Benjamin Peterson wrote:
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.
Are you sure this is a good idea for the stdlib ? id(obj) gives you the current storage address of an object in CPython and these can be reused over time. Using a key object to such a dict would not secure it's persistence throughout the usage lifetime in that dict, since d[id(key)] = 1 doesn't increase the refcount of key. Such reuse is bound to happen often for some Python object types, due to the free-lists we're using in CPython and the Python allocator which is optimized for reuse of fixed-size memory chunks. For experts, such a dictionary may have some value (e.g. to work around the hash() requirement of normal dictionaries), but I feel that newbies would have a hard time understanding all the implications. -- Marc-Andre Lemburg eGenix.com Professional Python Services directly from the Source (#1, May 30 2010)
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