Strange behavior of weak references (weakref module)
jason at jorendorff.com
Thu Dec 20 00:10:24 CET 2001
> I found this behavior (bug?) when I want to create weak reference to a
> instance method. It's hard to describe (using my poor English writing
> ability), so I just attach the way to reproduce the situation. And
> notice that the id() of a instance method reference changes after each
> function call!
This is correct behavior. Each time you do a.test, Python creates
a new "bound instance method" object.
>>> class A:
... def foo(self, word):
... print "foo:", word
>>> a = A()
>>> m = a.foo
<__main__.A instance at 0x00948648>
<function foo at 0x0092FD00>
You can create a "weak method reference" type that makes weak
references to both the object and the function.
def __init__(self, method):
self.self_ref = weakref.ref(method.im_self)
self.func_ref = weakref.ref(method.im_func)
def __call__(self, *args, **kwds):
im_self = self.self_ref()
im_func = self.func_ref()
if im_self is None or im_func is None:
# One or both of the references died. Do nothing.
args = (im_self,) + args
return im_func(*args, **kwds)
>>> a = A()
>>> z = WeakMethod(a.foo)
>>> del a # Get rid of a
>>> z("eggs") # Does nothing, because the instance a is gone.
## Jason Orendorff http://www.jorendorff.com/
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