[Python-ideas] An identity dict

[M.-A. Lemburg]

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
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