Weakref and __slots__
Without a
__weakref__variable for each instance, classes defining__slots__do not support weak references to its instances. If weak reference support is needed, then add__weakref__to the sequence of strings in the__slots__declaration.
From the Python documentation.
If you add __weakref__ to __slots__, your code will work:
>>> from weakref import ref>>>>>> class Klass(object):>>> __slots__ = ['foo', '__weakref__']>>> def __init__(self):>>> self.foo = 'bar'>>> k = Klass()>>> k => <__main__.Klass object at ...>>>> r = ref(k)>>> r => <weakref at ...; to 'Klass' at ...>
You have to add __weakref__ to the list of slots. It's one of the __slots__ quirks. Prior to 2.3, even this didn't work, but luckily your version isn't that old.