Circular graphs of collections.OrderedDict are uncollectable due to the presence of OrderedDict.__del__. >>> from collections import OrderedDict >>> import gc >>> left, right = OrderedDict(), OrderedDict() >>> left['other'] = right >>> right['other'] = left >>> assert not gc.garbage >>> del left, right >>> gc.collect() 10 >>> assert not gc.garbage Traceback (most recent call last): File "", line 1, in AssertionError Not an issue in 3.1.1, have not verified with 3.1.2.
This is an unfortunate aspect of using __del__. I don't see a way around it without reintroducing weak references. Of course, your code can also use weak ref proxies to avoid creating uncollectible circular garbage.
I find the behavior surprising compared to dict and other containers, where this is not an issue and weakrefs are not required in user code. I would not be surprised, however, to have to wait for a gc.collect() to clean up my cycles like I do for regular objects. For what it's worth, the pure-python alternative linked in the docs uses neither __del__ nor weakrefs, though it is undoubtably less efficient than this implementation. And the workaround 'del OrderedDict.__del__' seems to work as well, if one is willing to wait for a gc collection.
On #python-dev, Raymond, Amaury and Benjamin agreed that __del__ should be removed, with the rationale that this method should be used to release resources other than memory.
