On 2010-06-01 03:23, Raymond Hettinger wrote:
On May 31, 2010, at 5:45 PM, Benjamin Peterson wrote:
Essentially these are places where defined equality should not matter.
"should not matter" is the important part here. It might have been clearer to say "should be ignored" instead. I think Raymond is misunderstanding it.
Essentially, these are cases where an identity dictionary isn't necessary and would in-fact be worse performance-wise in every implementation except for PyPy which can compile the pure python code for indentity_dict.py.
It is necessary, because the objects involved might define their own __hash__ and __cmp__/__eq__, and these should *not* be used.
Sorry, but I think this idea is a total waste. Perhaps post it as a recipe, but it doesn't make sense to try to inject it into the standard library.
I don't think it is a total waste, but I have seen two ideas in this thread that I find more generally useful. One is "collections.keyfuncdict", which could be trivially used as an identitydict. The other is a WeakIdentityDict, which is a WeakKeyDict that uses only the identity of the keys for hashing/equality. These two are independent, one cannot be used to implement the other (unless collections.keyfuncdict grows an option to not keep strong refs to the keys, perhaps by providing the inverse keyfunc instead). Anyway, +0.1 on identitydict and +1 on each of collection.keyfuncdict and WeakIdentityDict. - Jacob