[Python-checkins] cpython: Issue #14631: Add a new :class:`weakref.WeakMethod` to simulate weak references
antoine.pitrou
python-checkins at python.org
Sat Nov 17 19:00:01 CET 2012
http://hg.python.org/cpython/rev/27c20650aeab
changeset: 80476:27c20650aeab
user: Antoine Pitrou <solipsis at pitrou.net>
date: Sat Nov 17 18:57:38 2012 +0100
summary:
Issue #14631: Add a new :class:`weakref.WeakMethod` to simulate weak references to bound methods.
files:
Doc/library/weakref.rst | 29 +++++
Lib/test/test_weakref.py | 140 +++++++++++++++++++++++++++
Lib/weakref.py | 56 ++++++++++-
Misc/NEWS | 3 +
4 files changed, 227 insertions(+), 1 deletions(-)
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -192,6 +192,35 @@
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()
+ <bound method C.method of <__main__.C object at 0x7fc859830220>>
+ >>> 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
--- a/Lib/test/test_weakref.py
+++ b/Lib/test/test_weakref.py
@@ -47,6 +47,11 @@
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 @@
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 @@
def test_main():
support.run_unittest(
ReferencesTestCase,
+ WeakMethodTestCase,
MappingTestCase,
WeakValueDictionaryTestCase,
WeakKeyDictionaryTestCase,
diff --git a/Lib/weakref.py b/Lib/weakref.py
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -27,7 +27,61 @@
__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
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -127,6 +127,9 @@
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.
--
Repository URL: http://hg.python.org/cpython
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