* In the examples you posted (such as http://codespeak.net/svn/pypy/trunk/pypy/tool/algo/graphlib.py ), it appears that PyPy already has an identity dict, so how are they helped by adding one to the collections module?
My purpose with those examples was to prove it as a generally useful utility.
* Most of the posted examples already work with regular dicts (which check
identity before they check equality) -- don't the other implementations already implement regular dicts which need to have identity-implied-equality in order to pass the test suite? I would expect the following snippet to work under all versions and implementations of Python:
>>> class A: ... pass >>> a = A() >>> d = {a: 10} >>> assert d[a] == 10 # uses a's identity for lookup
Yes, but that would be different if you have two "a"s with __eq__ defined to be equal and you want to hash them separately.
None of the presented examples take advantage of that property. All of them work with regular dictionaries. This proposal is still use case challenged. AFAICT from code searches, the idea of needing to override an existing __eq__ with an identity-only comparison seems to never come up. It would not even be popular as an ASPN recipe. Moreover, I think that including it in the standard library would be harmful. The language makes very few guarantees about object identity. In most cases a user would far better off using a regular dictionary. If a rare case arose where __eq__ needed to be overridden with an identity-only check, it is not hard to write d[id(obj)]=value. Strong -1 on including this in the standard library. Raymond P.S. ISTM that including subtly different variations of a data type does more harm than good. Understanding how to use an identity dictionary correctly requires understanding the nuances of object identity, how to keep the object alive outside the dictionary (even if the dictionary keeps it alive, a user still needs an external reference to be able to do a lookup), and knowing that the version proposed for CPython has dramatically worse speed/space performance than a regular dictionary. The very existence of an identity dictionary in collections is likely to distract a user away from a better solution using: d[id(obj)]=value.