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✏️ test_finalization.py

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"""
Tests for object finalization semantics, as outlined in PEP 442.
"""

import contextlib
import gc
import unittest
import weakref

try:
    from _testcapi import with_tp_del
except ImportError:
    def with_tp_del(cls):
        class C(object):
            def __new__(cls, *args, **kwargs):
                raise TypeError('requires _testcapi.with_tp_del')
        return C

try:
    from _testcapi import without_gc
except ImportError:
    def without_gc(cls):
        class C:
            def __new__(cls, *args, **kwargs):
                raise TypeError('requires _testcapi.without_gc')
        return C

from test import support

class NonGCSimpleBase:
    """
    The base class for all the objects under test, equipped with various
    testing features.
    """

survivors = []
    del_calls = []
    tp_del_calls = []
    errors = []

_cleaning = False

__slots__ = ()

@classmethod
    def _cleanup(cls):
        cls.survivors.clear()
        cls.errors.clear()
        gc.garbage.clear()
        gc.collect()
        cls.del_calls.clear()
        cls.tp_del_calls.clear()

@classmethod
    @contextlib.contextmanager
    def test(cls):
        """
        A context manager to use around all finalization tests.
        """
        with support.disable_gc():
            cls.del_calls.clear()
            cls.tp_del_calls.clear()
            NonGCSimpleBase._cleaning = False
            try:
                yield
                if cls.errors:
                    raise cls.errors[0]
            finally:
                NonGCSimpleBase._cleaning = True
                cls._cleanup()

def check_sanity(self):
        """
        Check the object is sane (non-broken).
        """

def __del__(self):
        """
        PEP 442 finalizer.  Record that this was called, check the
        object is in a sane state, and invoke a side effect.
        """
        try:
            if not self._cleaning:
                self.del_calls.append(id(self))
                self.check_sanity()
                self.side_effect()
        except Exception as e:
            self.errors.append(e)

def side_effect(self):
        """
        A side effect called on destruction.
        """

class SimpleBase(NonGCSimpleBase):

def __init__(self):
        self.id_ = id(self)

def check_sanity(self):
        assert self.id_ == id(self)

@without_gc
class NonGC(NonGCSimpleBase):
    __slots__ = ()

@without_gc
class NonGCResurrector(NonGCSimpleBase):
    __slots__ = ()

def side_effect(self):
        """
        Resurrect self by storing self in a class-wide list.
        """
        self.survivors.append(self)

class Simple(SimpleBase):
    pass

# Can't inherit from NonGCResurrector, in case importing without_gc fails.
class SimpleResurrector(SimpleBase):

def side_effect(self):
        """
        Resurrect self by storing self in a class-wide list.
        """
        self.survivors.append(self)

class TestBase:

def setUp(self):
        self.old_garbage = gc.garbage[:]
        gc.garbage[:] = []

def tearDown(self):
        # None of the tests here should put anything in gc.garbage
        try:
            self.assertEqual(gc.garbage, [])
        finally:
            del self.old_garbage
            gc.collect()

def assert_del_calls(self, ids):
        self.assertEqual(sorted(SimpleBase.del_calls), sorted(ids))

def assert_tp_del_calls(self, ids):
        self.assertEqual(sorted(SimpleBase.tp_del_calls), sorted(ids))

def assert_survivors(self, ids):
        self.assertEqual(sorted(id(x) for x in SimpleBase.survivors), sorted(ids))

def assert_garbage(self, ids):
        self.assertEqual(sorted(id(x) for x in gc.garbage), sorted(ids))

def clear_survivors(self):
        SimpleBase.survivors.clear()

class SimpleFinalizationTest(TestBase, unittest.TestCase):
    """
    Test finalization without refcycles.
    """

def test_simple(self):
        with SimpleBase.test():
            s = Simple()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])

def test_simple_resurrect(self):
        with SimpleBase.test():
            s = SimpleResurrector()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors(ids)
            self.assertIsNot(wr(), None)
            self.clear_survivors()
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
        self.assertIs(wr(), None)

@support.cpython_only
    def test_non_gc(self):
        with SimpleBase.test():
            s = NonGC()
            self.assertFalse(gc.is_tracked(s))
            ids = [id(s)]
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])

@support.cpython_only
    def test_non_gc_resurrect(self):
        with SimpleBase.test():
            s = NonGCResurrector()
            self.assertFalse(gc.is_tracked(s))
            ids = [id(s)]
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors(ids)
            self.clear_survivors()
            gc.collect()
            self.assert_del_calls(ids * 2)
            self.assert_survivors(ids)

class SelfCycleBase:

def __init__(self):
        super().__init__()
        self.ref = self

def check_sanity(self):
        super().check_sanity()
        assert self.ref is self

class SimpleSelfCycle(SelfCycleBase, Simple):
    pass

class SelfCycleResurrector(SelfCycleBase, SimpleResurrector):
    pass

class SuicidalSelfCycle(SelfCycleBase, Simple):

def side_effect(self):
        """
        Explicitly break the reference cycle.
        """
        self.ref = None

class SelfCycleFinalizationTest(TestBase, unittest.TestCase):
    """
    Test finalization of an object having a single cyclic reference to
    itself.
    """

def test_simple(self):
        with SimpleBase.test():
            s = SimpleSelfCycle()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])

def test_simple_resurrect(self):
        # Test that __del__ can resurrect the object being finalized.
        with SimpleBase.test():
            s = SelfCycleResurrector()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors(ids)
            # XXX is this desirable?
            self.assertIs(wr(), None)
            # When trying to destroy the object a second time, __del__
            # isn't called anymore (and the object isn't resurrected).
            self.clear_survivors()
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)

def test_simple_suicide(self):
        # Test the GC is able to deal with an object that kills its last
        # reference during __del__.
        with SimpleBase.test():
            s = SuicidalSelfCycle()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)

class ChainedBase:

def chain(self, left):
        self.suicided = False
        self.left = left
        left.right = self

def check_sanity(self):
        super().check_sanity()
        if self.suicided:
            assert self.left is None
            assert self.right is None
        else:
            left = self.left
            if left.suicided:
                assert left.right is None
            else:
                assert left.right is self
            right = self.right
            if right.suicided:
                assert right.left is None
            else:
                assert right.left is self

class SimpleChained(ChainedBase, Simple):
    pass

class ChainedResurrector(ChainedBase, SimpleResurrector):
    pass

class SuicidalChained(ChainedBase, Simple):

def side_effect(self):
        """
        Explicitly break the reference cycle.
        """
        self.suicided = True
        self.left = None
        self.right = None

class CycleChainFinalizationTest(TestBase, unittest.TestCase):
    """
    Test finalization of a cyclic chain.  These tests are similar in
    spirit to the self-cycle tests above, but the collectable object
    graph isn't trivial anymore.
    """

def build_chain(self, classes):
        nodes = [cls() for cls in classes]
        for i in range(len(nodes)):
            nodes[i].chain(nodes[i-1])
        return nodes

def check_non_resurrecting_chain(self, classes):
        N = len(classes)
        with SimpleBase.test():
            nodes = self.build_chain(classes)
            ids = [id(s) for s in nodes]
            wrs = [weakref.ref(s) for s in nodes]
            del nodes
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])
            self.assertEqual([wr() for wr in wrs], [None] * N)
            gc.collect()
            self.assert_del_calls(ids)

def check_resurrecting_chain(self, classes):
        N = len(classes)
        with SimpleBase.test():
            nodes = self.build_chain(classes)
            N = len(nodes)
            ids = [id(s) for s in nodes]
            survivor_ids = [id(s) for s in nodes if isinstance(s, SimpleResurrector)]
            wrs = [weakref.ref(s) for s in nodes]
            del nodes
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors(survivor_ids)
            # XXX desirable?
            self.assertEqual([wr() for wr in wrs], [None] * N)
            self.clear_survivors()
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_survivors([])

def test_homogenous(self):
        self.check_non_resurrecting_chain([SimpleChained] * 3)

def test_homogenous_resurrect(self):
        self.check_resurrecting_chain([ChainedResurrector] * 3)

def test_homogenous_suicidal(self):
        self.check_non_resurrecting_chain([SuicidalChained] * 3)

def test_heterogenous_suicidal_one(self):
        self.check_non_resurrecting_chain([SuicidalChained, SimpleChained] * 2)

def test_heterogenous_suicidal_two(self):
        self.check_non_resurrecting_chain(
            [SuicidalChained] * 2 + [SimpleChained] * 2)

def test_heterogenous_resurrect_one(self):
        self.check_resurrecting_chain([ChainedResurrector, SimpleChained] * 2)

def test_heterogenous_resurrect_two(self):
        self.check_resurrecting_chain(
            [ChainedResurrector, SimpleChained, SuicidalChained] * 2)

def test_heterogenous_resurrect_three(self):
        self.check_resurrecting_chain(
            [ChainedResurrector] * 2 + [SimpleChained] * 2 + [SuicidalChained] * 2)

# NOTE: the tp_del slot isn't automatically inherited, so we have to call
# with_tp_del() for each instantiated class.

class LegacyBase(SimpleBase):

def __del__(self):
        try:
            # Do not invoke side_effect here, since we are now exercising
            # the tp_del slot.
            if not self._cleaning:
                self.del_calls.append(id(self))
                self.check_sanity()
        except Exception as e:
            self.errors.append(e)

def __tp_del__(self):
        """
        Legacy (pre-PEP 442) finalizer, mapped to a tp_del slot.
        """
        try:
            if not self._cleaning:
                self.tp_del_calls.append(id(self))
                self.check_sanity()
                self.side_effect()
        except Exception as e:
            self.errors.append(e)

@with_tp_del
class Legacy(LegacyBase):
    pass

@with_tp_del
class LegacyResurrector(LegacyBase):

def side_effect(self):
        """
        Resurrect self by storing self in a class-wide list.
        """
        self.survivors.append(self)

@with_tp_del
class LegacySelfCycle(SelfCycleBase, LegacyBase):
    pass

@support.cpython_only
class LegacyFinalizationTest(TestBase, unittest.TestCase):
    """
    Test finalization of objects with a tp_del.
    """

def tearDown(self):
        # These tests need to clean up a bit more, since they create
        # uncollectable objects.
        gc.garbage.clear()
        gc.collect()
        super().tearDown()

def test_legacy(self):
        with SimpleBase.test():
            s = Legacy()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_tp_del_calls(ids)
            self.assert_survivors([])
            self.assertIs(wr(), None)
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_tp_del_calls(ids)

def test_legacy_resurrect(self):
        with SimpleBase.test():
            s = LegacyResurrector()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_tp_del_calls(ids)
            self.assert_survivors(ids)
            # weakrefs are cleared before tp_del is called.
            self.assertIs(wr(), None)
            self.clear_survivors()
            gc.collect()
            self.assert_del_calls(ids)
            self.assert_tp_del_calls(ids * 2)
            self.assert_survivors(ids)
        self.assertIs(wr(), None)

def test_legacy_self_cycle(self):
        # Self-cycles with legacy finalizers end up in gc.garbage.
        with SimpleBase.test():
            s = LegacySelfCycle()
            ids = [id(s)]
            wr = weakref.ref(s)
            del s
            gc.collect()
            self.assert_del_calls([])
            self.assert_tp_del_calls([])
            self.assert_survivors([])
            self.assert_garbage(ids)
            self.assertIsNot(wr(), None)
            # Break the cycle to allow collection
            gc.garbage[0].ref = None
        self.assert_garbage([])
        self.assertIs(wr(), None)

if __name__ == "__main__":
    unittest.main()

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✏️ test_finalization.py