[Python-Dev] gc ideas -- sparse memory

Antoine Pitrou solipsis at pitrou.net
Sat Dec 4 12:37:35 CET 2010

On Sat, 4 Dec 2010 15:06:45 +1100
Cameron Simpson <cs at zip.com.au> wrote:
> On 03Dec2010 18:15, James Y Knight <foom at fuhm.net> wrote:
> | On Dec 3, 2010, at 6:04 PM, Terry Reedy wrote:
> | > gc is implementation specific. CPython uses ref counting + cycle
> | > gc. A constraint on all implementations is that objects have a fixed,
> | > unique id during their lifetime. CPython uses the address as the id, so
> | > it cannot move objects. Other implementations do differently. Compacting
> | > gc requires an id to current address table or something.
> | 
> | It's somewhat unfortuante that python has this constraint, instead of
> | the looser: "objects have a fixed id during their lifetime", which is
> | much easier to implement, and practically as useful.
> Python doesn't have the constraint you imagine; it _does_ have "objects
> have a fixed id during their lifetime".
> _CPython_ has this constraint because it uses the address as the id,
> which is free and needs no papping or extra storage.

Well, the main reason CPython has this constraint is that the C API
mandates constant object pointers. That we can then reuse the pointer
for id() is only a consequence of this.



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