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

Christian Tismer tismer at stackless.com
Tue Jun 1 08:44:58 EDT 2004

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
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