From c3afba104aef5032e114b4f5cac0a3bbdfef2bba Mon Sep 17 00:00:00 2001 From: Antoine Pitrou Date: Sat, 17 Nov 2012 18:57:38 +0100 Subject: [PATCH] Issue #14631: Add a new :class:`weakref.WeakMethod` to simulate weak references to bound methods. --- Doc/library/weakref.rst | 29 ++++++++ Lib/test/test_weakref.py | 140 +++++++++++++++++++++++++++++++++++++++ Lib/weakref.py | 56 +++++++++++++++- Misc/NEWS | 3 + 4 files changed, 227 insertions(+), 1 deletion(-) diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst index 224f442edde..1bf6b587708 100644 --- a/Doc/library/weakref.rst +++ b/Doc/library/weakref.rst @@ -192,6 +192,35 @@ These method have the same issues as the and :meth:`keyrefs` method of discarded when no strong reference to it exists any more. +.. class:: WeakMethod(method) + + A custom :class:`ref` subclass which simulates a weak reference to a bound + method (i.e., a method defined on a class and looked up on an instance). + Since a bound method is ephemeral, a standard weak reference cannot keep + hold of it. :class:`WeakMethod` has special code to recreate the bound + method until either the object or the original function dies:: + + >>> class C: + ... def method(self): + ... print("method called!") + ... + >>> c = C() + >>> r = weakref.ref(c.method) + >>> r() + >>> r = weakref.WeakMethod(c.method) + >>> r() + > + >>> r()() + method called! + >>> del c + >>> gc.collect() + 0 + >>> r() + >>> + + .. versionadded:: 3.4 + + .. data:: ReferenceType The type object for weak references objects. diff --git a/Lib/test/test_weakref.py b/Lib/test/test_weakref.py index 92a47133cbc..dc6bf77572c 100644 --- a/Lib/test/test_weakref.py +++ b/Lib/test/test_weakref.py @@ -47,6 +47,11 @@ class Object: 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): @@ -880,6 +885,140 @@ class SubclassableWeakrefTestCase(TestBase): self.assertEqual(self.cbcalled, 0) +class WeakMethodTestCase(unittest.TestCase): + + def _subclass(self): + """Return a Object subclass overriding `some_method`.""" + class C(Object): + def some_method(self): + return 6 + return C + + def test_alive(self): + o = Object(1) + r = weakref.WeakMethod(o.some_method) + self.assertIsInstance(r, weakref.ReferenceType) + self.assertIsInstance(r(), type(o.some_method)) + self.assertIs(r().__self__, o) + self.assertIs(r().__func__, o.some_method.__func__) + self.assertEqual(r()(), 4) + + def test_object_dead(self): + o = Object(1) + r = weakref.WeakMethod(o.some_method) + del o + gc.collect() + self.assertIs(r(), None) + + def test_method_dead(self): + C = self._subclass() + o = C(1) + r = weakref.WeakMethod(o.some_method) + del C.some_method + gc.collect() + self.assertIs(r(), None) + + def test_callback_when_object_dead(self): + # Test callback behaviour when object dies first. + C = self._subclass() + calls = [] + def cb(arg): + calls.append(arg) + o = C(1) + r = weakref.WeakMethod(o.some_method, cb) + del o + gc.collect() + self.assertEqual(calls, [r]) + # Callback is only called once. + C.some_method = Object.some_method + gc.collect() + self.assertEqual(calls, [r]) + + def test_callback_when_method_dead(self): + # Test callback behaviour when method dies first. + C = self._subclass() + calls = [] + def cb(arg): + calls.append(arg) + o = C(1) + r = weakref.WeakMethod(o.some_method, cb) + del C.some_method + gc.collect() + self.assertEqual(calls, [r]) + # Callback is only called once. + del o + gc.collect() + self.assertEqual(calls, [r]) + + @support.cpython_only + def test_no_cycles(self): + # A WeakMethod doesn't create any reference cycle to itself. + o = Object(1) + def cb(_): + pass + r = weakref.WeakMethod(o.some_method, cb) + wr = weakref.ref(r) + del r + self.assertIs(wr(), None) + + def test_equality(self): + def _eq(a, b): + self.assertTrue(a == b) + self.assertFalse(a != b) + def _ne(a, b): + self.assertTrue(a != b) + self.assertFalse(a == b) + x = Object(1) + y = Object(1) + a = weakref.WeakMethod(x.some_method) + b = weakref.WeakMethod(y.some_method) + c = weakref.WeakMethod(x.other_method) + d = weakref.WeakMethod(y.other_method) + # Objects equal, same method + _eq(a, b) + _eq(c, d) + # Objects equal, different method + _ne(a, c) + _ne(a, d) + _ne(b, c) + _ne(b, d) + # Objects unequal, same or different method + z = Object(2) + e = weakref.WeakMethod(z.some_method) + f = weakref.WeakMethod(z.other_method) + _ne(a, e) + _ne(a, f) + _ne(b, e) + _ne(b, f) + del x, y, z + gc.collect() + # Dead WeakMethods compare by identity + refs = a, b, c, d, e, f + for q in refs: + for r in refs: + self.assertEqual(q == r, q is r) + self.assertEqual(q != r, q is not r) + + def test_hashing(self): + # Alive WeakMethods are hashable if the underlying object is + # hashable. + x = Object(1) + y = Object(1) + a = weakref.WeakMethod(x.some_method) + b = weakref.WeakMethod(y.some_method) + c = weakref.WeakMethod(y.other_method) + # Since WeakMethod objects are equal, the hashes should be equal. + self.assertEqual(hash(a), hash(b)) + ha = hash(a) + # Dead WeakMethods retain their old hash value + del x, y + gc.collect() + self.assertEqual(hash(a), ha) + self.assertEqual(hash(b), ha) + # If it wasn't hashed when alive, a dead WeakMethod cannot be hashed. + self.assertRaises(TypeError, hash, c) + + class MappingTestCase(TestBase): COUNT = 10 @@ -1455,6 +1594,7 @@ __test__ = {'libreftest' : libreftest} def test_main(): support.run_unittest( ReferencesTestCase, + WeakMethodTestCase, MappingTestCase, WeakValueDictionaryTestCase, WeakKeyDictionaryTestCase, diff --git a/Lib/weakref.py b/Lib/weakref.py index 339fcf4718d..8f9c107aa6f 100644 --- a/Lib/weakref.py +++ b/Lib/weakref.py @@ -27,7 +27,61 @@ ProxyTypes = (ProxyType, CallableProxyType) __all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs", "WeakKeyDictionary", "ReferenceType", "ProxyType", "CallableProxyType", "ProxyTypes", "WeakValueDictionary", - "WeakSet"] + "WeakSet", "WeakMethod"] + + +class WeakMethod(ref): + """ + A custom `weakref.ref` subclass which simulates a weak reference to + a bound method, working around the lifetime problem of bound methods. + """ + + __slots__ = "_func_ref", "_meth_type", "_alive", "__weakref__" + + def __new__(cls, meth, callback=None): + try: + obj = meth.__self__ + func = meth.__func__ + except AttributeError: + raise TypeError("argument should be a bound method, not {}" + .format(type(meth))) from None + def _cb(arg): + # The self-weakref trick is needed to avoid creating a reference + # cycle. + self = self_wr() + if self._alive: + self._alive = False + if callback is not None: + callback(self) + self = ref.__new__(cls, obj, _cb) + self._func_ref = ref(func, _cb) + self._meth_type = type(meth) + self._alive = True + self_wr = ref(self) + return self + + def __call__(self): + obj = super().__call__() + func = self._func_ref() + if obj is None or func is None: + return None + return self._meth_type(func, obj) + + def __eq__(self, other): + if isinstance(other, WeakMethod): + if not self._alive or not other._alive: + return self is other + return ref.__eq__(self, other) and self._func_ref == other._func_ref + return False + + def __ne__(self, other): + if isinstance(other, WeakMethod): + if not self._alive or not other._alive: + return self is not other + return ref.__ne__(self, other) or self._func_ref != other._func_ref + return True + + __hash__ = ref.__hash__ class WeakValueDictionary(collections.MutableMapping): diff --git a/Misc/NEWS b/Misc/NEWS index a544b8b0182..aa9f6c75507 100644 --- a/Misc/NEWS +++ b/Misc/NEWS @@ -127,6 +127,9 @@ Core and Builtins Library ------- +- Issue #14631: Add a new :class:`weakref.WeakMethod` to simulate weak + references to bound methods. + - Issue #16469: Fix exceptions from float -> Fraction and Decimal -> Fraction conversions for special values to be consistent with those for float -> int and Decimal -> int. Patch by Alexey Kachayev.