[Python-Dev] Why aren't more things weak referencable

[Christian Tismer]

Guido van Rossum wrote:

>>Same here. I would not vote to make strings or tuples or any other
>>tiny type weak-reffed in the first place.
>>Instead I would add the possible support to derived types, via
>>the __slot__ mechanism for instance.
>>There is a little coding necessary to make the generic code
>>handle the case of var-sized objects, but this is doable
>>and not very complicated.
>>
>>This may be really needed or not. if we can create the rule
>>"every derived type *can* have weak-refs", this is simpler
>>to memorize than "well, most can, some cannot".
> 
> 
> With some (considerable?) effort, slots on var-sized objects can
> certainly be supported -- the same approach as used for __dict__
> should work.  I expect it might slow down the normal case a bit,
> unless you define a new descriptor type just for this.

Yes, I didn't recognize the tricky implementation until you
said this. _PyObject_GetDictPtr looks at the end of the
object if type->tp_dictoffset is < 0.
This would become more complicated in the presence of weakref
and other slots. Well, fairly, they would simply all store
their offset from the end of the object.

A not so costly approach could be to change slots a little:
The slot offset is now computed on the beginning of the
object's structure. There could be either a type flag that
says "you must add an offset", or a new type slot which is either
a function that computes and additional offset, or NULL.
It would probably even suffice to check if varsize is nonzero
and then do a little computation to bump the offset.

cheers - chris
-- 
Christian Tismer             :^)   <mailto:tismer at stackless.com>
Mission Impossible 5oftware  :     Have a break! Take a ride on Python's
Johannes-Niemeyer-Weg 9a     :    *Starship* http://starship.python.net/
14109 Berlin                 :     PGP key -> http://wwwkeys.pgp.net/
work +49 30 89 09 53 34  home +49 30 802 86 56  mobile +49 173 24 18 776
PGP 0x57F3BF04       9064 F4E1 D754 C2FF 1619  305B C09C 5A3B 57F3 BF04
      whom do you want to sponsor today?   http://www.stackless.com/