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Issue #14631: Add a new :class:`weakref.WeakMethod` to simulate weak references to bound methods.

This commit is contained in:
Antoine Pitrou 2012-11-17 18:57:38 +01:00
parent 25bbe5e0bc
commit c3afba104a
4 changed files with 227 additions and 1 deletions
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29
Doc/library/weakref.rst
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@ -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()
<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.

140
Lib/test/test_weakref.py
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@ -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,

56
Lib/weakref.py
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@ -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):

3
Misc/NEWS
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@ -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.