[Python-Dev] gc ideas -- sparse memory

[Stephen J. Turnbull]

Steven D'Aprano writes:
 > Martin v. Löwis wrote:

 > >> It seems counter-productive to me to bother with an identity function
 > >> which doesn't meet that constraint. If id(x) == id(y) implies nothing
 > >> about x and y (they may, or may not, be the same object) then what's the
 > >> point?
 > > 
 > > See James' explanation: it would be possible to use this as the
 > > foundation of an identity hash table.
 > 
 > I'm afraid James' explanation didn't shed any light on the question to 
 > me. It seems to me that Java's IdentityHashValue[sic -- I think the 
 > correct function name is actually IdentityHashCode] is equivalent to 
 > Python's hash(), not to Python's id(), and claiming it is related to 
 > identity is misleading and confusing.

Not quite equivalent.  Python's hash() obeys a certain additional
constraint (that numeric values that compare equal have the same
hash), and IdentityHashValue presumably won't work at all on numbers
in Java (since they aren't objects).

Bikeshedding aside, I have to agree with you: there's no point in
calling such a function id() or IdentityAnything().  Everything you
need to know (including both the fact that it could be an efficient
way to search for a unique object, and the possible non-uniqueness of
the object assigned that code) is contained in the word "hash".