Created
August 15, 2022 15:35
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MCVE of how finalizers cannot depend on weak reference state in the presence of circular references.
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| from weakref import WeakValueDictionary | |
| import gc | |
| objs = WeakValueDictionary() | |
| class Registered: | |
| def __init__(self): | |
| objs[id(self)] = self | |
| def __del__(self): | |
| del objs[id(self)] | |
| class Simple(Registered): | |
| pass | |
| class Circular(Registered): | |
| def __init__(self): | |
| super().__init__() | |
| # create a circular reference | |
| def test(): | |
| print(self) | |
| self._test_fn = test | |
| def test_simple(): | |
| gc.disable() | |
| local_objs = [Simple() for a in range(10)] | |
| assert len(objs) == 10 | |
| local_objs = [] | |
| assert len(objs) == 0 | |
| def test_circular(): | |
| """ | |
| This raises because the object is not longer in "objs" by the time the object's | |
| __del__ is called - but this is only the case when there are circular references. | |
| """ | |
| gc.disable() | |
| local_objs = [Circular() for a in range(10)] | |
| assert len(objs) == 10 | |
| local_objs = [] | |
| assert len(objs) == 10 | |
| gc.collect() | |
| assert len(objs) == 0 | |
| if __name__ == "__main__": | |
| test_simple() | |
| test_circular() |
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