[Python-ideas] An identity dict

[Benjamin Peterson]

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