[Python-ideas] An identity dict

In the spirit of collections.OrderedDict and collections.defaultdict, I'd like to propose collections.identitydict. It would function just like a normal dictionary, but ignore hash values and comparison operators and merely lookup keys based on the key's id(). This dict is very useful for keep track of objects that should not be compared by normal comparison methods. For example, I would use an identitydict to hold weak references that would otherwise fallback to their referant object's hash. An advantage of formalizing this in collections would be to enable other Python implementations like PyPy, where id() is expensive, to provide an optimized identitydict. Regards, Benjamin