[Python-ideas] Identity dicts and sets

[Serhiy Storchaka]

On 03.01.13 20:32, Terry Reedy wrote:
> Yes, and my point was that we effectively already have such things.
>
> class Node(): pass
>
> Instances of Node wrap a dict as .__dict__, but are compared by wrapper
> identity rather than dict value. A set of such things is effectively an
> identity set. In 3.3+, if the instances all have the same attributes (if
> the .__dicts__ all have the same keys), there is only one (not too
> sparse) hashed list of keys for all instances and one corresponding (not
> too sparse) list of values for each instance.
>
> Also, which I did not say before: if one instead represents nodes by a
> unique integer or string or by a list that starts with such a unique
> identifier, then equality is again effectively identity and a set (or
> sequence) of such things is effectively an identity set. This
> corresponds to a standard database table where records have keys, so
> that the identity of records is not lost when reordered or removed from
> the table.

Not always you can choose node type. Sometimes nodes already exists and 
you should just work with them.

>  >> so I don't see what would be gained.
>
> You are proposing (yet-another) dict variation for use in *python* code.

In fact I think first of all about C code. Now using identity dict/set 
idiom is rather cumbersome in C code. With standard IdentityDict it 
should be so simple as using an ordinary dict.

> That requires more justification than a few percent speedup in
> specialized usages. It should make python programming substantially
> easier in multiple use cases. I do not yet see this in regard to graph
> algorithm.

Identity dict/set idiom used at least in followed stdlib modules: 
_threading_local, xmlrpc.client, json, lib2to3 (xrange fixer), copy, 
unittest.mock, idlelib (rpc, remote debugger and browser), ctypes, 
doctest, pickle, cProfile. May be it is implicitly used in some other 
places or can be used.