Question

I have a project where i’m trying to use weakrefs with callbacks, and I don’t understand what I’m doing wrong. I have created simplified test that shows the exact behavior i’m confused with.

Why is it that in this test test_a works as expected, but the weakref for self.MyCallbackB disappears between the class initialization and calling test_b? I thought like as long as the instance (a) exists, the reference to self.MyCallbackB should exist, but it doesn’t.

import weakref  class A(object):     def __init__(self):          def MyCallbackA():             print 'MyCallbackA'         self.MyCallbackA = MyCallbackA          self._testA = weakref.proxy(self.MyCallbackA)         self._testB = weakref.proxy(self.MyCallbackB)      def MyCallbackB(self):         print 'MyCallbackB'      def test_a(self):         self._testA()      def test_b(self):         self._testB()  if __name__ == '__main__':     a = A()         a.test_a()     a.test_b() 

Answer 1

You want a WeakMethod.

An explanation why your solution doesn’t work can be found in the discussion of the recipe:

Normal weakref.refs to bound methods don’t quite work the way one expects, because bound methods are first-class objects; weakrefs to bound methods are dead-on-arrival unless some other strong reference to the same bound method exists.