(self, other):
+                return 561
+        o = C()
+        p = weakref.proxy(o)
+        self.assertEqual(p @ 5, 1729)
+        self.assertEqual(5 @ p, -163)
+        p @= 5
+        self.assertEqual(p, 561)
+
+    # The PyWeakref_* C API is documented as allowing either NULL or
+    # None as the value for the callback, where either means "no
+    # callback".  The "no callback" ref and proxy objects are supposed
+    # to be shared so long as they exist by all callers so long as
+    # they are active.  In Python 2.3.3 and earlier, this guarantee
+    # was not honored, and was broken in different ways for
+    # PyWeakref_NewRef() and PyWeakref_NewProxy().  (Two tests.)
+
+    def test_shared_ref_without_callback(self):
+        self.check_shared_without_callback(weakref.ref)
+
+    def test_shared_proxy_without_callback(self):
+        self.check_shared_without_callback(weakref.proxy)
+
+    def check_shared_without_callback(self, makeref):
+        o = Object(1)
+        p1 = makeref(o, None)
+        p2 = makeref(o, None)
+        self.assertIs(p1, p2, "both callbacks were None in the C API")
+        del p1, p2
+        p1 = makeref(o)
+        p2 = makeref(o, None)
+        self.assertIs(p1, p2, "callbacks were NULL, None in the C API")
+        del p1, p2
+        p1 = makeref(o)
+        p2 = makeref(o)
+        self.assertIs(p1, p2, "both callbacks were NULL in the C API")
+        del p1, p2
+        p1 = makeref(o, None)
+        p2 = makeref(o)
+        self.assertIs(p1, p2, "callbacks were None, NULL in the C API")
+
+    def test_callable_proxy(self):
+        o = Callable()
+        ref1 = weakref.proxy(o)
+
+        self.check_proxy(o, ref1)
+
+        self.assertIs(type(ref1), weakref.CallableProxyType,
+                     "proxy is not of callable type")
+        ref1('twinkies!')
+        self.assertEqual(o.bar, 'twinkies!',
+                     "call through proxy not passed through to original")
+        ref1(x='Splat.')
+        self.assertEqual(o.bar, 'Splat.',
+                     "call through proxy not passed through to original")
+
+        # expect due to too few args
+        self.assertRaises(TypeError, ref1)
+
+        # expect due to too many args
+        self.assertRaises(TypeError, ref1, 1, 2, 3)
+
+    def check_proxy(self, o, proxy):
+        o.foo = 1
+        self.assertEqual(proxy.foo, 1,
+                     "proxy does not reflect attribute addition")
+        o.foo = 2
+        self.assertEqual(proxy.foo, 2,
+                     "proxy does not reflect attribute modification")
+        del o.foo
+        self.assertFalse(hasattr(proxy, 'foo'),
+                     "proxy does not reflect attribute removal")
+
+        proxy.foo = 1
+        self.assertEqual(o.foo, 1,
+                     "object does not reflect attribute addition via proxy")
+        proxy.foo = 2
+        self.assertEqual(o.foo, 2,
+            "object does not reflect attribute modification via proxy")
+        del proxy.foo
+        self.assertFalse(hasattr(o, 'foo'),
+                     "object does not reflect attribute removal via proxy")
+
+    def test_proxy_deletion(self):
+        # Test clearing of SF bug #762891
+        class Foo:
+            result = None
+            def __delitem__(self, accessor):
+                self.result = accessor
+        g = Foo()
+        f = weakref.proxy(g)
+        del f[0]
+        self.assertEqual(f.result, 0)
+
+    def test_proxy_bool(self):
+        # Test clearing of SF bug #1170766
+        class List(list): pass
+        lyst = List()
+        self.assertEqual(bool(weakref.proxy(lyst)), bool(lyst))
+
+    def test_proxy_iter(self):
+        # Test fails with a debug build of the interpreter
+        # (see bpo-38395).
+
+        obj = None
+
+        class MyObj:
+            def __iter__(self):
+                nonlocal obj
+                del obj
+                return NotImplemented
+
+        obj = MyObj()
+        p = weakref.proxy(obj)
+        with self.assertRaises(TypeError):
+            # "blech" in p calls MyObj.__iter__ through the proxy,
+            # without keeping a reference to the real object, so it
+            # can be killed in the middle of the call
+            "blech" in p
+
+    def test_getweakrefcount(self):
+        o = C()
+        ref1 = weakref.ref(o)
+        ref2 = weakref.ref(o, self.callback)
+        self.assertEqual(weakref.getweakrefcount(o), 2,
+                     "got wrong number of weak reference objects")
+
+        proxy1 = weakref.proxy(o)
+        proxy2 = weakref.proxy(o, self.callback)
+        self.assertEqual(weakref.getweakrefcount(o), 4,
+                     "got wrong number of weak reference objects")
+
+        del ref1, ref2, proxy1, proxy2
+        self.assertEqual(weakref.getweakrefcount(o), 0,
+                     "weak reference objects not unlinked from"
+                     " referent when discarded.")
+
+        # assumes ints do not support weakrefs
+        self.assertEqual(weakref.getweakrefcount(1), 0,
+                     "got wrong number of weak reference objects for int")
+
+    def test_getweakrefs(self):
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del ref1
+        self.assertEqual(weakref.getweakrefs(o), [ref2],
+                     "list of refs does not match")
+
+        o = C()
+        ref1 = weakref.ref(o, self.callback)
+        ref2 = weakref.ref(o, self.callback)
+        del ref2
+        self.assertEqual(weakref.getweakrefs(o), [ref1],
+                     "list of refs does not match")
+
+        del ref1
+        self.assertEqual(weakref.getweakrefs(o), [],
+                     "list of refs not cleared")
+
+        # assumes ints do not support weakrefs
+        self.assertEqual(weakref.getweakrefs(1), [],
+                     "list of refs does not match for int")
+
+    def test_newstyle_number_ops(self):
+        class F(float):
+            pass
+        f = F(2.0)
+        p = weakref.proxy(f)
+        self.assertEqual(p + 1.0, 3.0)
+        self.assertEqual(1.0 + p, 3.0)  # this used to SEGV
+
+    def test_callbacks_protected(self):
+        # Callbacks protected from already-set exceptions?
+        # Regression test for SF bug #478534.
+        class BogusError(Exception):
+            pass
+        data = {}
+        def remove(k):
+            del data[k]
+        def encapsulate():
+            f = lambda : ()
+            data[weakref.ref(f, remove)] = None
+            raise BogusError
+        try:
+            encapsulate()
+        except BogusError:
+            pass
+        else:
+            self.fail("exception not properly restored")
+        try:
+            encapsulate()
+        except BogusError:
+            pass
+        else:
+            self.fail("exception not properly restored")
+
+    def test_sf_bug_840829(self):
+        # "weakref callbacks and gc corrupt memory"
+        # subtype_dealloc erroneously exposed a new-style instance
+        # already in the process of getting deallocated to gc,
+        # causing double-deallocation if the instance had a weakref
+        # callback that triggered gc.
+        # If the bug exists, there probably won't be an obvious symptom
+        # in a release build.  In a debug build, a segfault will occur
+        # when the second attempt to remove the instance from the "list
+        # of all objects" occurs.
+
+        import gc
+
+        class C(object):
+            pass
+
+        c = C()
+        wr = weakref.ref(c, lambda ignore: gc.collect())
+        del c
+
+        # There endeth the first part.  It gets worse.
+        del wr
+
+        c1 = C()
+        c1.i = C()
+        wr = weakref.ref(c1.i, lambda ignore: gc.collect())
+
+        c2 = C()
+        c2.c1 = c1
+        del c1  # still alive because c2 points to it
+
+        # Now when subtype_dealloc gets called on c2, it's not enough just
+        # that c2 is immune from gc while the weakref callbacks associated
+        # with c2 execute (there are none in this 2nd half of the test, btw).
+        # subtype_dealloc goes on to call the base classes' deallocs too,
+        # so any gc triggered by weakref callbacks associated with anything
+        # torn down by a base class dealloc can also trigger double
+        # deallocation of c2.
+        del c2
+
+    def test_callback_in_cycle_1(self):
+        import gc
+
+        class J(object):
+            pass
+
+        class II(object):
+            def acallback(self, ignore):
+                self.J
+
+        I = II()
+        I.J = J
+        I.wr = weakref.ref(J, I.acallback)
+
+        # Now J and II are each in a self-cycle (as all new-style class
+        # objects are, since their __mro__ points back to them).  I holds
+        # both a weak reference (I.wr) and a strong reference (I.J) to class
+        # J.  I is also in a cycle (I.wr points to a weakref that references
+        # I.acallback).  When we del these three, they all become trash, but
+        # the cycles prevent any of them from getting cleaned up immediately.
+        # Instead they have to wait for cyclic gc to deduce that they're
+        # trash.
+        #
+        # gc used to call tp_clear on all of them, and the order in which
+        # it does that is pretty accidental.  The exact order in which we
+        # built up these things manages to provoke gc into running tp_clear
+        # in just the right order (I last).  Calling tp_clear on II leaves
+        # behind an insane class object (its __mro__ becomes NULL).  Calling
+        # tp_clear on J breaks its self-cycle, but J doesn't get deleted
+        # just then because of the strong reference from I.J.  Calling
+        # tp_clear on I starts to clear I's __dict__, and just happens to
+        # clear I.J first -- I.wr is still intact.  That removes the last
+        # reference to J, which triggers the weakref callback.  The callback
+        # tries to do "self.J", and instances of new-style classes look up
+        # attributes ("J") in the class dict first.  The class (II) wants to
+        # search II.__mro__, but that's NULL.   The result was a segfault in
+        # a release build, and an assert failure in a debug build.
+        del I, J, II
+        gc.collect()
+
+    def test_callback_in_cycle_2(self):
+        import gc
+
+        # This is just like test_callback_in_cycle_1, except that II is an
+        # old-style class.  The symptom is different then:  an instance of an
+        # old-style class looks in its own __dict__ first.  'J' happens to
+        # get cleared from I.__dict__ before 'wr', and 'J' was never in II's
+        # __dict__, so the attribute isn't found.  The difference is that
+        # the old-style II doesn't have a NULL __mro__ (it doesn't have any
+        # __mro__), so no segfault occurs.  Instead it got:
+        #    test_callback_in_cycle_2 (__main__.ReferencesTestCase) ...
+        #    Exception exceptions.AttributeError:
+        #   "II instance has no attribute 'J'" in <bound method II.acallback
+        #       of <?.II instance at 0x00B9B4B8>> ignored
+
+        class J(object):
+            pass
+
+        class II:
+            def acallback(self, ignore):
+                self.J
+
+        I = II()
+        I.J = J
+        I.wr = weakref.ref(J, I.acallback)
+
+        del I, J, II
+        gc.collect()
+
+    def test_callback_in_cycle_3(self):
+        import gc
+
+        # This one broke the first patch that fixed the last two.  In this
+        # case, the objects reachable from the callback aren't also reachable
+        # from the object (c1) *triggering* the callback:  you can get to
+        # c1 from c2, but not vice-versa.  The result was that c2's __dict__
+        # got tp_clear'ed by the time the c2.cb callback got invoked.
+
+        class C:
+            def cb(self, ignore):
+                self.me
+                self.c1
+                self.wr
+
+        c1, c2 = C(), C()
+
+        c2.me = c2
+        c2.c1 = c1
+        c2.wr = weakref.ref(c1, c2.cb)
+
+        del c1, c2
+        gc.collect()
+
+    def test_callback_in_cycle_4(self):
+        import gc
+
+        # Like test_callback_in_cycle_3, except c2 and c1 have different
+        # classes.  c2's class (C) isn't reachable from c1 then, so protecting
+        # objects reachable from the dying object (c1) isn't enough to stop
+        # c2's class (C) from getting tp_clear'ed before c2.cb is invoked.
+        # The result was a segfault (C.__mro__ was NULL when the callback
+        # tried to look up self.me).
+
+        class C(object):
+            def cb(self, ignore):
+                self.me
+                self.c1
+                self.wr
+
+        class D:
+            pass
+
+        c1, c2 = D(), C()
+
+        c2.me = c2
+        c2.c1 = c1
+        c2.wr = weakref.ref(c1, c2.cb)
+
+        del c1, c2, C, D
+        gc.collect()
+
+    @support.requires_type_collecting
+    def test_callback_in_cycle_resurrection(self):
+        import gc
+
+        # Do something nasty in a weakref callback:  resurrect objects
+        # from dead cycles.  For this to be attempted, the weakref and
+        # its callback must also be part of the cyclic trash (else the
+        # objects reachable via the callback couldn't be in cyclic trash
+        # to begin with -- the callback would act like an external root).
+        # But gc clears trash weakrefs with callbacks early now, which
+        # disables the callbacks, so the callbacks shouldn't get called
+        # at all (and so nothing actually gets resurrected).
+
+        alist = []
+        class C(object):
+            def __init__(self, value):
+                self.attribute = value
+
+            def acallback(self, ignore):
+                alist.append(self.c)
+
+        c1, c2 = C(1), C(2)
+        c1.c = c2
+        c2.c = c1
+        c1.wr = weakref.ref(c2, c1.acallback)
+        c2.wr = weakref.ref(c1, c2.acallback)
+
+        def C_went_away(ignore):
+            alist.append("C went away")
+        wr = weakref.ref(C, C_went_away)
+
+        del c1, c2, C   # make them all trash
+        self.assertEqual(alist, [])  # del isn't enough to reclaim anything
+
+        gc.collect()
+        # c1.wr and c2.wr were part of the cyclic trash, so should have
+        # been cleared without their callbacks executing.  OTOH, the weakref
+        # to C is bound to a function local (wr), and wasn't trash, so that
+        # callback should have been invoked when C went away.
+        self.assertEqual(alist, ["C went away"])
+        # The remaining weakref should be dead now (its callback ran).
+        self.assertEqual(wr(), None)
+
+        del alist[:]
+        gc.collect()
+        self.assertEqual(alist, [])
+
+    def test_callbacks_on_callback(self):
+        import gc
+
+        # Set up weakref callbacks *on* weakref callbacks.
+        alist = []
+        def safe_callback(ignore):
+            alist.append("safe_callback called")
+
+        class C(object):
+            def cb(self, ignore):
+                alist.append("cb called")
+
+        c, d = C(), C()
+        c.other = d
+        d.other = c
+        callback = c.cb
+        c.wr = weakref.ref(d, callback)     # this won't trigger
+        d.wr = weakref.ref(callback, d.cb)  # ditto
+        external_wr = weakref.ref(callback, safe_callback)  # but this will
+        self.assertIs(external_wr(), callback)
+
+        # The weakrefs attached to c and d should get cleared, so that
+        # C.cb is never called.  But external_wr isn't part of the cyclic
+        # trash, and no cyclic trash is reachable from it, so safe_callback
+        # should get invoked when the bound method object callback (c.cb)
+        # -- which is itself a callback, and also part of the cyclic trash --
+        # gets reclaimed at the end of gc.
+
+        del callback, c, d, C
+        self.assertEqual(alist, [])  # del isn't enough to clean up cycles
+        gc.collect()
+        self.assertEqual(alist, ["safe_callback called"])
+        self.assertEqual(external_wr(), None)
+
+        del alist[:]
+        gc.collect()
+        self.assertEqual(alist, [])
+
+    def test_gc_during_ref_creation(self):
+        self.check_gc_during_creation(weakref.ref)
+
+    def test_gc_during_proxy_creation(self):
+        self.check_gc_during_creation(weakref.proxy)
+
+    def check_gc_during_creation(self, makeref):
+        thresholds = gc.get_threshold()
+        gc.set_threshold(1, 1, 1)
+        gc.collect()
+        class A:
+            pass
+
+        def callback(*args):
+            pass
+
+        referenced = A()
+
+        a = A()
+        a.a = a
+        a.wr = makeref(referenced)
+
+        try:
+            # now make sure the object and the ref get labeled as
+            # cyclic trash:
+            a = A()
+            weakref.ref(referenced, callback)
+
+        finally:
+            gc.set_threshold(*thresholds)
+
+    def test_ref_created_during_del(self):
+        # Bug #1377858
+        # A weakref created in an object's __del__() would crash the
+        # interpreter when the weakref was cleaned up since it would refer to
+        # non-existent memory.  This test should not segfault the interpreter.
+        class Target(object):
+            def __del__(self):
+                global ref_from_del
+                ref_from_del = weakref.ref(self)
+
+        w = Target()
+
+    def test_init(self):
+        # Issue 3634
+        # <weakref to class>.__init__() doesn't check errors correctly
+        r = weakref.ref(Exception)
+        self.assertRaises(TypeError, r.__init__, 0, 0, 0, 0, 0)
+        # No exception should be raised here
+        gc.collect()
+
+    def test_classes(self):
+        # Check that classes are weakrefable.
+        class A(object):
+            pass
+        l = []
+        weakref.ref(int)
+        a = weakref.ref(A, l.append)
+        A = None
+        gc.collect()
+        self.assertEqual(a(), None)
+        self.assertEqual(l, [a])
+
+    def test_equality(self):
+        # Alive weakrefs defer equality testing to their underlying object.
+        x = Object(1)
+        y = Object(1)
+        z = Object(2)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        c = weakref.ref(z)
+        d = weakref.ref(x)
+        # Note how we directly test the operators here, to stress both
+        # __eq__ and __ne__.
+        self.assertTrue(a == b)
+        self.assertFalse(a != b)
+        self.assertFalse(a == c)
+        self.assertTrue(a != c)
+        self.assertTrue(a == d)
+        self.assertFalse(a != d)
+        del x, y, z
+        gc.collect()
+        for r in a, b, c:
+            # Sanity check
+            self.assertIs(r(), None)
+        # Dead weakrefs compare by identity: whether `a` and `d` are the
+        # same weakref object is an implementation detail, since they pointed
+        # to the same original object and didn't have a callback.
+        # (see issue #16453).
+        self.assertFalse(a == b)
+        self.assertTrue(a != b)
+        self.assertFalse(a == c)
+        self.assertTrue(a != c)
+        self.assertEqual(a == d, a is d)
+        self.assertEqual(a != d, a is not d)
+
+    def test_ordering(self):
+        # weakrefs cannot be ordered, even if the underlying objects can.
+        ops = [operator.lt, operator.gt, operator.le, operator.ge]
+        x = Object(1)
+        y = Object(1)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        for op in ops:
+            self.assertRaises(TypeError, op, a, b)
+        # Same when dead.
+        del x, y
+        gc.collect()
+        for op in ops:
+            self.assertRaises(TypeError, op, a, b)
+
+    def test_hashing(self):
+        # Alive weakrefs hash the same as the underlying object
+        x = Object(42)
+        y = Object(42)
+        a = weakref.ref(x)
+        b = weakref.ref(y)
+        self.assertEqual(hash(a), hash(42))
+        del x, y
+        gc.collect()
+        # Dead weakrefs:
+        # - retain their hash is they were hashed when alive;
+        # - otherwise, cannot be hashed.
+        self.assertEqual(hash(a), hash(42))
+        self.assertRaises(TypeError, hash, b)
+
+    def test_trashcan_16602(self):
+        # Issue #16602: when a weakref's target was part of a long
+        # deallocation chain, the trashcan mechanism could delay clearing
+        # of the weakref and make the target object visible from outside
+        # code even though its refcount had dropped to 0.  A crash ensued.
+        class C:
+            def __init__(self, parent):
+                if not parent:
+                    return
+                wself = weakref.ref(self)
+                def cb(wparent):
+                    o = wself()
+                self.wparent = weakref.ref(parent, cb)
+
+        d = weakref.WeakKeyDictionary()
+        root = c = C(None)
+        for n in range(100):
+            d[c] = c = C(c)
+        del root
+        gc.collect()
+
+    def test_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        self.assertIs(ref1.__callback__, callback)
+
+        ref2 = weakref.ref(x)
+        self.assertIsNone(ref2.__callback__)
+
+    def test_callback_attribute_after_deletion(self):
+        x = Object(1)
+        ref = weakref.ref(x, self.callback)
+        self.assertIsNotNone(ref.__callback__)
+        del x
+        support.gc_collect()
+        self.assertIsNone(ref.__callback__)
+
+    def test_set_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        with self.assertRaises(AttributeError):
+            ref1.__callback__ = lambda ref: None
+
+    def test_callback_gcs(self):
+        class ObjectWithDel(Object):
+            def __del__(self): pass
+        x = ObjectWithDel(1)
+        ref1 = weakref.ref(x, lambda ref: support.gc_collect())
+        del x
+        support.gc_collect()
+
+
+class SubclassableWeakrefTestCase(TestBase):
+
+    def test_subclass_refs(self):
+        class MyRef(weakref.ref):
+            def __init__(self, ob, callback=None, value=42):
+                self.value = value
+                super().__init__(ob, callback)
+            def __call__(self):
+                self.called = True
+                return super().__call__()
+        o = Object("foo")
+        mr = MyRef(o, value=24)
+        self.assertIs(mr(), o)
+        self.assertTrue(mr.called)
+        self.assertEqual(mr.value, 24)
+        del o
+        self.assertIsNone(mr())
+        self.assertTrue(mr.called)
+
+    def test_subclass_refs_dont_replace_standard_refs(self):
+        class MyRef(weakref.ref):
+            pass
+        o = Object(42)
+        r1 = MyRef(o)
+        r2 = weakref.ref(o)
+        self.assertIsNot(r1, r2)
+        self.assertEqual(weakref.getweakrefs(o), [r2, r1])
+        self.assertEqual(weakref.getweakrefcount(o), 2)
+        r3 = MyRef(o)
+        self.assertEqual(weakref.getweakrefcount(o), 3)
+        refs = weakref.getweakrefs(o)
+        self.assertEqual(len(refs), 3)
+        self.assertIs(r2, refs[0])
+        self.assertIn(r1, refs[1:])
+        self.assertIn(r3, refs[1:])
+
+    def test_subclass_refs_dont_conflate_callbacks(self):
+        class MyRef(weakref.ref):
+            pass
+        o = Object(42)
+        r1 = MyRef(o, id)
+        r2 = MyRef(o, str)
+        self.assertIsNot(r1, r2)
+        refs = weakref.getweakrefs(o)
+        self.assertIn(r1, refs)
+        self.assertIn(r2, refs)
+
+    def test_subclass_refs_with_slots(self):
+        class MyRef(weakref.ref):
+            __slots__ = "slot1", "slot2"
+            def __new__(type, ob, callback, slot1, slot2):
+                return weakref.ref.__new__(type, ob, callback)
+            def __init__(self, ob, callback, slot1, slot2):
+                self.slot1 = slot1
+                self.slot2 = slot2
+            def meth(self):
+                return self.slot1 + self.slot2
+        o = Object(42)
+        r = MyRef(o, None, "abc", "def")
+        self.assertEqual(r.slot1, "abc")
+        self.assertEqual(r.slot2, "def")
+        self.assertEqual(r.meth(), "abcdef")
+        self.assertFalse(hasattr(r, "__dict__"))
+
+    def test_subclass_refs_with_cycle(self):
+        """Confirm https://bugs.python.org/issue3100 is fixed."""
+        # An instance of a weakref subclass can have attributes.
+        # If such a weakref holds the only strong reference to the object,
+        # deleting the weakref will delete the object. In this case,
+        # the callback must not be called, because the ref object is
+        # being deleted.
+        class MyRef(weakref.ref):
+            pass
+
+        # Use a local callback, for "regrtest -R::"
+        # to detect refcounting problems
+        def callback(w):
+            self.cbcalled += 1
+
+        o = C()
+        r1 = MyRef(o, callback)
+        r1.o = o
+        del o
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+        # Same test, with two weakrefs to the same object
+        # (since code paths are different)
+        o = C()
+        r1 = MyRef(o, callback)
+        r2 = MyRef(o, callback)
+        r1.r = r2
+        r2.o = o
+        del o
+        del r2
+
+        del r1 # Used to crash here
+
+        self.assertEqual(self.cbcalled, 0)
+
+
+class WeakMethodTestCase(unittest.TestCase):
+
+    def _subclass(self):
+        """Return an Object subclass overriding `some_method`."""
+        class C(Object):
+            def some_method(self):
+                return 6
+        return C
+
+    def test_alive(self):
+        o = Object(1)
+        r = weakref.WeakMethod(o.some_method)
+        self.assertIsInstance(r, weakref.ReferenceType)
+        self.assertIsInstance(r(), type(o.some_method))
+        self.assertIs(r().__self__, o)
+        self.assertIs(r().__func__, o.some_method.__func__)
+        self.assertEqual(r()(), 4)
+
+    def test_object_dead(self):
+        o = Object(1)
+        r = weakref.WeakMethod(o.some_method)
+        del o
+        gc.collect()
+        self.assertIs(r(), None)
+
+    def test_method_dead(self):
+        C = self._subclass()
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method)
+        del C.some_method
+        gc.collect()
+        self.assertIs(r(), None)
+
+    def test_callback_when_object_dead(self):
+        # Test callback behaviour when object dies first.
+        C = self._subclass()
+        calls = []
+        def cb(arg):
+            calls.append(arg)
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method, cb)
+        del o
+        gc.collect()
+        self.assertEqual(calls, [r])
+        # Callback is only called once.
+        C.some_method = Object.some_method
+        gc.collect()
+        self.assertEqual(calls, [r])
+
+    def test_callback_when_method_dead(self):
+        # Test callback behaviour when method dies first.
+        C = self._subclass()
+        calls = []
+        def cb(arg):
+            calls.append(arg)
+        o = C(1)
+        r = weakref.WeakMethod(o.some_method, cb)
+        del C.some_method
+        gc.collect()
+        self.assertEqual(calls, [r])
+        # Callback is only called once.
+        del o
+        gc.collect()
+        self.assertEqual(calls, [r])
+
+    @support.cpython_only
+    def test_no_cycles(self):
+        # A WeakMethod doesn't create any reference cycle to itself.
+        o = Object(1)
+        def cb(_):
+            pass
+        r = weakref.WeakMethod(o.some_method, cb)
+        wr = weakref.ref(r)
+        del r
+        self.assertIs(wr(), None)
+
+    def test_equality(self):
+        def _eq(a, b):
+            self.assertTrue(a == b)
+            self.assertFalse(a != b)
+        def _ne(a, b):
+            self.assertTrue(a != b)
+            self.assertFalse(a == b)
+        x = Object(1)
+        y = Object(1)
+        a = weakref.WeakMethod(x.some_method)
+        b = weakref.WeakMethod(y.some_method)
+        c = weakref.WeakMethod(x.other_method)
+        d = weakref.WeakMethod(y.other_method)
+        # Objects equal, same method
+        _eq(a, b)
+        _eq(c, d)
+        # Objects equal, different method
+        _ne(a, c)
+        _ne(a, d)
+        _ne(b, c)
+        _ne(b, d)
+        # Objects unequal, same or different method
+        z = Object(2)
+        e = weakref.WeakMethod(z.some_method)
+        f = weakref.WeakMethod(z.other_method)
+        _ne(a, e)
+        _ne(a, f)
+        _ne(b, e)
+        _ne(b, f)
+        del x, y, z
+        gc.collect()
+        # Dead WeakMethods compare by identity
+        refs = a, b, c, d, e, f
+        for q in refs:
+            for r in refs:
+                self.assertEqual(q == r, q is r)
+                self.assertEqual(q != r, q is not r)
+
+    def test_hashing(self):
+        # Alive WeakMethods are hashable if the underlying object is
+        # hashable.
+        x = Object(1)
+        y = Object(1)
+        a = weakref.WeakMethod(x.some_method)
+        b = weakref.WeakMethod(y.some_method)
+        c = weakref.WeakMethod(y.other_method)
+        # Since WeakMethod objects are equal, the hashes should be equal.
+        self.assertEqual(hash(a), hash(b))
+        ha = hash(a)
+        # Dead WeakMethods retain their old hash value
+        del x, y
+        gc.collect()
+        self.assertEqual(hash(a), ha)
+        self.assertEqual(hash(b), ha)
+        # If it wasn't hashed when alive, a dead WeakMethod cannot be hashed.
+        self.assertRaises(TypeError, hash, c)
+
+
+class MappingTestCase(TestBase):
+
+    COUNT = 10
+
+    def check_len_cycles(self, dict_type, cons):
+        N = 20
+        items = [RefCycle() for i in range(N)]
+        dct = dict_type(cons(o) for o in items)
+        # Keep an iterator alive
+        it = dct.items()
+        try:
+            next(it)
+        except StopIteration:
+            pass
+        del items
+        gc.collect()
+        n1 = len(dct)
+        del it
+        gc.collect()
+        n2 = len(dct)
+        # one item may be kept alive inside the iterator
+        self.assertIn(n1, (0, 1))
+        self.assertEqual(n2, 0)
+
+    def test_weak_keyed_len_cycles(self):
+        self.check_len_cycles(weakref.WeakKeyDictionary, lambda k: (k, 1))
+
+    def test_weak_valued_len_cycles(self):
+        self.check_len_cycles(weakref.WeakValueDictionary, lambda k: (1, k))
+
+    def check_len_race(self, dict_type, cons):
+        # Extended sanity checks for len() in the face of cyclic collection
+        self.addCleanup(gc.set_threshold, *gc.get_threshold())
+        for th in range(1, 100):
+            N = 20
+            gc.collect(0)
+            gc.set_threshold(th, th, th)
+            items = [RefCycle() for i in range(N)]
+            dct = dict_type(cons(o) for o in items)
+            del items
+            # All items will be collected at next garbage collection pass
+            it = dct.items()
+            try:
+                next(it)
+            except StopIteration:
+                pass
+            n1 = len(dct)
+            del it
+            n2 = len(dct)
+            self.assertGreaterEqual(n1, 0)
+            self.assertLessEqual(n1, N)
+            self.assertGreaterEqual(n2, 0)
+            self.assertLessEqual(n2, n1)
+
+    def test_weak_keyed_len_race(self):
+        self.check_len_race(weakref.WeakKeyDictionary, lambda k: (k, 1))
+
+    def test_weak_valued_len_race(self):
+        self.check_len_race(weakref.WeakValueDictionary, lambda k: (1, k))
+
+    def test_weak_values(self):
+        #
+        #  This exercises d.copy(), d.items(), d[], del d[], len(d).
+        #
+        dict, objects = self.make_weak_valued_dict()
+        for o in objects:
+            self.assertEqual(weakref.getweakrefcount(o), 1)
+            self.assertIs(o, dict[o.arg],
+                         "wrong object returned by weak dict!")
+        items1 = list(dict.items())
+        items2 = list(dict.copy().items())
+        items1.sort()
+        items2.sort()
+        self.assertEqual(items1, items2,
+                     "cloning of weak-valued dictionary did not work!")
+        del items1, items2
+        self.assertEqual(len(dict), self.COUNT)
+        del objects[0]
+        self.assertEqual(len(dict), self.COUNT - 1,
+                     "deleting object did not cause dictionary update")
+        del objects, o
+        self.assertEqual(len(dict), 0,
+                     "deleting the values did not clear the dictionary")
+        # regression on SF bug #447152:
+        dict = weakref.WeakValueDictionary()
+        self.assertRaises(KeyError, dict.__getitem__, 1)
+        dict[2] = C()
+        self.assertRaises(KeyError, dict.__getitem__, 2)
+
+    def test_weak_keys(self):
+        #
+        #  This exercises d.copy(), d.items(), d[] = v, d[], del d[],
+        #  len(d), k in d.
+        #
+        dict, objects = self.make_weak_keyed_dict()
+        for o in objects:
+            self.assertEqual(weakref.getweakrefcount(o), 1,
+                         "wrong number of weak references to %r!" % o)
+            self.assertIs(o.arg, dict[o],
+                         "wrong object returned by weak dict!")
+        items1 = dict.items()
+        items2 = dict.copy().items()
+        self.assertEqual(set(items1), set(items2),
+                     "cloning of weak-keyed dictionary did not work!")
+        del items1, items2
+        self.assertEqual(len(dict), self.COUNT)
+        del objects[0]
+        self.assertEqual(len(dict), (self.COUNT - 1),
+                     "deleting object did not cause dictionary update")
+        del objects, o
+        self.assertEqual(len(dict), 0,
+                     "deleting the keys did not clear the dictionary")
+        o = Object(42)
+        dict[o] = "What is the meaning of the universe?"
+        self.assertIn(o, dict)
+        self.assertNotIn(34, dict)
+
+    def test_weak_keyed_iters(self):
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_iters(dict)
+
+        # Test keyrefs()
+        refs = dict.keyrefs()
+        self.assertEqual(len(refs), len(objects))
+        objects2 = list(objects)
+        for wr in refs:
+            ob = wr()
+            self.assertIn(ob, dict)
+            self.assertIn(ob, dict)
+            self.assertEqual(ob.arg, dict[ob])
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+        # Test iterkeyrefs()
+        objects2 = list(objects)
+        self.assertEqual(len(list(dict.keyrefs())), len(objects))
+        for wr in dict.keyrefs():
+            ob = wr()
+            self.assertIn(ob, dict)
+            self.assertIn(ob, dict)
+            self.assertEqual(ob.arg, dict[ob])
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+    def test_weak_valued_iters(self):
+        dict, objects = self.make_weak_valued_dict()
+        self.check_iters(dict)
+
+        # Test valuerefs()
+        refs = dict.valuerefs()
+        self.assertEqual(len(refs), len(objects))
+        objects2 = list(objects)
+        for wr in refs:
+            ob = wr()
+            self.assertEqual(ob, dict[ob.arg])
+            self.assertEqual(ob.arg, dict[ob.arg].arg)
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+        # Test itervaluerefs()
+        objects2 = list(objects)
+        self.assertEqual(len(list(dict.itervaluerefs())), len(objects))
+        for wr in dict.itervaluerefs():
+            ob = wr()
+            self.assertEqual(ob, dict[ob.arg])
+            self.assertEqual(ob.arg, dict[ob.arg].arg)
+            objects2.remove(ob)
+        self.assertEqual(len(objects2), 0)
+
+    def check_iters(self, dict):
+        # item iterator:
+        items = list(dict.items())
+        for item in dict.items():
+            items.remove(item)
+        self.assertFalse(items, "items() did not touch all items")
+
+        # key iterator, via __iter__():
+        keys = list(dict.keys())
+        for k in dict:
+            keys.remove(k)
+        self.assertFalse(keys, "__iter__() did not touch all keys")
+
+        # key iterator, via iterkeys():
+        keys = list(dict.keys())
+        for k in dict.keys():
+            keys.remove(k)
+        self.assertFalse(keys, "iterkeys() did not touch all keys")
+
+        # value iterator:
+        values = list(dict.values())
+        for v in dict.values():
+            values.remove(v)
+        self.assertFalse(values,
+                     "itervalues() did not touch all values")
+
+    def check_weak_destroy_while_iterating(self, dict, objects, iter_name):
+        n = len(dict)
+        it = iter(getattr(dict, iter_name)())
+        next(it)             # Trigger internal iteration
+        # Destroy an object
+        del objects[-1]
+        gc.collect()    # just in case
+        # We have removed either the first consumed object, or another one
+        self.assertIn(len(list(it)), [len(objects), len(objects) - 1])
+        del it
+        # The removal has been committed
+        self.assertEqual(len(dict), n - 1)
+
+    def check_weak_destroy_and_mutate_while_iterating(self, dict, testcontext):
+        # Check that we can explicitly mutate the weak dict without
+        # interfering with delayed removal.
+        # `testcontext` should create an iterator, destroy one of the
+        # weakref'ed objects and then return a new key/value pair corresponding
+        # to the destroyed object.
+        with testcontext() as (k, v):
+            self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.__delitem__, k)
+        self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.pop, k)
+        self.assertNotIn(k, dict)
+        with testcontext() as (k, v):
+            dict[k] = v
+        self.assertEqual(dict[k], v)
+        ddict = copy.copy(dict)
+        with testcontext() as (k, v):
+            dict.update(ddict)
+        self.assertEqual(dict, ddict)
+        with testcontext() as (k, v):
+            dict.clear()
+        self.assertEqual(len(dict), 0)
+
+    def check_weak_del_and_len_while_iterating(self, dict, testcontext):
+        # Check that len() works when both iterating and removing keys
+        # explicitly through various means (.pop(), .clear()...), while
+        # implicit mutation is deferred because an iterator is alive.
+        # (each call to testcontext() should schedule one item for removal
+        #  for this test to work properly)
+        o = Object(123456)
+        with testcontext():
+            n = len(dict)
+            # Since underlaying dict is ordered, first item is popped
+            dict.pop(next(dict.keys()))
+            self.assertEqual(len(dict), n - 1)
+            dict[o] = o
+            self.assertEqual(len(dict), n)
+        # last item in objects is removed from dict in context shutdown
+        with testcontext():
+            self.assertEqual(len(dict), n - 1)
+            # Then, (o, o) is popped
+            dict.popitem()
+            self.assertEqual(len(dict), n - 2)
+        with testcontext():
+            self.assertEqual(len(dict), n - 3)
+            del dict[next(dict.keys())]
+            self.assertEqual(len(dict), n - 4)
+        with testcontext():
+            self.assertEqual(len(dict), n - 5)
+            dict.popitem()
+            self.assertEqual(len(dict), n - 6)
+        with testcontext():
+            dict.clear()
+            self.assertEqual(len(dict), 0)
+        self.assertEqual(len(dict), 0)
+
+    def test_weak_keys_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'keyrefs')
+        dict, objects = self.make_weak_keyed_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                v = objects.pop().arg
+                gc.collect()      # just in case
+                yield Object(v), v
+            finally:
+                it = None           # should commit all removals
+                gc.collect()
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+        # Issue #21173: len() fragile when keys are both implicitly and
+        # explicitly removed.
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_weak_del_and_len_while_iterating(dict, testcontext)
+
+    def test_weak_values_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_valued_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'itervaluerefs')
+        self.check_weak_destroy_while_iterating(dict, objects, 'valuerefs')
+        dict, objects = self.make_weak_valued_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                k = objects.pop().arg
+                gc.collect()      # just in case
+                yield k, Object(k)
+            finally:
+                it = None           # should commit all removals
+                gc.collect()
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+        dict, objects = self.make_weak_valued_dict()
+        self.check_weak_del_and_len_while_iterating(dict, testcontext)
+
+    def test_make_weak_keyed_dict_from_dict(self):
+        o = Object(3)
+        dict = weakref.WeakKeyDictionary({o:364})
+        self.assertEqual(dict[o], 364)
+
+    def test_make_weak_keyed_dict_from_weak_keyed_dict(self):
+        o = Object(3)
+        dict = weakref.WeakKeyDictionary({o:364})
+        dict2 = weakref.WeakKeyDictionary(dict)
+        self.assertEqual(dict[o], 364)
+
+    def make_weak_keyed_dict(self):
+        dict = weakref.WeakKeyDictionary()
+        objects = list(map(Object, range(self.COUNT)))
+        for o in objects:
+            dict[o] = o.arg
+        return dict, objects
+
+    def test_make_weak_valued_dict_from_dict(self):
+        o = Object(3)
+        dict = weakref.WeakValueDictionary({364:o})
+        self.assertEqual(dict[364], o)
+
+    def test_make_weak_valued_dict_from_weak_valued_dict(self):
+        o = Object(3)
+        dict = weakref.WeakValueDictionary({364:o})
+        dict2 = weakref.WeakValueDictionary(dict)
+        self.assertEqual(dict[364], o)
+
+    def test_make_weak_valued_dict_misc(self):
+        # errors
+        self.assertRaises(TypeError, weakref.WeakValueDictionary.__init__)
+        self.assertRaises(TypeError, weakref.WeakValueDictionary, {}, {})
+        self.assertRaises(TypeError, weakref.WeakValueDictionary, (), ())
+        # special keyword arguments
+        o = Object(3)
+        for kw in 'self', 'dict', 'other', 'iterable':
+            d = weakref.WeakValueDictionary(**{kw: o})
+            self.assertEqual(list(d.keys()), [kw])
+            self.assertEqual(d[kw], o)
+
+    def make_weak_valued_dict(self):
+        dict = weakref.WeakValueDictionary()
+        objects = list(map(Object, range(self.COUNT)))
+        for o in objects:
+            dict[o.arg] = o
+        return dict, objects
+
+    def check_popitem(self, klass, key1, value1, key2, value2):
+        weakdict = klass()
+        weakdict[key1] = value1
+        weakdict[key2] = value2
+        self.assertEqual(len(weakdict), 2)
+        k, v = weakdict.popitem()
+        self.assertEqual(len(weakdict), 1)
+        if k is key1:
+            self.assertIs(v, value1)
+        else:
+            self.assertIs(v, value2)
+        k, v = weakdict.popitem()
+        self.assertEqual(len(weakdict), 0)
+        if k is key1:
+            self.assertIs(v, value1)
+        else:
+            self.assertIs(v, value2)
+
+    def test_weak_valued_dict_popitem(self):
+        self.check_popitem(weakref.WeakValueDictionary,
+                           "key1", C(), "key2", C())
+
+    def test_weak_keyed_dict_popitem(self):
+        self.check_popitem(weakref.WeakKeyDictionary,
+                           C(), "value 1", C(), "value 2")
+
+    def check_setdefault(self, klass, key, value1, value2):
+        self.assertIsNot(value1, value2,
+                     "invalid test"
+                     " -- value parameters must be distinct objects")
+        weakdict = klass()
+        o = weakdict.setdefault(key, value1)
+        self.assertIs(o, value1)
+        self.assertIn(key, weakdict)
+        self.assertIs(weakdict.get(key), value1)
+        self.assertIs(weakdict[key], value1)
+
+        o = weakdict.setdefault(key, value2)
+        self.assertIs(o, value1)
+        self.assertIn(key, weakdict)
+        self.assertIs(weakdict.get(key), value1)
+        self.assertIs(weakdict[key], value1)
+
+    def test_weak_valued_dict_setdefault(self):
+        self.check_setdefault(weakref.WeakValueDictionary,
+                              "key", C(), C())
+
+    def test_weak_keyed_dict_setdefault(self):
+        self.check_setdefault(weakref.WeakKeyDictionary,
+                              C(), "value 1", "value 2")
+
+    def check_update(self, klass, dict):
+        #
+        #  This exercises d.update(), len(d), d.keys(), k in d,
+        #  d.get(), d[].
+        #
+        weakdict = klass()
+        weakdict.update(dict)
+        self.assertEqual(len(weakdict), len(dict))
+        for k in weakdict.keys():
+            self.assertIn(k, dict, "mysterious new key appeared in weak dict")
+            v = dict.get(k)
+            self.assertIs(v, weakdict[k])
+            self.assertIs(v, weakdict.get(k))
+        for k in dict.keys():
+            self.assertIn(k, weakdict, "original key disappeared in weak dict")
+            v = dict[k]
+            self.assertIs(v, weakdict[k])
+            self.assertIs(v, weakdict.get(k))
+
+    def test_weak_valued_dict_update(self):
+        self.check_update(weakref.WeakValueDictionary,
+                          {1: C(), 'a': C(), C(): C()})
+        # errors
+        self.assertRaises(TypeError, weakref.WeakValueDictionary.update)
+        d = weakref.WeakValueDictionary()
+        self.assertRaises(TypeError, d.update, {}, {})
+        self.assertRaises(TypeError, d.update, (), ())
+        self.assertEqual(list(d.keys()), [])
+        # special keyword arguments
+        o = Object(3)
+        for kw in 'self', 'dict', 'other', 'iterable':
+            d = weakref.WeakValueDictionary()
+            d.update(**{kw: o})
+            self.assertEqual(list(d.keys()), [kw])
+            self.assertEqual(d[kw], o)
+
+    def test_weak_keyed_dict_update(self):
+        self.check_update(weakref.WeakKeyDictionary,
+                          {C(): 1, C(): 2, C(): 3})
+
+    def test_weak_keyed_delitem(self):
+        d = weakref.WeakKeyDictionary()
+        o1 = Object('1')
+        o2 = Object('2')
+        d[o1] = 'something'
+        d[o2] = 'something'
+        self.assertEqual(len(d), 2)
+        del d[o1]
+        self.assertEqual(len(d), 1)
+        self.assertEqual(list(d.keys()), [o2])
+
+    def test_weak_valued_delitem(self):
+        d = weakref.WeakValueDictionary()
+        o1 = Object('1')
+        o2 = Object('2')
+        d['something'] = o1
+        d['something else'] = o2
+        self.assertEqual(len(d), 2)
+        del d['something']
+        self.assertEqual(len(d), 1)
+        self.assertEqual(list(d.items()), [('something else', o2)])
+
+    def test_weak_keyed_bad_delitem(self):
+        d = weakref.WeakKeyDictionary()
+        o = Object('1')
+        # An attempt to delete an object that isn't there should raise
+        # KeyError.  It didn't before 2.3.
+        self.assertRaises(KeyError, d.__delitem__, o)
+        self.assertRaises(KeyError, d.__getitem__, o)
+
+        # If a key isn't of a weakly referencable type, __getitem__ and
+        # __setitem__ raise TypeError.  __delitem__ should too.
+        self.assertRaises(TypeError, d.__delitem__,  13)
+        self.assertRaises(TypeError, d.__getitem__,  13)
+        self.assertRaises(TypeError, d.__setitem__,  13, 13)
+
+    def test_weak_keyed_cascading_deletes(self):
+        # SF bug 742860.  For some reason, before 2.3 __delitem__ iterated
+        # over the keys via self.data.iterkeys().  If things vanished from
+        # the dict during this (or got added), that caused a RuntimeError.
+
+        d = weakref.WeakKeyDictionary()
+        mutate = False
+
+        class C(object):
+            def __init__(self, i):
+                self.value = i
+            def __hash__(self):
+                return hash(self.value)
+            def __eq__(self, other):
+                if mutate:
+                    # Side effect that mutates the dict, by removing the
+                    # last strong reference to a key.
+                    del objs[-1]
+                return self.value == other.value
+
+        objs = [C(i) for i in range(4)]
+        for o in objs:
+            d[o] = o.value
+        del o   # now the only strong references to keys are in objs
+        # Find the order in which iterkeys sees the keys.
+        objs = list(d.keys())
+        # Reverse it, so that the iteration implementation of __delitem__
+        # has to keep looping to find the first object we delete.
+        objs.reverse()
+
+        # Turn on mutation in C.__eq__.  The first time through the loop,
+        # under the iterkeys() business the first comparison will delete
+        # the last item iterkeys() would see, and that causes a
+        #     RuntimeError: dictionary changed size during iteration
+        # when the iterkeys() loop goes around to try comparing the next
+        # key.  After this was fixed, it just deletes the last object *our*
+        # "for o in obj" loop would have gotten to.
+        mutate = True
+        count = 0
+        for o in objs:
+            count += 1
+            del d[o]
+        self.assertEqual(len(d), 0)
+        self.assertEqual(count, 2)
+
+    def test_make_weak_valued_dict_repr(self):
+        dict = weakref.WeakValueDictionary()
+        self.assertRegex(repr(dict), '<WeakValueDictionary at 0x.*>')
+
+    def test_make_weak_keyed_dict_repr(self):
+        dict = weakref.WeakKeyDictionary()
+        self.assertRegex(repr(dict), '<WeakKeyDictionary at 0x.*>')
+
+    def test_threaded_weak_valued_setdefault(self):
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(100000):
+                x = d.setdefault(10, RefCycle())
+                self.assertIsNot(x, None)  # we never put None in there!
+                del x
+
+    def test_threaded_weak_valued_pop(self):
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(100000):
+                d[10] = RefCycle()
+                x = d.pop(10, 10)
+                self.assertIsNot(x, None)  # we never put None in there!
+
+    def test_threaded_weak_valued_consistency(self):
+        # Issue #28427: old keys should not remove new values from
+        # WeakValueDictionary when collecting from another thread.
+        d = weakref.WeakValueDictionary()
+        with collect_in_thread():
+            for i in range(200000):
+                o = RefCycle()
+                d[10] = o
+                # o is still alive, so the dict can't be empty
+                self.assertEqual(len(d), 1)
+                o = None  # lose ref
+
+    def check_threaded_weak_dict_copy(self, type_, deepcopy):
+        # `type_` should be either WeakKeyDictionary or WeakValueDictionary.
+        # `deepcopy` should be either True or False.
+        exc = []
+
+        class DummyKey:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        class DummyValue:
+            def __init__(self, ctr):
+                self.ctr = ctr
+
+        def dict_copy(d, exc):
+            try:
+                if deepcopy is True:
+                    _ = copy.deepcopy(d)
+                else:
+                    _ = d.copy()
+            except Exception as ex:
+                exc.append(ex)
+
+        def pop_and_collect(lst):
+            gc_ctr = 0
+            while lst:
+                i = random.randint(0, len(lst) - 1)
+                gc_ctr += 1
+                lst.pop(i)
+                if gc_ctr % 10000 == 0:
+                    gc.collect()  # just in case
+
+        self.assertIn(type_, (weakref.WeakKeyDictionary, weakref.WeakValueDictionary))
+
+        d = type_()
+        keys = []
+        values = []
+        # Initialize d with many entries
+        for i in range(70000):
+            k, v = DummyKey(i), DummyValue(i)
+            keys.append(k)
+            values.append(v)
+            d[k] = v
+            del k
+            del v
+
+        t_copy = threading.Thread(target=dict_copy, args=(d, exc,))
+        if type_ is weakref.WeakKeyDictionary:
+            t_collect = threading.Thread(target=pop_and_collect, args=(keys,))
+        else:  # weakref.WeakValueDictionary
+            t_collect = threading.Thread(target=pop_and_collect, args=(values,))
+
+        t_copy.start()
+        t_collect.start()
+
+        t_copy.join()
+        t_collect.join()
+
+        # Test exceptions
+        if exc:
+            raise exc[0]
+
+    def test_threaded_weak_key_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, False)
+
+    def test_threaded_weak_key_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakKeyDictionary, True)
+
+    def test_threaded_weak_value_dict_copy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, False)
+
+    def test_threaded_weak_value_dict_deepcopy(self):
+        # Issue #35615: Weakref keys or values getting GC'ed during dict
+        # copying should not result in a crash.
+        self.check_threaded_weak_dict_copy(weakref.WeakValueDictionary, True)
+
+    @support.cpython_only
+    def test_remove_closure(self):
+        d = weakref.WeakValueDictionary()
+        self.assertIsNone(d._remove.__closure__)
+
+
+from test import mapping_tests
+
+class WeakValueDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
+    """Check that WeakValueDictionary conforms to the mapping protocol"""
+    __ref = {"key1":Object(1), "key2":Object(2), "key3":Object(3)}
+    type2test = weakref.WeakValueDictionary
+    def _reference(self):
+        return self.__ref.copy()
+
+class WeakKeyDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
+    """Check that WeakKeyDictionary conforms to the mapping protocol"""
+    __ref = {Object("key1"):1, Object("key2"):2, Object("key3"):3}
+    type2test = weakref.WeakKeyDictionary
+    def _reference(self):
+        return self.__ref.copy()
+
+
+class FinalizeTestCase(unittest.TestCase):
+
+    class A:
+        pass
+
+    def _collect_if_necessary(self):
+        # we create no ref-cycles so in CPython no gc should be needed
+        if sys.implementation.name != 'cpython':
+            support.gc_collect()
+
+    def test_finalize(self):
+        def add(x,y,z):
+            res.append(x + y + z)
+            return x + y + z
+
+        a = self.A()
+
+        res = []
+        f = weakref.finalize(a, add, 67, 43, z=89)
+        self.assertEqual(f.alive, True)
+        self.assertEqual(f.peek(), (a, add, (67,43), {'z':89}))
+        self.assertEqual(f(), 199)
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [199])
+
+        res = []
+        f = weakref.finalize(a, add, 67, 43, 89)
+        self.assertEqual(f.peek(), (a, add, (67,43,89), {}))
+        self.assertEqual(f.detach(), (a, add, (67,43,89), {}))
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [])
+
+        res = []
+        f = weakref.finalize(a, add, x=67, y=43, z=89)
+        del a
+        self._collect_if_necessary()
+        self.assertEqual(f(), None)
+        self.assertEqual(f(), None)
+        self.assertEqual(f.peek(), None)
+        self.assertEqual(f.detach(), None)
+        self.assertEqual(f.alive, False)
+        self.assertEqual(res, [199])
+
+    def test_arg_errors(self):
+        def fin(*args, **kwargs):
+            res.append((args, kwargs))
+
+        a = self.A()
+
+        res = []
+        f = weakref.finalize(a, fin, 1, 2, func=3, obj=4)
+        self.assertEqual(f.peek(), (a, fin, (1, 2), {'func': 3, 'obj': 4}))
+        f()
+        self.assertEqual(res, [((1, 2), {'func': 3, 'obj': 4})])
+
+        res = []
+        with self.assertWarns(DeprecationWarning):
+            f = weakref.finalize(a, func=fin, arg=1)
+        self.assertEqual(f.peek(), (a, fin, (), {'arg': 1}))
+        f()
+        self.assertEqual(res, [((), {'arg': 1})])
+
+        res = []
+        with self.assertWarns(DeprecationWarning):
+            f = weakref.finalize(obj=a, func=fin, arg=1)
+        self.assertEqual(f.peek(), (a, fin, (), {'arg': 1}))
+        f()
+        self.assertEqual(res, [((), {'arg': 1})])
+
+        self.assertRaises(TypeError, weakref.finalize, a)
+        self.assertRaises(TypeError, weakref.finalize)
+
+    def test_order(self):
+        a = self.A()
+        res = []
+
+        f1 = weakref.finalize(a, res.append, 'f1')
+        f2 = weakref.finalize(a, res.append, 'f2')
+        f3 = weakref.finalize(a, res.append, 'f3')
+        f4 = weakref.finalize(a, res.append, 'f4')
+        f5 = weakref.finalize(a, res.append, 'f5')
+
+        # make sure finalizers can keep themselves alive
+        del f1, f4
+
+        self.assertTrue(f2.alive)
+        self.assertTrue(f3.alive)
+        self.assertTrue(f5.alive)
+
+        self.assertTrue(f5.detach())
+        self.assertFalse(f5.alive)
+
+        f5()                       # nothing because previously unregistered
+        res.append('A')
+        f3()                       # => res.append('f3')
+        self.assertFalse(f3.alive)
+        res.append('B')
+        f3()                       # nothing because previously called
+        res.append('C')
+        del a
+        self._collect_if_necessary()
+                                   # => res.append('f4')
+                                   # => res.append('f2')
+                                   # => res.append('f1')
+        self.assertFalse(f2.alive)
+        res.append('D')
+        f2()                       # nothing because previously called by gc
+
+        expected = ['A', 'f3', 'B', 'C', 'f4', 'f2', 'f1', 'D']
+        self.assertEqual(res, expected)
+
+    def test_all_freed(self):
+        # we want a weakrefable subclass of weakref.finalize
+        class MyFinalizer(weakref.finalize):
+            pass
+
+        a = self.A()
+        res = []
+        def callback():
+            res.append(123)
+        f = MyFinalizer(a, callback)
+
+        wr_callback = weakref.ref(callback)
+        wr_f = weakref.ref(f)
+        del callback, f
+
+        self.assertIsNotNone(wr_callback())
+        self.assertIsNotNone(wr_f())
+
+        del a
+        self._collect_if_necessary()
+
+        self.assertIsNone(wr_callback())
+        self.assertIsNone(wr_f())
+        self.assertEqual(res, [123])
+
+    @classmethod
+    def run_in_child(cls):
+        def error():
+            # Create an atexit finalizer from inside a finalizer called
+            # at exit.  This should be the next to be run.
+            g1 = weakref.finalize(cls, print, 'g1')
+            print('f3 error')
+            1/0
+
+        # cls should stay alive till atexit callbacks run
+        f1 = weakref.finalize(cls, print, 'f1', _global_var)
+        f2 = weakref.finalize(cls, print, 'f2', _global_var)
+        f3 = weakref.finalize(cls, error)
+        f4 = weakref.finalize(cls, print, 'f4', _global_var)
+
+        assert f1.atexit == True
+        f2.atexit = False
+        assert f3.atexit == True
+        assert f4.atexit == True
+
+    def test_atexit(self):
+        prog = ('from test.test_weakref import FinalizeTestCase;'+
+                'FinalizeTestCase.run_in_child()')
+        rc, out, err = script_helper.assert_python_ok('-c', prog)
+        out = out.decode('ascii').splitlines()
+        self.assertEqual(out, ['f4 foobar', 'f3 error', 'g1', 'f1 foobar'])
+        self.assertTrue(b'ZeroDivisionError' in err)
+
+
+libreftest = """ Doctest for examples in the library reference: weakref.rst
+
+>>> import weakref
+>>> class Dict(dict):
+...     pass
+...
+>>> obj = Dict(red=1, green=2, blue=3)   # this object is weak referencable
+>>> r = weakref.ref(obj)
+>>> print(r() is obj)
+True
+
+>>> import weakref
+>>> class Object:
+...     pass
+...
+>>> o = Object()
+>>> r = weakref.ref(o)
+>>> o2 = r()
+>>> o is o2
+True
+>>> del o, o2
+>>> print(r())
+None
+
+>>> import weakref
+>>> class ExtendedRef(weakref.ref):
+...     def __init__(self, ob, callback=None, **annotations):
+...         super().__init__(ob, callback)
+...         self.__counter = 0
+...         for k, v in annotations.items():
+...             setattr(self, k, v)
+...     def __call__(self):
+...         '''Return a pair containing the referent and the number of
+...         times the reference has been called.
+...         '''
+...         ob = super().__call__()
+...         if ob is not None:
+...             self.__counter += 1
+...             ob = (ob, self.__counter)
+...         return ob
+...
+>>> class A:   # not in docs from here, just testing the ExtendedRef
+...     pass
+...
+>>> a = A()
+>>> r = ExtendedRef(a, foo=1, bar="baz")
+>>> r.foo
+1
+>>> r.bar
+'baz'
+>>> r()[1]
+1
+>>> r()[1]
+2
+>>> r()[0] is a
+True
+
+
+>>> import weakref
+>>> _id2obj_dict = weakref.WeakValueDictionary()
+>>> def remember(obj):
+...     oid = id(obj)
+...     _id2obj_dict[oid] = obj
+...     return oid
+...
+>>> def id2obj(oid):
+...     return _id2obj_dict[oid]
+...
+>>> a = A()             # from here, just testing
+>>> a_id = remember(a)
+>>> id2obj(a_id) is a
+True
+>>> del a
+>>> try:
+...     id2obj(a_id)
+... except KeyError:
+...     print('OK')
+... else:
+...     print('WeakValueDictionary error')
+OK
+
+"""
+
+__test__ = {'libreftest' : libreftest}
+
+def test_main():
+    support.run_unittest(
+        ReferencesTestCase,
+        WeakMethodTestCase,
+        MappingTestCase,
+        WeakValueDictionaryTestCase,
+        WeakKeyDictionaryTestCase,
+        SubclassableWeakrefTestCase,
+        FinalizeTestCase,
+        )
+    support.run_doctest(sys.modules[__name__])
+
+
+if __name__ == "__main__":
+    test_main()