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In Python weakref document( http://docs.python.org/library/weakref.html ), it says that
Several built-in types such as list and dict do not directly support weak references but can add support through subclassing
I think creating weakref for big dict could be useful in some real cases. I'm wondering what's the reason behind that implementation ?
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The weakref module allows the Python programmer to create weak references to objects. In the following, the term referent means the object which is referred to by a weak reference.
A WeakValueDictionary d is a mapping that references its values weakly. When the reference count of a value v in d goes to 0 , all items of d such that d [ k ] is v disappear. adict is used to initialize the mapping.
Most of the built-in types are not directly weak referenceable (e.g. str, int, float, list, dict, None), and there are a few that cannot even be made so by sub-classing (e.g. tuples in CPython).
Some details about the underlying implementation of weakrefs for several built-in types can be found in this March-2005 python-list post by Raymond Hettinger.
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