Issue #15528: Add weakref.finalize to support finalization using

weakref callbacks.

This is 2e446e87ac5b except that collections/__init__.py has been
modified to import proxy from _weakref instead of weakref.  This
eliminates an import cycle which seems to cause a problem on Unix but
not Windows.

Doc/library/weakref.rst

@@ -51,10 +51,15 @@ garbage collection.  :class:`WeakSet` implements the :class:`set` interface,
 but keeps weak references to its elements, just like a
 :class:`WeakKeyDictionary` does.
 
-Most programs should find that using one of these weak container types is all
-they need -- it's not usually necessary to create your own weak references
-directly.  The low-level machinery used by the weak dictionary implementations
-is exposed by the :mod:`weakref` module for the benefit of advanced uses.
+:class:`finalize` provides a straight forward way to register a
+cleanup function to be called when an object is garbage collected.
+This is simpler to use than setting up a callback function on a raw
+weak reference.
+
+Most programs should find that using one of these weak container types
+or :class:`finalize` is all they need -- it's not usually necessary to
+create your own weak references directly.  The low-level machinery is
+exposed by the :mod:`weakref` module for the benefit of advanced uses.
 
 Not all objects can be weakly referenced; those objects which can include class
 instances, functions written in Python (but not in C), instance methods, sets,
@@ -117,7 +122,16 @@ Extension types can easily be made to support weak references; see
       weakref.  If there is no callback or if the referent of the weakref is
       no longer alive then this attribute will have value ``None``.
 
-   .. versionadded:: 3.4
+   .. note::
+
+      Like :meth:`__del__` methods, weak reference callbacks can be
+      called during interpreter shutdown when module globals have been
+      overwritten with :const:`None`.  This can make writing robust
+      weak reference callbacks a challenge.  Callbacks registered
+      using :class:`finalize` do not have to worry about this issue
+      because they will not be run after module teardown has begun.
+
+   .. versionchanged:: 3.4
       Added the :attr:`__callback__` attribute.
 
 
@@ -229,6 +243,66 @@ These method have the same issues as the and :meth:`keyrefs` method of
 
    .. versionadded:: 3.4
 
+.. class:: finalize(obj, func, *args, **kwargs)
+
+   Return a callable finalizer object which will be called when *obj*
+   is garbage collected.  A finalizer is *alive* until it is called
+   (either explicitly or at garbage collection), and after that it is
+   *dead*.  Calling a live finalizer returns the result of evaluating
+   ``func(*arg, **kwargs)``, whereas calling a dead finalizer returns
+   :const:`None`.
+
+   Exceptions raised by finalizer callbacks during garbage collection
+   will be shown on the standard error output, but cannot be
+   propagated.  They are handled in the same way as exceptions raised
+   from an object's :meth:`__del__` method or a weak reference's
+   callback.
+
+   When the program exits, each remaining live finalizer is called
+   unless its :attr:`atexit` attribute has been set to false.  They
+   are called in reverse order of creation.
+
+   A finalizer will never invoke its callback during the later part of
+   the interpreter shutdown when module globals are liable to have
+   been replaced by :const:`None`.
+
+   .. method:: __call__()
+
+      If *self* is alive then mark it as dead and return the result of
+      calling ``func(*args, **kwargs)``.  If *self* is dead then return
+      :const:`None`.
+
+   .. method:: detach()
+
+      If *self* is alive then mark it as dead and return the tuple
+      ``(obj, func, args, kwargs)``.  If *self* is dead then return
+      :const:`None`.
+
+   .. method:: peek()
+
+      If *self* is alive then return the tuple ``(obj, func, args,
+      kwargs)``.  If *self* is dead then return :const:`None`.
+
+   .. attribute:: alive
+
+      Property which is true if the finalizer is alive, false otherwise.
+
+   .. attribute:: atexit
+
+      A writable boolean property which by default is true.  When the
+      program exits, it calls all remaining live finalizers for which
+      :attr:`.atexit` is true.  They are called in reverse order of
+      creation.
+
+   .. note::
+
+      It is important to ensure that *func*, *args* and *kwargs* do
+      not own any references to *obj*, either directly or indirectly,
+      since otherwise *obj* will never be garbage collected.  In
+      particular, *func* should not be a bound method of *obj*.
+
+   .. versionadded:: 3.4
+
 
 .. data:: ReferenceType
 
@@ -365,3 +439,134 @@ objects can still be retrieved by ID if they do.
    def id2obj(oid):
        return _id2obj_dict[oid]
 
+
+.. _finalize-examples:
+
+Finalizer Objects
+-----------------
+
+Often one uses :class:`finalize` to register a callback without
+bothering to keep the returned finalizer object.  For instance
+
+    >>> import weakref
+    >>> class Object:
+    ...     pass
+    ...
+    >>> kenny = Object()
+    >>> weakref.finalize(kenny, print, "You killed Kenny!")  #doctest:+ELLIPSIS
+    <finalize object at ...; for 'Object' at ...>
+    >>> del kenny
+    You killed Kenny!
+
+The finalizer can be called directly as well.  However the finalizer
+will invoke the callback at most once.
+
+    >>> def callback(x, y, z):
+    ...     print("CALLBACK")
+    ...     return x + y + z
+    ...
+    >>> obj = Object()
+    >>> f = weakref.finalize(obj, callback, 1, 2, z=3)
+    >>> assert f.alive
+    >>> assert f() == 6
+    CALLBACK
+    >>> assert not f.alive
+    >>> f()                     # callback not called because finalizer dead
+    >>> del obj                 # callback not called because finalizer dead
+
+You can unregister a finalizer using its :meth:`~finalize.detach`
+method.  This kills the finalizer and returns the arguments passed to
+the constructor when it was created.
+
+    >>> obj = Object()
+    >>> f = weakref.finalize(obj, callback, 1, 2, z=3)
+    >>> f.detach()                                           #doctest:+ELLIPSIS
+    (<__main__.Object object ...>, <function callback ...>, (1, 2), {'z': 3})
+    >>> newobj, func, args, kwargs = _
+    >>> assert not f.alive
+    >>> assert newobj is obj
+    >>> assert func(*args, **kwargs) == 6
+    CALLBACK
+
+Unless you set the :attr:`~finalize.atexit` attribute to
+:const:`False`, a finalizer will be called when the program exit if it
+is still alive.  For instance
+
+    >>> obj = Object()
+    >>> weakref.finalize(obj, print, "obj dead or exiting")  #doctest:+ELLIPSIS
+    <finalize object at ...; for 'Object' at ...>
+    >>> exit()                                               #doctest:+SKIP
+    obj dead or exiting
+
+
+Comparing finalizers with :meth:`__del__` methods
+-------------------------------------------------
+
+Suppose we want to create a class whose instances represent temporary
+directories.  The directories should be deleted with their contents
+when the first of the following events occurs:
+
+* the object is garbage collected,
+* the object's :meth:`remove` method is called, or
+* the program exits.
+
+We might try to implement the class using a :meth:`__del__` method as
+follows::
+
+    class TempDir:
+        def __init__(self):
+            self.name = tempfile.mkdtemp()
+
+        def remove(self):
+            if self.name is not None:
+                shutil.rmtree(self.name)
+                self.name = None
+
+        @property
+        def removed(self):
+            return self.name is None
+
+        def __del__(self):
+            self.remove()
+
+This solution has a couple of serious problems:
+
+* There is no guarantee that the object will be garbage collected
+  before the program exists, so the directory might be left.  This is
+  because reference cycles containing an object with a :meth:`__del__`
+  method can never be collected.  And even if the :class:`TempDir`
+  object is not itself part of a reference cycle, it may still be kept
+  alive by some unkown uncollectable reference cycle.
+
+* The :meth:`__del__` method may be called at shutdown after the
+  :mod:`shutil` module has been cleaned up, in which case
+  :attr:`shutil.rmtree` will have been replaced by :const:`None`.
+  This will cause the :meth:`__del__` method to fail and the directory
+  will not be removed.
+
+Using finalizers we can avoid these problems::
+
+    class TempDir:
+        def __init__(self):
+            self.name = tempfile.mkdtemp()
+            self._finalizer = weakref.finalize(self, shutil.rmtree, self.name)
+
+        def remove(self):
+            self._finalizer()
+
+        @property
+        def removed(self):
+            return not self._finalizer.alive
+
+Defined like this, even if a :class:`TempDir` object is part of a
+reference cycle, that reference cycle can still be garbage collected.
+If the object never gets garbage collected the finalizer will still be
+called at exit.
+
+.. note::
+
+   If you create a finalizer object in a daemonic thread just as the
+   the program exits then there is the possibility that the finalizer
+   does not get called at exit.  However, in a daemonic thread
+   :func:`atexit.register`, ``try: ... finally: ...`` and ``with: ...``
+   do not guarantee that cleanup occurs either.

Lib/collections/__init__.py

@@ -12,7 +12,7 @@ from operator import itemgetter as _itemgetter, eq as _eq
 from keyword import iskeyword as _iskeyword
 import sys as _sys
 import heapq as _heapq
-from weakref import proxy as _proxy
+from _weakref import proxy as _proxy
 from itertools import repeat as _repeat, chain as _chain, starmap as _starmap
 from reprlib import recursive_repr as _recursive_repr
 

Lib/test/test_weakref.py

@@ -7,11 +7,15 @@ import operator
 import contextlib
 import copy
 
-from test import support
+from test import support, script_helper
 
 # Used in ReferencesTestCase.test_ref_created_during_del() .
 ref_from_del = None
 
+# Used by FinalizeTestCase as a global that may be replaced by None
+# when the interpreter shuts down.
+_global_var = 'foobar'
+
 class C:
     def method(self):
         pass
@@ -1551,6 +1555,151 @@ class WeakKeyDictionaryTestCase(mapping_tests.BasicTestMappingProtocol):
     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_order(self):
+        a = self.A()
+        res = []
+
+        f1 = weakref.finalize(a, res.append, 'f1')
+        f2 = weakref.finalize(a, res.append, 'f2')
+        f3 = weakref.finalize(a, res.append, 'f3')
+        f4 = weakref.finalize(a, res.append, 'f4')
+        f5 = weakref.finalize(a, res.append, 'f5')
+
+        # make sure finalizers can keep themselves alive
+        del f1, f4
+
+        self.assertTrue(f2.alive)
+        self.assertTrue(f3.alive)
+        self.assertTrue(f5.alive)
+
+        self.assertTrue(f5.detach())
+        self.assertFalse(f5.alive)
+
+        f5()                       # nothing because previously unregistered
+        res.append('A')
+        f3()                       # => res.append('f3')
+        self.assertFalse(f3.alive)
+        res.append('B')
+        f3()                       # nothing because previously called
+        res.append('C')
+        del a
+        self._collect_if_necessary()
+                                   # => res.append('f4')
+                                   # => res.append('f2')
+                                   # => res.append('f1')
+        self.assertFalse(f2.alive)
+        res.append('D')
+        f2()                       # nothing because previously called by gc
+
+        expected = ['A', 'f3', 'B', 'C', 'f4', 'f2', 'f1', 'D']
+        self.assertEqual(res, expected)
+
+    def test_all_freed(self):
+        # we want a weakrefable subclass of weakref.finalize
+        class MyFinalizer(weakref.finalize):
+            pass
+
+        a = self.A()
+        res = []
+        def callback():
+            res.append(123)
+        f = MyFinalizer(a, callback)
+
+        wr_callback = weakref.ref(callback)
+        wr_f = weakref.ref(f)
+        del callback, f
+
+        self.assertIsNotNone(wr_callback())
+        self.assertIsNotNone(wr_f())
+
+        del a
+        self._collect_if_necessary()
+
+        self.assertIsNone(wr_callback())
+        self.assertIsNone(wr_f())
+        self.assertEqual(res, [123])
+
+    @classmethod
+    def run_in_child(cls):
+        def error():
+            # Create an atexit finalizer from inside a finalizer called
+            # at exit.  This should be the next to be run.
+            g1 = weakref.finalize(cls, print, 'g1')
+            print('f3 error')
+            1/0
+
+        # cls should stay alive till atexit callbacks run
+        f1 = weakref.finalize(cls, print, 'f1', _global_var)
+        f2 = weakref.finalize(cls, print, 'f2', _global_var)
+        f3 = weakref.finalize(cls, error)
+        f4 = weakref.finalize(cls, print, 'f4', _global_var)
+
+        assert f1.atexit == True
+        f2.atexit = False
+        assert f3.atexit == True
+        assert f4.atexit == True
+
+    def test_atexit(self):
+        prog = ('from test.test_weakref import FinalizeTestCase;'+
+                'FinalizeTestCase.run_in_child()')
+        rc, out, err = script_helper.assert_python_ok('-c', prog)
+        out = out.decode('ascii').splitlines()
+        self.assertEqual(out, ['f4 foobar', 'f3 error', 'g1', 'f1 foobar'])
+        self.assertTrue(b'ZeroDivisionError' in err)
+
+
 libreftest = """ Doctest for examples in the library reference: weakref.rst
 
 >>> import weakref
@@ -1644,6 +1793,7 @@ def test_main():
         WeakValueDictionaryTestCase,
         WeakKeyDictionaryTestCase,
         SubclassableWeakrefTestCase,
+        FinalizeTestCase,
         )
     support.run_doctest(sys.modules[__name__])
 

Lib/weakref.py

@@ -21,13 +21,16 @@ from _weakref import (
 from _weakrefset import WeakSet, _IterationGuard
 
 import collections  # Import after _weakref to avoid circular import.
+import sys
+import itertools
+import atexit
 
 ProxyTypes = (ProxyType, CallableProxyType)
 
 __all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
            "WeakKeyDictionary", "ReferenceType", "ProxyType",
            "CallableProxyType", "ProxyTypes", "WeakValueDictionary",
-           "WeakSet", "WeakMethod"]
+           "WeakSet", "WeakMethod", "finalize"]
 
 
 class WeakMethod(ref):
@@ -436,3 +439,135 @@ class WeakKeyDictionary(collections.MutableMapping):
                 d[ref(key, self._remove)] = value
         if len(kwargs):
             self.update(kwargs)
+
+
+class finalize:
+    """Class for finalization of weakrefable objects
+
+    finalize(obj, func, *args, **kwargs) returns a callable finalizer
+    object which will be called when obj is garbage collected. The
+    first time the finalizer is called it evaluates func(*arg, **kwargs)
+    and returns the result. After this the finalizer is dead, and
+    calling it just returns None.
+
+    When the program exits any remaining finalizers for which the
+    atexit attribute is true will be run in reverse order of creation.
+    By default atexit is true.
+    """
+
+    # Finalizer objects don't have any state of their own.  They are
+    # just used as keys to lookup _Info objects in the registry.  This
+    # ensures that they cannot be part of a ref-cycle.
+
+    __slots__ = ()
+    _registry = {}
+    _shutdown = False
+    _index_iter = itertools.count()
+    _dirty = False
+
+    class _Info:
+        __slots__ = ("weakref", "func", "args", "kwargs", "atexit", "index")
+
+    def __init__(self, obj, func, *args, **kwargs):
+        info = self._Info()
+        info.weakref = ref(obj, self)
+        info.func = func
+        info.args = args
+        info.kwargs = kwargs or None
+        info.atexit = True
+        info.index = next(self._index_iter)
+        self._registry[self] = info
+        finalize._dirty = True
+
+    def __call__(self, _=None):
+        """If alive then mark as dead and return func(*args, **kwargs);
+        otherwise return None"""
+        info = self._registry.pop(self, None)
+        if info and not self._shutdown:
+            return info.func(*info.args, **(info.kwargs or {}))
+
+    def detach(self):
+        """If alive then mark as dead and return (obj, func, args, kwargs);
+        otherwise return None"""
+        info = self._registry.get(self)
+        obj = info and info.weakref()
+        if obj is not None and self._registry.pop(self, None):
+            return (obj, info.func, info.args, info.kwargs or {})
+
+    def peek(self):
+        """If alive then return (obj, func, args, kwargs);
+        otherwise return None"""
+        info = self._registry.get(self)
+        obj = info and info.weakref()
+        if obj is not None:
+            return (obj, info.func, info.args, info.kwargs or {})
+
+    @property
+    def alive(self):
+        """Whether finalizer is alive"""
+        return self in self._registry
+
+    @property
+    def atexit(self):
+        """Whether finalizer should be called at exit"""
+        info = self._registry.get(self)
+        return bool(info) and info.atexit
+
+    @atexit.setter
+    def atexit(self, value):
+        info = self._registry.get(self)
+        if info:
+            info.atexit = bool(value)
+
+    def __repr__(self):
+        info = self._registry.get(self)
+        obj = info and info.weakref()
+        if obj is None:
+            return '<%s object at %#x; dead>' % (type(self).__name__, id(self))
+        else:
+            return '<%s object at %#x; for %r at %#x>' % \
+                (type(self).__name__, id(self), type(obj).__name__, id(obj))
+
+    @classmethod
+    def _select_for_exit(cls):
+        # Return live finalizers marked for exit, oldest first
+        L = [(f,i) for (f,i) in cls._registry.items() if i.atexit]
+        L.sort(key=lambda item:item[1].index)
+        return [f for (f,i) in L]
+
+    @classmethod
+    def _exitfunc(cls):
+        # At shutdown invoke finalizers for which atexit is true.
+        # This is called once all other non-daemonic threads have been
+        # joined.
+        reenable_gc = False
+        try:
+            if cls._registry:
+                import gc
+                if gc.isenabled():
+                    reenable_gc = True
+                    gc.disable()
+                pending = None
+                while True:
+                    if pending is None or finalize._dirty:
+                        pending = cls._select_for_exit()
+                        finalize._dirty = False
+                    if not pending:
+                        break
+                    f = pending.pop()
+                    try:
+                        # gc is disabled, so (assuming no daemonic
+                        # threads) the following is the only line in
+                        # this function which might trigger creation
+                        # of a new finalizer
+                        f()
+                    except Exception:
+                        sys.excepthook(*sys.exc_info())
+                    assert f not in cls._registry
+        finally:
+            # prevent any more finalizers from executing during shutdown
+            finalize._shutdown = True
+            if reenable_gc:
+                gc.enable()
+
+atexit.register(finalize._exitfunc)

Misc/NEWS

@@ -74,6 +74,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #15528: Add weakref.finalize to support finalization using
+  weakref callbacks.
+
 - Issue #14173: Avoid crashing when reading a signal handler during
   interpreter shutdown.