cpython/Lib/test/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
import textwrap

from test import support
from test.support import script_helper, ALWAYS_EQ, suppress_immortalization
from test.support import gc_collect
from test.support import import_helper
from test.support import threading_helper
from test.support import is_wasi, Py_DEBUG

# 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=0.005):
    """
    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)

    @support.cpython_only
    def test_ref_repr(self):
        obj = C()
        ref = weakref.ref(obj)
        regex = (
            rf"<weakref at 0x[0-9a-fA-F]+; "
            rf"to '{'' if __name__ == '__main__' else C.__module__ + '.'}{C.__qualname__}' "
            rf"at 0x[0-9a-fA-F]+>"
        )
        self.assertRegex(repr(ref), regex)

        obj = None
        gc_collect()
        self.assertRegex(repr(ref),
                         rf'<weakref at 0x[0-9a-fA-F]+; dead>')

        # test type with __name__
        class WithName:
            @property
            def __name__(self):
                return "custom_name"

        obj2 = WithName()
        ref2 = weakref.ref(obj2)
        regex = (
            rf"<weakref at 0x[0-9a-fA-F]+; "
            rf"to '{'' if __name__ == '__main__' else WithName.__module__ + '.'}"
            rf"{WithName.__qualname__}' "
            rf"at 0x[0-9a-fA-F]+ +\(custom_name\)>"
        )
        self.assertRegex(repr(ref2), regex)

    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):
        _testcapi = import_helper.import_module("_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)

    @support.cpython_only
    def test_proxy_repr(self):
        obj = C()
        ref = weakref.proxy(obj, self.callback)
        regex = (
            rf"<weakproxy at 0x[0-9a-fA-F]+; "
            rf"to '{'' if __name__ == '__main__' else C.__module__ + '.'}{C.__qualname__}' "
            rf"at 0x[0-9a-fA-F]+>"
        )
        self.assertRegex(repr(ref), regex)

        obj = None
        gc_collect()
        self.assertRegex(repr(ref),
                         rf'<weakproxy at 0x[0-9a-fA-F]+; dead>')

    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

    @suppress_immortalization()
    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()

    @suppress_immortalization()
    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()

    @suppress_immortalization()
    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)

    @unittest.skipIf(is_wasi and Py_DEBUG, "requires deep stack")
    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()

    @support.cpython_only
    def test_no_memory_when_clearing(self):
        # gh-118331: Make sure we do not raise an exception from the destructor
        # when clearing weakrefs if allocating the intermediate tuple fails.
        code = textwrap.dedent("""
        import _testcapi
        import weakref

        class TestObj:
            pass

        def callback(obj):
            pass

        obj = TestObj()
        # The choice of 50 is arbitrary, but must be large enough to ensure
        # the allocation won't be serviced by the free list.
        wrs = [weakref.ref(obj, callback) for _ in range(50)]
        _testcapi.set_nomemory(0)
        del obj
        """).strip()
        res, _ = script_helper.run_python_until_end("-c", code)
        stderr = res.err.decode("ascii", "backslashreplace")
        self.assertNotRegex(stderr, "_Py_Dealloc: Deallocator of type 'TestObj'")


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

    if support.check_sanitizer(thread=True) and support.Py_GIL_DISABLED:
        # Reduce iteration count to get acceptable latency
        NUM_THREADED_ITERATIONS = 1000
    else:
        NUM_THREADED_ITERATIONS = 100000

    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(self.NUM_THREADED_ITERATIONS):
                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(self.NUM_THREADED_ITERATIONS):
                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(2 * self.NUM_THREADED_ITERATIONS):
                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

    @support.cpython_only
    def test_weak_valued_consistency(self):
        # A single-threaded, deterministic repro for issue #28427: old keys
        # should not remove new values from WeakValueDictionary. This relies on
        # an implementation detail of CPython's WeakValueDictionary (its
        # underlying dictionary of KeyedRefs) to reproduce the issue.
        d = weakref.WeakValueDictionary()
        with support.disable_gc():
            d[10] = RefCycle()
            # Keep the KeyedRef alive after it's replaced so that GC will invoke
            # the callback.
            wr = d.data[10]
            # Replace the value with something that isn't cyclic garbage
            o = RefCycle()
            d[10] = o
            # Trigger GC, which will invoke the callback for `wr`
            gc.collect()
            self.assertEqual(len(d), 1)

    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()
    @support.requires_resource('cpu')
    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()
    @support.requires_resource('cpu')
    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()
    @support.requires_resource('cpu')
    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()
    @support.requires_resource('cpu')
    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()