[Python-Dev] gc ideas -- sparse memory

Steven D'Aprano steve at pearwood.info
Sat Dec 4 07:48:13 CET 2010

James Y Knight wrote:
> On Dec 3, 2010, at 10:50 PM, Terry Reedy wrote:
>> On 12/3/2010 7:46 PM, James Y Knight wrote:
>>> Sure they are. This is what Java provides you, for example. If you
>>> have fixed, but potentially non-unique ids (in Java you get this
>>> using "identityHashCode()"), you can still make an identity
>> I do not see the point of calling a (non-unique) hash value the identity
> My point was simply that a) it's an unfortunate constraint on potential GC implementations that objects need to have a fixed and unique id in Python, and b) that it's not actually necessary to have such a constraint (in the abstract sense of required; obviously it's a requirement upon Python *today*, due to existing code which depends upon that promise). 

I'm afraid I don't follow you. Unless you're suggesting some sort of 
esoteric object system whereby objects *don't* have identity (e.g. where 
objects are emergent properties of some sort of distributed, 
non-localised "information"), any object naturally has an identity -- 

It seems to me that an identify function must obey one constraint:

* Two objects which exist simultaneously have the same identity if
   and only if they are the same object i.e. id(x) == id(y) if and
   only if x is y.

Other than that, an implementation is free to make id() behave any way 
they like. CPython uses the memory location of the object, but Jython 
and IronPython use an incrementing counter which is never re-used for 
the lifetime of the Python process. CPython's implementation implies 
that objects may not be moved in memory, but that's not a language 
constraint, that's an implementation issue.

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? Why would you bother using that function when you could just use 
x == y instead?

> Would you be happier if I had said "it's unfortunate that Python has an "id" function instead of an "identityHashValue" function? I suppose that's what I really meant. Python the language would not have been harmed had it had from the start an identityHashValue() function instead of an id() function. In the CPython implementation, it may even have had the exact same behavior, but would've allowed other implementations more flexibility.

No, because I don't see what the point of identityHashValue or why you 
would ever bother to use it.


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